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GEOL6379-Applied Biostratigraphy - Day6 09-10-2022-Part1
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00:14 why? Or is that like Come on now. That's not what
00:30 want. Where is that sitting sitting ? I can't and shake please.
00:56 the only solution. Mhm. There's still, that's the best we
01:21 . What about all this? You get rid of it. Never seen
01:29 gonna it's gonna be in the Alright, so in that case do
01:42 this way. Okay, so we here yesterday talking about the Wetherill Concordia
02:04 . The points on the red line equal in age to the two
02:10 Um But in some cases we will points that fall off that line.
02:17 And so how do we deal with ? And as I mentioned this this
02:21 less of a problem these days as talk about in a little bit but
02:26 lots of papers out there that will this. So if you're ever going
02:29 look at a paper that's more than years old, you're going to have
02:32 probably deal with this as well. have points which um as I
02:37 you know, we'll make Concordia here it's the discord e a. That
02:40 have to interpret sometimes if all the land right on top of Concordia that
02:45 not a problem. Well, it's a big problem. Um So as
02:52 we've got these two intercepts but unfortunately the upper intercept is the age of
02:57 of crystallization and sometimes the lower intercept the age of crystallization and I should
03:02 out that I'm saying age of crystallization because I'm talking about uranium lead in
03:09 if we were looking at uranium lead some other middle like appetite, it
03:14 be the age of crystallization, it be the age of retaining closure because
03:19 closure temperature of latin appetite is only 500 degrees. The closure temperature of
03:24 zircon is eight or 900 degrees, is essentially the same thing as
03:29 but you can date other phases as . Talk about and so age of
03:35 is really specific to zircon. And and and and the way of always
03:41 around that and never making a mistake never talk about crystallization, just talk
03:46 the time of obtaining a closed system this case they happen to be the
03:50 thing. So we talked about this here where we have a sample which
03:59 is perturbed and this determines can be growth of news er cons on top
04:07 old ones and what we have been that line is a continuum between fully
04:13 bits and fully young bits, you , and so we may have
04:17 you can have a grain, here's zircon and you can have an overgrowth
04:23 just sort of looks like that or could have an old bit and it's
04:30 overgrowth, it looks like that both these parts be referred to as the
04:39 component, um same concept going It's just that you have, you
04:44 , a different mix. So there's mixing line. Um and as I
04:49 , it can also you get a mixing line if you have an episode
04:54 what's called lead loss and I'll describe Los in a minute. But just
04:57 the purposes of this diagram, they the same. So That's what would
05:04 like if you had a zircons that say 1700 million years old and then
05:12 a an event like this, you all those points towards the origin.
05:16 if this event of lead loss or growth was over, then these things
05:22 continue to evolve as they would and they would move up to this
05:27 then you would have a line, discord a line that would go between
05:32 2800, 20 912:00 because it was hundreds when this sample was at 12
05:44 17, that this sample was down . So so the additional 1200 million
05:52 are is this And this notice that is a good point, notice that
05:59 distance on this line is not this is 500 million years. So
06:04 this again, that's because of the the unequal rates of decay. So
06:14 how you could, that's how it look if the upper intercept was the
06:19 age and the lower intercept was some event. Um I skipped over this
06:26 time I go ahead and mention there's another diagram. If we look
06:29 this diagram, this has 207 over 35 to 6/2 38 you can get
06:35 same information by plotting this diagram, is which is called the terra Wasser
06:41 diagram because it was first introduced in paper by Terror and Wasser Burg.
06:45 this is different in that here we the 2 38/2 06, which is
06:49 the Y axis of the other And here we have 20 6/2 oh
06:54 . And remember we showed how the genic 206207 ratio is also a function
07:00 the age, but we have two measures of the age here, just
07:03 we do on the other way. reason that this might be good in
07:06 ways is because we have eliminated from diagram, the isotope that is generally
07:11 hardest to measure Uranium 2 35 is small. Um So some people
07:19 will do this. So if you this diagram, it's equivalent to this
07:25 . Uh but this diagram, the , the weather will diagram is still
07:30 a great deal more often. Um the same, the same story would
07:35 up on the terra Wasser burg diagram just that the directions of them in
07:40 diagram, old ages go up this instead of this one. Okay,
07:50 now let's discuss this issue of lead . You might think, well what's
07:54 could I thought you said that the temperature of lead in Zircon was really
07:58 hot and it is and yet there evidences for what looks, you know
08:06 geo chemically like lead loss. And You know, I recommended a book
08:13 you all. I don't know if one of you picked it up.
08:15 a really good book, but you , it's $140. But they have
08:20 really interesting point to make about lead in this book. I love
08:23 They say that the the advances of lead geo chronology have not been made
08:30 understanding Led Los but by learning ways avoid it. So that you can
08:35 analyze samples that don't exhibit this very anymore because we can be much more
08:39 about which samples we choose. But not like anybody's figured out lead
08:44 Uh people have offered these various reasons maybe what you have is a change
08:49 the crystal because of radiation damage when crystal is um when when when radio
08:57 , when when uranium decays it shoots alpha particles and then occasionally it shoots
09:03 this fishing, which makes it all that damages the crystal. And that
09:09 the acidity changes everything about the And we'll see this really matters when
09:14 comes to heli update that can change closure temperature by a lot because the
09:20 you bombard this lattice with radiation, more it changes. And so maybe
09:28 and so once you lose the you know, led to the closure
09:33 of lead may go away. That's meta monetization. Um Another similar process
09:40 be called dilated C when you have pores in a crystal that are formed
09:45 filled at high pressure. When that is released, the fluid inside those
09:52 are still under the hydrostatic pressure that were formed at. And then they
09:56 crack open the crystal physically changing it again, perhaps changing the diffuse it
10:02 , changing the closure temperature. Weathering shouldn't be a big deal. You
10:08 lose lead but it's not gonna affect the isotopic composition of the lead.
10:14 And then some folks will say, maybe it's metamorphosis, episodic loss due
10:18 some thermal disturbance. But once again this sort of goes against the the
10:24 of the observational data and all of experimental data that says for nice,
10:30 zircons, the closure temperatures is super . So these are just thoughts about
10:37 to get lost out of a zircon a world in which you shouldn't have
10:41 at all. But apparently it might . Um we know that meta mechanization
10:47 something that's a real thing and if not looked at a lot of thin
10:52 , maybe you've never seen this but what this is is a bio
10:56 and there's a little zircon inside the type you produce these radiation halos around
11:01 Zircon and what this shows. And can, you know, look how
11:05 like several diameters of zircon away. this shows us is that bio tide
11:10 much more susceptible to radiation damage than is. You can but that's not
11:16 happening in the bio type that's happening the economy and moving out. But
11:22 we can see it better in in type is happening in the market
11:27 Okay, so let's look at some data. Now, here's again,
11:32 sample from the llano uplift. I you one from yesterday and this gave
11:38 same age, but this is zircons a granite from central texas. And
11:44 have 1235 fractions on here, and of them are recorded, but they
11:51 up a fairly nice low line Um And the upper intercept is
11:58 And lower intercept is something we don't see it down there and we can
12:04 the lower intercept because in this case a little consequence because we're not gonna
12:10 it to interpret some geologic events. Now you might say, well,
12:16 am I ignoring it? If if of the possibilities is the lower intercept
12:20 be the crystallization, but we can this lower intercept because of the regional
12:26 , this is a granite on which Canberra and sandstone. So how old
12:34 this granite be younger, older, got to be older than Cameron.
12:52 lower intercept is younger than Cameron. so that's just impossible from the,
12:57 the regional geology. And so clearly only option is to be older than
13:02 , that's our upper intercept. So by applying the, you know,
13:07 the cross cutting relationships or principle of , we know that this lower intercept
13:13 out of bounds. So yes, got oper intercept can be the answer
13:18 into the exception to the answer. if you were given this without any
13:22 context, you might worry a little . Now it is also the case
13:28 unless you have, unless you have on the Concordia diagrams, that sort
13:36 , is that which you never You generally have data? You can
13:40 down here or plot down here, lead loss or the inheritance.
13:45 sometimes, well, you certainly, rarely have data that. It's what's
13:49 up here. You don't have anything . And so that's a that's a
13:53 . Now, I'll show you an in a minute where we have more
13:57 down here than we might have one two points up here. But in
14:04 , the points cluster near the right , the the crystallization intercept. But
14:10 speaking, we have to consider But but by applying some geologic
14:15 that's usually easy. Okay, um that is an example of of this
14:25 loss or perhaps juvenile. This probably isn't juvenile. Now, of course
14:29 can examine your zircons in the in in the microscope and look for this
14:34 of character. And if you say like this, well then you're gonna
14:38 a mixing between old and young. don't think I would guess that if
14:42 you know, if we had access the zircons, we wouldn't see that
14:46 looks like a kind of lead loss . And I should point out again
14:51 this is a paper from 1992 using old data on purpose to show you
14:56 old things because again, you might see quite this much discordance these
15:01 But and and you know, these fractions were defined by some some
15:07 They decided in the lab probably magnetic , maybe some other hand picking choice
15:13 color or shape or size. Um they are almost certainly not one Zurich
15:18 at a time story mixing them And it's probably some of them had
15:25 of these problems that we discussed earlier that would pull them down this
15:30 Um In the simple interpretation, it pull them down to the to the
15:35 that the lower intercept would be the at which this array was developed.
15:40 know, they were all up here zircons, but they were pulled down
15:44 some towards some point. Well they pulled down towards the origin point,
15:50 happened. And then the whole thing evolved up to this. Now,
15:54 this couldn't be people down to the , it doesn't look like it.
15:59 But if it's pulled down to the , then we just say, well
16:01 most of the lead loss occurred Um but people didn't go all this
16:09 to figure out when the lead loss place in this granite. I want
16:12 know the geologic history of central not something fluid flow events, the
16:17 or the Jurassic or whenever. So one way to interpret things to
16:25 The other way, we have to a moment and look at this.
16:29 is a relationship that's been shown by that that shows that the dissolution of
16:36 in uh frenetic melt. It's not easy and it is dependent on the
16:43 and the composition of the magma. this is the equation that you get
16:48 this paper and what you have over is the log of the distribution coefficient
16:54 zirconium as it is partitioned in a or in a liquid. And uh
17:02 that means is that under certain zirconium will stay in the liquid and
17:07 sort or it'll stay in a Zurich Zurich on crystal or in other cases
17:13 will go to the liquid, This is a parameter that illustrates the
17:19 potential of a mineral and it's dependent temperature and it's dependent dependent on
17:27 this this this value. M whereas , some measure of the composition and
17:35 for for many compositions of frenetic Um the temperature has to be remarkably
17:41 in order for that distribution coefficient to to be less than one. And
17:47 it's not less than one, that that you're going to get situations like
17:51 because the zircons will not entirely they won't melt fast enough to where
17:55 move from melting conditions to crystallizing And so you can look at a
18:02 . If you have, if you you have the geochemistry of your
18:06 you can calculate the saturation temperature and even without looking at a microscope,
18:12 can say the likelihood of whether all zircons that were here before is melted
18:17 this grant. And it doesn't happen often. As I said, zircons
18:22 and are hard to get rid And in this case that can be
18:27 problem because if you're interested in the age of the current granite, you
18:33 want to be confused by by having come from the previous history but there
18:39 have it but we can get around . Here's an example of something where
18:44 have a bunch of points. There's point here, there's two points
18:48 12 and once again these data defined line which has an intercept upper and
18:57 . The lower intercepts about 66 and operator stepped is 1400. Um And
19:05 which is the right answer. How is this, granted? Well,
19:09 , we try to apply this Most of the data are down
19:13 so that's probably a clue that that's thing. But more importantly,
19:18 all you gotta do is apply geologic the the field relationship between this
19:26 which was collected in Southeastern Arizona, rock intrudes paleozoic sedimentary rocks. So
19:37 has to be based on that. granite must be younger than the rocks
19:45 intruding and music can't be this The paleozoic rocks are four or 500
19:51 old. This would make it older the rocks that intrudes. That doesn't
19:56 . And so, okay, it's late cretaceous plutonium which is intruding.
20:01 paleozoic rocks perfectly fine. And so you go. I mean, I
20:07 you this this this this diagram without geologic context, you'd have to
20:11 well, this is likely to be 66 million year old granite with some
20:16 in it. But it could be or granite that has uh, you
20:22 , a profound episode of lead loss , that's that's that's quite unlikely.
20:29 theoretically possible. Um Another good thing this diagram is if you look to
20:36 if you look to the older basement Arizona, you'll find evidence that most
20:41 those, you know, really bottom the crust. Rocks are about
20:46 Not all, but some of them of them are about 1400 million years
20:50 . There's a 1400 and there's a million component down there. And so
20:55 looks like the rocks that were melted the cretaceous to produce this granite,
20:59 of that 1400 million year component makes sense. And furthermore, it looks
21:05 that was the only component that was since we have such a nice straight
21:10 . So um None of these points concordant, but they might make a
21:16 line and combine that with the Graphic evidence that's a 66 million year
21:22 rock. Um Here's another example, comes from the Idaho battle if and
21:31 get the same story, although a bit a little bit more extreme
21:35 we have four fractions. Again, is old data, but the concept
21:40 still important. Um four fractions which a lower intercept of 47 and an
21:46 intercept of 1770. Again, we that these rocks, this rock isn't
21:52 because of the rocks that it intrudes layered, fan or rocks, so
21:59 can't be a protozoa rock. It's Eocene rock Which fits with the whole
22:04 geology and the Strata Graffiti. And , the basement rocks of this region
22:10 about 1700. So this shows that chemistry, we would predict that the
22:16 of this magma is one that is sufficient even at high temperatures or perhaps
22:22 competent for this temperature for this composition magma. We can put an upper
22:27 on how chaotic God because it didn't all the previous circum. And you'll
22:33 this a lot because it's hard and , it's hard to melt dessert
22:38 And that's why you'll frequently have uh inherited component. Sometimes there might just
22:44 a whiff of it, you may melted most of them and you might
22:47 only get something like this. Um you'll you'll you'll still have to deal
22:52 it. That is if you're analyzing whole zircon, I'm I'm pretty sure
22:59 have some pictures coming up, but in case, I don't know
23:02 you know, I keep referring to new data. Why why don't we
23:06 about, you know, the um most many laboratories now have the
23:12 to analyze not this whole zircon or not this holder con you can go
23:17 there and either with a laser beam an ion beam interrogate just that little
23:23 of the Zircon. And you that's the that's the great transition of
23:29 40 or 50 years ago. You not only could you have to measure
23:33 whole thing, you have to measure one and this one together with another
23:36 or 20 zircons to get enough lead measure. Now we can shoot that
23:41 a laser or an ion beam. stuff is is uh put into a
23:49 and analyze and get pretty good And so now, and this is
23:55 you don't have to worry about discord data very often because nobody's going to
24:00 the whole shovel full of their They're going to take a circle like
24:04 . And if they're interested in the earth part, they'll just avoid this
24:08 . They won't pick a crystal like . You know, you can see
24:12 in, in the microscope usually pretty . So they'll avoid that because even
24:17 you have a very thin rim, gonna be mixing the two together,
24:22 you've got a big enough rim. now you may want to shoot this
24:26 on its own to see what it , but you won't use it.
24:30 , and, and that will produce points that on a Concordia diagram might
24:35 , might be here and here or here. Um, and that means
24:41 may be interesting and valuable, but , you don't have to worry about
24:47 points. Um, Here's an example even when you have single crystals,
24:54 might get a complication. uh this a paper from 1998 and this is
25:01 a granite in near Mount Everest. well we see bunches or cons and
25:09 plot mostly discordant except for this one here. The rest of them are
25:15 , but they don't form a nice line. But if we, if
25:21 put the limits of how it is here. We see one sort of
25:27 that goes up to 500 and another that goes out to 2.5 billion.
25:33 they both come back to the single down here, just a wee bit
25:36 20. And so the interpretation of granite is it's a 20 million year
25:41 granite which was melting a variety of types that had a variety of previous
25:49 . And this makes sense from the in the Himalayas we have quite a
25:52 of or division flu thons, but also a bunch of our key and
25:57 as well, so To have a between 502 billion is not surprising.
26:06 And so you know here we have example where just one age of thing
26:09 melted. I read an example where variety of ages were melted and I
26:14 remember if every one of these crystals is single crystal, I don't think
26:18 is, I think this one was helps us see that it's on Concordia
26:24 but of course, well that means it's not only a single crystal,
26:29 it's one of its one of these of single crystals that maybe had almost
26:34 inherited but you know, and so can, when you can just because
26:38 have an inherited opponent doesn't mean every in your granite will have that because
26:43 you can just nuclear wait a new and grow a brand new user con
26:46 like you would otherwise. But if old zircons hanging out, it's more
26:52 for that new zircon to grow on previously existing nuclear nation spot. But
26:57 you get lucky and you find one didn't do that grow on its
27:01 you'll get that. Um, so next? Okay, Here's another
27:11 Um this comes from some work I'm right now and this is from
27:18 from a zircon. This is from relied dike in new Mexico and we've
27:24 two scales, one that shows all data and one that suits in the
27:28 all the data. We got a of points down here and one way
27:32 here resume. If we ignore that , we just zoom in on
27:36 We look at these points here and got a bunch of points here that
27:42 going off the gear and then this which plots pretty much at Concordia.
27:48 perfect, it's not on but a bit. And so you've got a
27:53 of a problem here. And what are you gonna define this? Because
27:57 mean there's not an obvious bunch of to put on here. I mean
28:03 , you know, this one has advantage of being concordant and these ones
28:08 the advantage of having nice straight This isn't concorde id perfectly and there's
28:15 obvious line among all those points. so what I said that we should
28:20 and it will take, we'll take point that along with this point produces
28:27 oldest age. We take this point might have an intercept down here.
28:32 we're gonna take the oldest age as most conservative. It could be younger
28:36 that, but it won't be older whatever this line is. That's why
28:39 two are colored in blue because they're only two points that define this
28:44 So this line suggests that we have million year, although it's 1200 plus
28:49 -212000 million year old inherited component. we have an age of about
28:59 Regionally. This was a bit of , but but the rock these dates
29:04 these rocks haven't really been dated. lot of people just looked at him
29:07 I think they called them. Can't what that map called them. I
29:14 it was a scene question mark. didn't know, you know, there
29:18 some other Ec rocks, you 100 miles away. So they said
29:22 were you see, well, it's easy. It's my A.
29:27 And this this this intercept here some between 3400. That makes sense for
29:34 region. There's quite a lot of for this 1000 million year olds are
29:39 there. The Grenville age cross where might be in 1400 million year old
29:43 12 seems a little bit hard. edges here are better, but you
29:47 , this is a fairly uncertain value on just two points that are themselves
29:54 . So you know, we we zircons as being the best and they
30:00 but they don't come without complications. are all these points spread out along
30:05 ? Probably this problem here. Um so this is a, you
30:12 some people would look at these data maybe come up with a different interpretation
30:18 that's fine, you know, but for you to know, is that
30:22 that sometimes things are complicated. This this is one of the more complicated
30:27 . Even with modern data, this this was collected just a couple years
30:32 . And so there's no sort of about old machines and millions shoveling the
30:37 here. These are these are ones cons at a time and still there's
30:41 complication. Oh well I already I showed you that. Okay,
30:50 here's some of the pictures of what was talking about. So here's a
30:54 a paper from 2000 showing a There's your scale, that's 100
31:00 So that's a typical size of a and you can see that optically there's
31:05 there's a there's a there's a core a rim on this thing And these
31:10 white circles show the areas that were with a laser. I think it
31:14 a laser, there's two kinds of , we'll talk about that. Um
31:19 it was these little bits observe con know if that's 100 million, that's
31:25 micron, this is maybe only like microns across how wonderful that we can
31:30 a good age there. And what see is the core of that
31:35 This is from some nice in The core of that crystal is 1900
31:40 2000 million years old. But the where they got a little bit bigger
31:45 and where we put this, put the laser beam right on the spot
31:50 . They get an age of And so that's a lot more information
31:56 analyzing this whole thing and getting an of what, what what do you
32:00 you analyze this whole thing, what you think? You get? You
32:03 an age of about 1800, And nothing, nothing happened 1800 million
32:10 ago to this rock. Now, you had enough of these crystals,
32:14 that they make some nice line, understand that 1800 didn't mean anything.
32:19 , first of all, it would 1800, it would be a discordant
32:22 . It wouldn't be 1800 concordant. but if you had enough, and
32:28 it was a simple case, you be able to see the mixing line
32:30 200 and and 2000. But with modern techniques, there you go,
32:38 , there are different things. And on what you're trying to sort
32:43 Well, you've learned it. Like said, if you're if you're if
32:46 was a volcanic rock, I think have my next example. Yeah,
32:51 get to that example in a Uh when you're, when you're doing
32:58 rocks for example, you're doing it to understand the age of crystallization,
33:02 so you don't care about this stuff in this case, maybe you're studying
33:07 metamorphic history of the Brazilian Shield. , that that might become very
33:12 but in terms of figuring out the youngest bit, using this granite to
33:16 you something about crosscutting relationships when this moved, all that kind of
33:21 The 1900 just gets in the Um Here's a picture that's really
33:28 It doesn't have any ages on but it just shows shows very well
33:32 this could work. Once again, a core and a more juvenile overgrowth
33:40 that core. Look at that course crystal there. Um Why do you
33:45 it's so rounded? Are Zircons derek start out rounded like that. What
34:00 make it rounded? No, You knew the answer to this when
34:09 were taking physical geology. Not, complicated what makes things round?
34:20 this is weathering and erosion. So that probably is is a zircon that
34:25 melted out of a sedimentary rock. was a Detroit als er con which
34:30 was melted or the sandstone was melted produced this new zircon is associated with
34:37 uh well, whatever rock this it's in the alps, it might
34:41 a might be a metamorphic rock, the block that was metamorphose was a
34:46 rock. See that because of the rounded zircon, but then a new
34:50 of zircon grew on top of If we were to analyze this whole
34:54 , we'd get a slightly older age the new than the than the growth
34:58 we were to be able to analyze two separately, we'd learn them
35:04 Um Here are some volcanic zircons that to have no overgrowth in them
35:10 And that's, you know, if looking for the edge of the graph
35:14 the, of the eruption, this great. You know, we
35:21 we don't usually care to have it way. It's just often that
35:24 But sometimes you get away without Um and these are the data from
35:30 zircons, no, no evidence of in them. But you still get
35:34 little bit of spread here from 3 to 3 80. Now they've gone
35:40 and taken the weighted mean of all those to get the age of 3
35:43 because they're thinking that maybe that spread just analytical uncertainty. Uh you
35:50 another option is to say that there lead loss and you take the oldest
35:55 77 or the other option is to that this is just a little whiff
35:59 inheritance and we're gonna take the youngest and say all the other ones are
36:03 . So you know, even with bunch of data which all fall on
36:08 you still are challenge sometimes with an . Are these is this just the
36:13 variation of a magnetic system? And should take the average or is this
36:18 magmatic system that has had some lead for some inheritance to it? In
36:22 case we should take the extremes of data. I can't tell by looking
36:27 this diagram these guys chose in the , I'm pretty sure to take the
36:31 of need and I think that was they were looking for this was a
36:36 rock that was at the I don't the devonian mississippian boundary maybe. And
36:42 think they already had an idea that cancer was. Well actually these these
36:48 , I can't remember why they um an example of of of a
36:58 the igneous core and an UN's own rim. You can see these little
37:05 here which these are these donations are small changes in chemistry of the
37:11 Um They're not individual overgrowth but maybe the magma is depleting in some
37:17 the crystal changes a little bit and up in these images. That only
37:22 in igneous systems. It doesn't happen metamorphic systems. And so we can
37:26 that that is a metamorphic overgrowth on of this and if you were you
37:36 , if you wanted to know something , you were looking at that Zircon
37:40 to try to study it would be . You might not, this is
37:46 kind of zircon that would only be looked at if you're interested in metamorphic
37:50 , if you're interested in sedimentary rocks even igneous rocks nowadays, you just
37:54 that one aside. Um Here's here's an example. I was telling
38:00 about another project I worked on a years ago, where we were,
38:04 were very interested in the age of rye light because it helped us um
38:09 understand the timing of deformation. This in a fold and we want to
38:14 when the folding took place. Um this was one of the uppermost units
38:19 the folds. We wanted to date . Um And and we actually dated
38:25 two ways. We may talk about other way, but this is we
38:28 it by our by five uranium lead on one place and then another place
38:34 the strata. Graphic sequence. We it by Argon 40 39. Um
38:40 here we got this age of million years, 34.7 and 34.7 plus
38:48 -4. I should point out that is not an outstanding precision. And
38:55 because we're using this technique. This the laser population of mass spectrometry dr
39:03 ablation, inductive lee coupled mass spectrometry talk about the techniques in a
39:08 And this technique has the advantage of fast and inexpensive but it's not but
39:14 not high precision. And remember, know in geochemistry as with everything in
39:19 there's fast, there's cheap and there's , you only get to choose two
39:23 those. Right? This is fast cheap. It's not outstandingly good.
39:29 another technique that gives you much better it's and it's also kind of fast
39:34 it's a lot more expensive. Um anyway this is this is okay for
39:41 technique and I'm sorry this is I'm turn these lights off just down so
39:45 can really see this better. Here the zircons and you see these little
39:50 in here. Those are the places that the laser went in and got
39:55 age and then that that curve over is the is the hist a gram
40:00 the probability density function of these Now you'll notice that you know these
40:07 this is all the data. I think I have pictures on my on
40:12 power point just now of of all other we had access to I collected
40:16 sample about this big and from there was able to get hundreds observed on
40:22 they had quite a range of color size and and many of them had
40:27 in them. And some of them evidence for uh for zoning Ignored all
40:32 are the 50 cons that were analyzed went into the paper. How was
40:37 not now it's five that we analyzed think like 16 times or something like
40:42 . I don't know, maybe not many. And you can see that
40:45 individual data aren't very good. 34 minus 1.6. 35 plus plus
40:51 you know, that uncertainty is But when you pull them all together
40:55 use the magic of the of the me, You get an average of
41:02 plus or -14. So this would a an example of a pretty modern
41:08 to certainly into dating a Riley. get some of the nicest crystals you
41:12 find after, you know, and can you can do that with a
41:16 about this big and you know I collected a sample about this big
41:24 I collected it on the road by truck. If I had to carry
41:30 for 10 miles, it might have this big. Uh but you
41:38 the point is that these are just five zircons we decided to look at
41:42 having had a look at a couple . That's the modern way Just and
41:48 and you see we zap trying to in some cases, the tip of
41:52 crystal here is here staying away. the points to the middle, that's
41:59 , that's 35 maybe. That's probably . So, but we we picked
42:04 that didn't have any evidence optically of . And then we made sure to
42:09 the tip of the crystals which is likely to be away from any core
42:14 . And with that we got a assigned an age. And this became
42:20 for our paper because But I said was all about the structure. We
42:25 know that that at least some of folding in this fold is younger than
42:31 . And that was valuable. And so there's, you know,
42:33 you're doing some sort of structural problem , knowing the age of the youngest
42:39 , is that, you know how problem with holding this is in southwestern
42:43 Mexico. And the problem with this part of the larger my progeny.
42:48 thing that affected the Rocky Mountains and Laramie Androgyny, you know, is
42:53 referred to as something that happened in Mexico in particular between about 40 and
42:57 million years ago, 80 and 40 years ago. But that 40 was
43:02 a guess because the problem was, that most of the rocks that are
43:07 in this region are cretaceous cretaceous rocks are folded that are thrust over other
43:13 rocks, you have cretaceous rocks that folded. But all that means,
43:17 course, is the defamation is younger predacious. How much younger? You
43:21 ? Well here we actually found a , there's cretaceous rocks that are
43:25 There's rocks that are folded. But in this place, there's also this
43:31 younger volcanic rock that sits at the of all these rocks that are
43:36 And so we were able to re how should I say, We offered
43:41 new interpretation of this geologic event? that at least in this place,
43:47 not 80-40, it's something to, , something less than 35. So
43:54 needs that sort of thing once in while. Okay, so that's
44:01 Um The there are other minerals and second probably the second most common mineral
44:10 you'll see dated will be mona's Um And this is usually because zircon
44:17 not available, but you can get out of the granite uh quite
44:22 And you may see this talked about it's also it's also a good example
44:27 the complications that we might have to about. It's a common accessory mineral
44:31 I said in in in fell sick . Um And it also apparently has
44:36 very high closure temperature. So it you much of the same information that
44:40 get out of here. Um Mom , you know, are a phosphate
44:46 , which means they're not as durable zircon, this nice silicate mineral.
44:51 So they're more likely to be measured you won't see modest sites in sand
44:55 very much they'll break down. But a rock like a highlighter of
45:00 That might be really good. You'll that they have Syrian lantana and neodymium
45:05 are all rare earth elements. There's a story um in there. Um
45:10 it so it's the thorium that's going be an interesting issue here.
45:16 Any mineral that that puts thorium in major chemical composition. That means that
45:22 can substitute the uranium because remember thorium were really close together, just like
45:27 can substitute uranium for meconium, we substitute uranium for thorium even better.
45:36 we're actually going to discuss mondesi in context of uranium lead dating. Not
45:41 lead dating. Even though there will more thorium in the rock than
45:44 Because as I said, we get to uranium to play with. Whereas
45:50 thorium is just the one thorium decays lead. Whereas now we get the
45:54 uranium to the two leads. So can use our Concordia diagram. Uh
45:58 other reason is because as I the geochemistry of measuring thorium is more
46:03 than measuring Iran. Um So we're not pay attention to thorium very
46:11 But but we are going to pay again. Let's go back and look
46:15 the decay of uranium 238. Got this going here and here's the part
46:21 the decay of uranium 2 38. to pay attention to to 38 decays
46:26 thorium 2 30 for for optimum to for uranium 2 34 and then we
46:32 to thorium 2 30. Now this is important because thorium 230 has a
46:38 life, 75,000 years pretty long. imagine we have a magma and it's
46:48 some uranium in it. Well that is going to be decaying. You
46:52 whether it's in the magma or Remember we said that radioactive decay is
46:56 of pressure or temperature magmatic magmatic temperatures change anything. So even in the
47:01 some uranium is out there and occasionally decays away and then it will be
47:05 that. That thorium 2 34 will the journey down to lead to a
47:11 but it has to stop in all places. Well what if we get
47:14 thorium 2 30 It's got a half for 75,000 years that thorium 2:30 is
47:19 be relatively long lived. Well go to mona's why does that really love
47:27 . And so when the Monocacy is it will incorporate whatever thorium is around
47:36 it will do it will like it story um better than uranium because uranium
47:40 not in the in the in the there. But because we've got uranium
47:48 the magma we're making this second kind story Thorium 2 30 came from uranium
47:56 2 32 is regular thorium it's just . If we're gonna do uranium lead
48:03 of this mineral we have to be to be concerned to this because man
48:07 it preferentially incorporates story um over there will be substantial quantities of thorium
48:15 30 at crystallization. If there's uranium the magma, we have to particularly
48:21 about magnets that have a high uranium ratio. If there's no uranium in
48:27 magma, then there'll be no thorium 30 to worry about. But the
48:30 uranium we have in this, the of thorium 2 30 to thorium 2
48:36 goes out. Why is that And well, I guess I just
48:39 just said that why is that Because what we would be doing then
48:45 gonna be gonna be crystallizing a bunch thorium 2 30 into the magma.
48:50 normally we think now that thorium 2 is eventually going to become lead to
48:55 . But normally when we calculate the of something, we take the amount
48:59 lead to six, the amount of uranium to 38. And we get
49:04 ratio, but that assumes that all the thorns of the lead to a
49:09 came from Uranium 2 38. That in this crystal, but in this
49:15 some of that led to a six from uranium 2 38 that was just
49:20 out in the magma never made it this crystal. So we actually gave
49:25 part of this story. Um part this led to a six. Got
49:30 head start, it's called unsupported lead it's not the lead that came from
49:35 uranium that was in this crystal, was lead that came from the uranium
49:40 was in the Magna that decay to 230 that then got into crystal,
49:45 ultimate parent is not represented in this . And so that will lead to
49:51 that plot up here. It's called Discordance because we have a we have
49:56 excess of lead to six, not its way up here. And that's
50:03 for the amount of lead to uranium 38 we have, we would come
50:09 we get an age that's much So how are we going to deal
50:14 that? Well, one option when see that up there is to just
50:20 it, ignore that and just use 75 a. Just pull it straight
50:24 and say that, you know, this should be is a matter of
50:29 , this this uh how much um Extra Dorian was jammed in.
50:34 it's hard to know exactly how much was. I mean, there's a
50:37 , so there's an equation, if know the the thorium uranium of your
50:43 and the thorium uranium of your you can calculate this stuff, but
50:48 don't always know this, you So rather than going to all that
50:53 , you just might say, here's the situation where we're gonna have
50:57 use the lesser of these to remember lead 2-35 is the isotope that's almost
51:03 . So this is the harder thing mention. But you do that,
51:10 I'm showing here are real data. is another granite from Nepal. And
51:15 just gonna use the the 207 to age in this case. And if
51:20 do then we can say, well this granite is about 24 a half
51:24 years old. And because we know closure temperature of lead in Montecito is
51:28 high, we can make all the interpretations we would have made as if
51:31 got the concordance their content. Um another example that's interesting because we've got
51:41 things going on in this. This the same paper that we looked at
51:43 minute ago from that granite from Mount . And here we go. Here
51:49 have the Montecito Zircons before. Here have Mona's eats. And if we
51:54 in on this, we see that are some points here that are reverse
51:59 discord there just a little bit above Concordia line, then we have this
52:05 that's kind of on Concordia. But if we actually zoom out, we
52:10 that there are more points. This , this one, this one form
52:15 line and it goes up to 4 plus or minus something And that's remember
52:22 is our same same granite where we the zircons down there at 20 going
52:26 to 500 and the other one going to two billion. Well, the
52:31 this was interpreted then is that this also evidence of a 20 million year
52:35 crystallization age because we're gonna take the age here, that's going to be
52:41 little bit above 20. And in because there's this and this other this
52:46 bit is an inherited component going up 4 71. So you can have
52:52 in mona's sites too. And in because this inherited component, we don't
52:59 let's go to Excuse me. So this because we're drawing a line
53:08 , we really don't know is this this uh is this only does it
53:14 any inheritance in this point? If doesn't then you can drop it straight
53:18 and get the get the But remember we got from the was value more
53:23 this. So maybe there's a there's slight bit of inheritance in this bit
53:27 this line should really go to here we drop it. The more
53:32 the more evidence we have for something this. Uh huh. So we've
53:37 reverse discordance and inheritance going on at same time. Uh But it's another
53:43 , it's got geochemical similarities. Got little complication because it likes thorium more
53:48 uranium but we're not actually measuring the . We're doing uranium. You
53:52 And none of this analysis here of granite was any thorium measured. But
53:57 became an important concern because because we a lot of thorium during crystallization and
54:06 more the more uranium is in the relative to thorium, the more likely
54:12 going to have this problem. Um , well we're almost done with uranium
54:24 . So I'll move on to that . So just an illustration of how
54:31 zircons are seeing through geologic problems and learning about the older events we turn
54:38 this rock which I referred to earlier the oldest rock or the oldest
54:44 Well, this is the oldest This is the casting nice from Northwest
54:49 in Canada. And these are the of a bunch of, of analyses
54:55 mostly spot analyses of zircons. Now is an old paper 1989. But
55:03 paper made quite a stir because it one of the first papers to be
55:08 using this new technology iron probe dating could do these tiny little spots.
55:14 then. They were using these other spectrometers, much less sensitive. You
55:17 to toss mini zircons in at a . And there was evidence from lots
55:24 reasons that this rock was very And so they wanted to apply this
55:27 technique to this very old rock because talked about all these ways in which
55:32 know, if you have metamorphoses um re gross, you're going to have
55:35 old rock look a little younger. if you can tease apart the zircons
55:40 look at them individually. Like I for those rai lights you can and
55:43 this case here, these guys are to figure out what's the oldest rock
55:47 know about. They care about the more than the rims. When I
55:50 doing that, doing that highlight I didn't care about the course.
55:54 want to know the ribs. So you choose to pay attention to,
55:57 depends on what your studies about this about. You know, oldest
56:01 And get your paper in the picture the paper kind of stuff. And
56:05 still the oldest rock. We know zircons. That this this was the
56:09 paper, there's some other papers that put some zircons more up here.
56:14 by looking at all of these zircons I think these these these these
56:19 these were the multi zircon analysis from papers. But then this this paper
56:24 in and looked at single points and to push points up to here.
56:31 um And they said a subsequent paper some more zircons that pushes it up
56:35 here. And so this oldest rock we know about has an agent of
56:40 4.1 billion. And it has come this more modern techniques of being able
56:45 look at little bits. And then is the paper, this is an
56:50 of the same sort of approach, through these are zircons, not from
56:54 the, not the oldest rock, the oldest stuff. These are individual
56:59 from the sandstone. And again, we do the individuals, we don't
57:03 them up and of course you would want to mix them up if you're
57:06 about a sandstone because that's, that's guaranteed of putting things together that don't
57:13 together. We'll talk a lot about . Maybe today. Um, you
57:18 , the ability to measure grains one a time opened up this whole new
57:22 of looking at sandstone, whole great to understand. But you know,
57:27 when back when the technology wasn't so . Well, we could just assume
57:32 all from the same granite. So know, averaging together makes sense,
57:36 ? They have the same history, have the same chemistry. We'll just
57:39 them together because we can't avoid They never did that with sand
57:43 They didn't say, well, we can't measure singles or cons,
57:47 we'll measure them together. They waited whole, the whole, the whole
57:51 approach of looking at sand stones had wait until the mass spectrometers got good
57:57 to look at grains one at a . Uh, here's what we
58:01 This is one of those first this is a sandstone. It's anarchy
58:04 sandstone, but they were again trying to work on this. What's
58:09 oldest bit we got problems and they some of these individuals are cons back
58:16 about 4.4 billion and this by picking out one at a time looking at
58:21 cores. Not worrying about the Um There's two groups that have worked
58:26 this mostly this is the group from and the other group from U.
58:31 . L. A. Have done more than anybody else. Um
58:34 C. L. A. Um The guy in charge from
58:37 C. L. A. Group my PhD supervisor and he's told me
58:40 you know they there's thousands, you they've looked at hundreds of thousands of
58:46 from this from this thing because now can automate it. They can set
58:49 up zap zap but nobody come home then come back in the morning.
58:53 500 zircons have been analyzed. Uh what they usually do is set it
58:58 to do it really fast And and precision and they you know they do
59:04 overnight and seven or eight of them older than 3.8. They go back
59:11 those and do three and do it well and find out whether it's 3.8
59:15 3.9 or 4.4. But only about of the zircons are older than four
59:21 . Uh but tend to so get of the old ones that had to
59:26 at literally hundreds of thousands of But you know nowadays they do
59:32 Okay I think. Oh so that's Zircons and old rocks. Just we
59:38 point out that you can date other by uranium lead. Here's a here's
59:42 example of a, of a paper looked at speen, although some people
59:48 call steen tight night. Um and should point out that the the substitution
59:56 spine has tight tedium in it. what uranium is substituted for is
60:02 titanium is also plus four. So can date scene and here's some here's
60:07 data that gives a from a from sort of granted I think from Argentina
60:12 an age of 4 76. you date other minerals like appetite and a
60:20 of others. Um Spin and appetite the more common minerals in rocks like
60:25 . It's been shown through some good , mostly that the closure temperature of
60:31 in spain and appetite is much lower it is in zircon. Zircons,
60:34 know, super high Speaking in the or around four or 500. Um
60:40 once again, if you're looking at rapidly cool granite or a real light
60:45 something like that, it's just fine date the scene. But if you're
60:49 at a metamorphic rock or a very cool granite, it's probably going to
60:54 you a different age than the Zircons gonna start keeping time at 800
60:59 and the and the spin's gonna start time at 500 degrees. And if
61:04 a slowly cool sample, that could a long time between. And of
61:09 if you're interested in the applet history this mountain, that's valuable information.
61:17 . Just a little bit about measurement . Um Nowadays you'll see three things
61:23 are done. Um the first is thermal ionization mass spectrometer. And that's
61:29 technique that has been used for 60 years. It's just that the
61:34 get better and better and better and . And so even now thermal ionization
61:39 spectrometry or Timms tIM's will um still you know, pretty good results and
61:45 don't need to put in, you , a shovel full of zircons.
61:49 can now do them one at a . But the disadvantage of tim's is
61:55 you have to take the sample and it in some acid and dissolve it
62:00 then treat that liquid. Certain way put that that liquid through an ion
62:07 column and you gather the uranium and lead and then you put it on
62:10 filament that then you heat it up analyze it by heating it up.
62:14 where the thermal ionization comes from. This has very high temporal resolution.
62:21 a very precise technique. This is you're going to get the really,
62:25 know, pleasure -10% sort of stuff the machine is very good and very
62:31 . And you can and if you you have a big enough sample you'll
62:35 really good results good and good in of precise. The disadvantage is you
62:40 to dissolve the whole crystal or you to dissolve whatever it is you're
62:44 you have to take that thing and it in there. So if there's
62:46 variation in that thing you're using it be averaged out. Um It also
62:52 more time. You know, you you gotta take your sample, you've
62:55 to dissolve it in acid, you to put it through the columns.
62:58 mean that that could take a couple weeks. Um And because of all
63:03 extra stuff, there's the cost of and the machine, the tIM's machine
63:08 pretty expensive to buy. So you say this has high temporal precision,
63:12 spatial resolution. It's not a fast , takes a lot of money to
63:18 the machine and keep it operating. . And another said that technique That
63:25 that concept of Tim's has been back years. It's just that they do
63:30 better now. But these other two are not old things. The
63:36 secondary ionization mass spectrometry that started in , started with that paper I showed
63:41 here that 1989 paper. But um didn't take off until maybe, Well
63:50 guess it sort of took off in 90s and there are a few of
63:54 but there and there are, you , you want to find a laboratory
63:58 does thermal ionization mass spectrometer, they're hard to find. Um because although
64:04 expensive they're not super expensive. You a machine that you might need a
64:08 this size maybe not not this size this size and you know some some
64:14 things and the machine itself might cost don't know five or $600,000. The
64:21 machines will take a room this size I don't think you're gonna get any
64:27 out of $3 million. So you've to be able to you know,
64:31 with a you know sort of $3 $4 million dollar outlay. Which is
64:35 there aren't very many of these around when we get to the cost of
64:39 last one. But why would you $3 million? Well because of the
64:44 high spatial resolution. The what we in the secondary ionization is a is
64:50 beam of ions either season or oxygen which are hitting the crystal. And
64:56 will cause the the um cloud of to come out here and they can
65:01 shot into a plasma. But the that that system can analyze. Just
65:07 tiny amount of material. And the of these beams to liberate. Just
65:11 tiny amount of material means that you really start looking at geochemical variations on
65:16 scale. Just a few microns. the what's the you know so you
65:19 might see what's the history of this as it grew or even if you've
65:24 just a teeny little rim, You like you're interested in the age of
65:28 right light but it's mostly core, if you've only got a 10 mil
65:33 spot, you know, so it it has unparalleled spatial resolutions um and
65:42 temporal resolution is still pretty good. precision on those ages. Not
65:46 Maybe not quite as good as tim's it comes with this other thing.
65:51 problem with secondary with sims is it's relatively slow. Maybe not as slow
65:55 tim's and as I said, it's high set up costs. There
66:00 you know, I don't know how sims labs there are in the world
66:03 days. There's still not very I mean you know people people have
66:09 here and there but you hear you know, you hear about laboratories
66:13 are trying to get sims. They on getting the money for sims for
66:17 , 15 years and never get it it's, you know, you have
66:20 convince, you know, your university your National Science Foundation or something to
66:26 . So there's one at U. . L. A. There's one
66:29 stanford. I'm not that all the States I think it might be.
66:37 they started in Australia, there's one Canberra, there's one in Perth the
66:42 that makes smokes of them now is paris. So there's several in
66:46 there's one in London or Cambridge. in Cambridge. Uh there's a couple
66:53 japan I think. But that's all there there you know $4 or $5
67:00 dollars just to just to open the and run. But they're nice then
67:06 the inductive lee coupled plasma mass spectrometry I. C. P.
67:10 S. And that is the data came from the pilot study I showed
67:16 was done here at U. Of . Uh And there are a lot
67:19 of these I. C. M. S. Labs around these
67:22 because um although the temporal precision is as good. You see that you
67:28 when we when we did that we some nice crystals and we zapped it
67:33 times. We still got an uncertainty about what was that? 35 plus
67:38 -15. That's one part in. ? Is that? One part in
67:48 ? Right. That's not a That's that's 1.5. That's between one
67:53 2%. Okay. Why do we up with that? Because these machines
68:00 pretty cheap? The cost of one these things is not three million.
68:04 not 600,000. It's like 250,000. it's much easier to say oh I
68:09 one of those and you can and the other thing that the other thing
68:12 really nice about it is it's You can get 300 of those little
68:18 in an afternoon. So fast. gotta and the spatial resolution is still
68:23 good. It's not quite as good sims. Sims might get down to
68:27 microns. Uh I C P. . S. It's more like 25
68:31 its best. And you can make bigger if you want. So it's
68:36 good. So if you were, know, had a sample and you
68:42 to be dated, you would go one of these kinds of labs.
68:46 If you're interested in fast, the . C. P. M.
68:49 . Is the way to go. you're interested in the ultimate precision,
68:53 the other two were better. Uh again you got fast, you got
68:57 and you've got good, you can't all three of those. Uh And
69:03 this this illustrates stuff. Um Well that there we already talked about
69:09 Oh this is just to point out I just said this this is the
69:13 is the I. C. M. S. Data and you
69:16 35 plus 9.4. That's okay. it was fast and cheap.
69:23 You want something else, You pay more money. Be more patient
69:27 get better. Um This just shows history of publication of these sorts of
69:33 . Look at the tim's. So have what we have in the red
69:36 are the reported precision of the The blue line is the reported sample
69:42 that was used And then on the we have the publication year. So
69:48 Tim's we go back to the 1970s you see that back in 1970,
69:53 were having to look at million micrograms analyze it and analyze a whole gram
70:01 material. That's a lot of And when they did that they got
70:06 reported precision of about one and a . So back in 1970 you
70:12 you know, you you know, 1.5%. Everybody said okay fine.
70:17 you have to when you look at data you have to remember they were
70:19 at you know, half a gram more of material. That means they
70:23 averaging stuff together and that comes with . But please we can analyze what
70:43 , today, that's because of the of the machines. They had they
70:50 to analyze this much weight up 10 to the 5th, 10 to
70:54 sixth range of my microgram. This a million micrograms for a grant.
71:02 . They had to they had to it wasn't the number of zircons,
71:06 was the weight of the circle. if they if you didn't have half
71:10 gram of material analysis was not likely be very good because of the sensitivity
71:16 the machine. We needed a big . And so that's why I always
71:20 shovel the Zircons in there because you , and and I mean if you
71:24 a single zircon that was an inch . That would have been fun.
71:28 garcons are generally small. So that you needed a fuser. Cons a
71:33 dozen, maybe 100 depending on what up to. Which means you have
71:38 collect samples that were the size of table, bring them home. You
71:42 , I had to get back to drawing and then process them. So
71:47 a lot more work to get all zircons because you couldn't analyze to reserve
71:51 or three zircons. You needed you a quarter of a gram of
71:54 If you were lucky. Now this that over time the amount of material
71:59 went down from half a gram you know, 10 micrograms nowadays.
72:05 that's you know, now you can at a single zircon and analyze
72:08 You still have to put it in , but you've dropped the necessary amount
72:13 material by a factor of almost a . That's good. And while doing
72:20 , not only because sensitivity went up the precision of the And now we're
72:25 about decisions that are a quarter of percent, pretty really darn good.
72:31 you don't. But compare that to sample weight on say sims. Here
72:38 go for a million down to Well, ever since the beginning of
72:43 we were starting at a value of six. And that hasn't changed very
72:46 . We need about shooting it's not . This nano here we're talking about
72:53 micrograms. Even in 1980. A machine. This $5 million machine was
73:01 analyzing just continue a little bit. got just a few nanograms and that
73:05 changed and the but but here's the off instead of this one or 1%
73:11 better. We started at about 4% now we've moved down about 3%.
73:17 by able to were able to analyze a tedious little bit of samples.
73:22 so that means you're not mixing anything and pick it right and then compare
73:27 to I. C. P. . S. And here we are
73:30 we're back to micro laser spot Not quite same as as a as
73:37 grams. But it gives you a of the size of things, laser
73:41 diameter goes from about 50 microns to 30. And the reported precision has
73:48 from from 15 back in the in in the mid-90s to a precision of
73:53 2%. And that's what I showed in the in the data that we
73:57 from you of age a couple years is about 2%. Um So there
74:02 your choices. Um And of course would only be talking about this.
74:07 can just talk about the modern here I C p M. S.
74:11 2% since two or 3% tim's quarter a percent. But you're not analyzing
74:20 same thing here, you're analyzing some of of of the equivalent of 10
74:26 here. You're analyzing the equivalent of nanograms but with a greater I'm
74:36 So as you consider your needs of project, you know, and you
74:45 I can film for these for these of things. Often I've dealt with
74:50 companies, I used to run an lab. I don't anymore. We're
74:54 talk about argon next. I used run an argon lab and they used
74:58 oil companies used to call me up say we've got a sample and I
75:03 learned far too late that you can oil companies just about whatever you
75:07 I don't care. You know, drilling a well right $100 million
75:12 And you say, well, would be okay if you paid $5000 for
75:17 ? Um so if you're an oil and you don't really have to pay
75:21 too much about how much is this to cost? This will cost the
75:26 . These will cost more. This , this is faster. This
75:31 you know, relatively speaking kind of but it might be appropriate. Plus
75:37 -2%. Might be plenty good for of lots of of problems. Um
75:46 then, and, and that applies data mostly applies to zircons. But
75:52 , as I said, we can , we can date other things I
75:55 seen already, I mentioned appetite you can also do root eel.
75:59 Delhi is a mineral that you'll find Mayfair rocks? It's a silica under
76:05 zircon? It's just zirconium oxide. If there's cilic around you make sure
76:10 but if there's not cilic around, less silica around like in a basalt
76:15 make uh and then Xena time is sort of phosphate that you might
76:21 You'll notice that all of those minerals either phosphates or they are silicates or
76:28 that have zirconium or titanium because that's and titanium are the things that uranium
76:36 substitute for. Think that's the end that. Yeah. Um Any questions
76:46 uranium lead dating. Zircons. um now I'm going through all
76:54 So that's our first main technique. go through some other techniques and then
76:58 go through applications. We'll talk you know, ideas and so I
77:03 to finish today, getting through all the, I will get through all
77:08 techniques and then probably talk about the dating today. And then next friday
77:14 be mostly all about applications about, know, let's and so next
77:20 you need, you know, you to come really knowing what the closure
77:23 of the systems are. You this is throwing a lot of information
77:27 you, you know, what's the champion of this. What do you
77:30 that that, that graph I showed had like 10 things on it.
77:34 you need to be studying this next are sort of, what are the
77:39 temperature of zircon, What are the temperature of feldspar and so forth.
77:44 when we talk about applications, that's stuff you have to know to know
77:48 which technique to apply to which but we're not, you know,
77:52 haven't given you all that information I'm just pointing out that's where we're
77:55 next. So that's the end of . Let's take a break. It's
78:01 what time is it? It's Um let's come back at 9:55.
78:16 , so our next topic, it's be the potassium argon system, although
78:37 we'll see that we're gonna use a of potassium argon dating that's not used
78:42 . This is a picture of what want. The best potassium argon lab
78:48 ever been to. I said I to have a lab um not as
78:53 as this one here at U. H but we shut that down a
78:55 years ago. This is the lab New Mexico Tech run by my friend
79:01 Heisler. We both went to graduate together and it's really anyway, it's
79:07 great lap. Um And that's how , that's the mess. That's actually
79:13 actually two different mass spectrometers put together there. Um So what's going on
79:20 ? We have um potassium. Now gonna pay attention to? There are
79:26 isotopes of potassium, 39 40 and 39 and 40 or 39 and 41
79:33 stable. See most and most of potassium is 39, but just a
79:40 little bit 100% of all the potassium there is potassium 40 and it's
79:49 has a half life of one and quarter billion years. Um The reason
79:55 put I put it in red because radioactive, I put 39 in red
79:59 we're gonna use it in a very way. Our potassium 41 plays no
80:05 here. Um there are two different in which potassium 40 decays um About
80:16 of the time decays by beta decay calcium 40. And about 11% of
80:23 time it decays by electron capture to 40. When we measure the amount
80:31 potassium, We have to take into that most of the potassium went to
80:37 else. But we know that it's a ratio of 89-11. And so
80:41 can account for that. But if just took the amount of potassium that
80:45 there now, and and the amount Argon was there. Now you get
80:48 very different answer because there used to , you know, back when that
80:52 that when that sample started it had more potassium 89% of the decays have
80:57 to something else that we don't measure we get the ar but we know
81:02 so we take care of that. because we've got these two decays,
81:07 the total half life is the sum the two different. Uh So the
81:15 K. Constant, you know, these values for these two things,
81:20 we put the whole thing together. the effective half life of the system
81:25 one in a quarter billion years. when we measure potassium and argon,
81:29 can use that, that decay And that's a really convenient uh Past
81:38 , half life, that's a billion has gone through only four, you
81:42 , only about 3.5 half lives since age of the earth. So we
81:47 really gotten down to that unusual but it's also not a super,
81:54 not a super short, I mean is, it is long enough to
81:59 be used for old rocks, but is short enough that you can use
82:03 for pretty young rocks. And indeed you, because it's potassium and we
82:07 minerals like potassium feldspar that are you know, full of potassium,
82:14 can date things that are really, young there, the eruption of Mount
82:19 , the historical eruption in 67, . Well known eruption that was dated
82:25 this technique And they got, it 67 a. D. they got
82:30 date of like 2000 plus or -300 . It's the rare instance where we
82:36 exactly what the right answer is and got it within uncertainty. So you
82:40 date rocks that are just thousands of old with this technique and also billions
82:45 years old. So that's very So, you know, we know
82:52 potassium 40 decays to Argon 40. gonna have to pay attention to the
82:56 isotopes of argon. Then There are other naturally occurring stable isotopes, Argon
83:02 and Argon 38. Argon 37 and 39 are not naturally occurring because they
83:09 such short half lives. But we're to make them in the nuclear reactor
83:13 use them to our advantage. But won't get to that for a
83:17 But just in terms of argon, argon dating 37 and 39 are not
83:24 . Um You see that we have all of our argon is Argon 40
83:32 of the argon. We're breathing right is Argon 40. There's a small
83:36 of 36 an even smaller amount of . So again, we have the
83:44 life of one and a quarter I said all this already because we
83:49 have up to 14% K. To insanity. I said this already the
83:54 . R. System can be used almost the entirety of Earth history.
83:59 right then Age equation same as before , daughters to begin with parents Atlanta
84:07 . Nice one. Um However, got a little bit of an issue
84:15 that we have to account for the of 40 that decays to argon.
84:21 So if we assume that there's no to begin with. And that's a
84:24 good assumption for a noble gas. We're going to look at the cake
84:31 here as a ratio of cake constant argon to the total decay constant.
84:37 with that we can just re arrange tea and we get the equation that
84:41 equals one over lambda times the log all this. Where this is what
84:45 measure in the lab to measure the of argon 40 radio genic. Uh
84:51 uh fantastic for it. Uh If like, as in other applications,
85:01 can do um an ice across. we will use the argon 36 as
85:11 state doesn't as the no normalizing isotope , well, we don't have much
85:17 . Um I didn't mention this but For art for for for Rubidium
85:23 , 86 ratios. Why didn't we rubidium 86? The reason we chose
85:30 , you know, there are other of strontium? The reason we chose
85:34 is it's the one that's closest in to most 87 samples. Remember remember
85:40 initial ratios we were talking about had about .7. Mass spectrometers are really
85:46 did talk about this maybe did mass are really good at measuring ratios about
85:51 . Uh And so we chose a isotope that will give us ratios near
85:57 . You can choose another one we don't have a choice. We're
86:00 have a ratio near 300 40-36 I . That's enough of that. So
86:14 potassium argon would proceed much like other . Except the problem we have a
86:20 in potassium argon dating that we didn't to worry about in any of these
86:24 things. And that's because the geochemistry potassium argon are very different. They
86:31 be measured on the same machinery. is a noble gas, potassium is
86:38 metal. Just can't be done. so you have to split the sample
86:43 . You have to use some of sample to measure the potassium. Take
86:48 of the sample over here. Different measure the argon and then get the
86:52 of the two concentrations. Remember I that Mass spectrometers are better at measuring
87:00 than concentrations. But here we have figure out a concentration of potassium 40
87:05 a concentration of radio genic art on . Make a quotient at the
87:10 So not only do we having to a quotient of concentrations, but we're
87:14 to do it by splitting the sample our And once we do that,
87:17 have to assume that the amount of that we measure on here concentration of
87:24 here that we measured is the same the concentration of argon. We measured
87:27 the potassium bit and vice versa. measured potassium over here. We have
87:31 assume that that's representative of the potassium over here. Now the samples are
87:36 and they came from right next to other. Well, what can you
87:40 ? But that that's a that's a . Right? Whereas in all of
87:44 other machines machines, we put the and strontium the same apparatus, You
87:49 measure them at the same time. you're measuring them of the same
87:54 You don't have to break the crystal half and put it over at the
87:59 least that there's not the sampling handling . Well, how are we going
88:02 break a crystalline or we're going to two separate christians. And that's a
88:07 issue of potassium argon analysis which will rid of very quickly. Um
88:14 they all and this goes back to to the various ways of measuring potassium
88:20 there never were very good and but but that's how you would measure protest
88:29 to measure argon, you heat, heat the sample up and argon will
88:35 be lifted from the sample and eventually have to melt the sample because in
88:40 some crystals particularly felt spars uh at the all the argon will eventually go
88:48 the vapor phase. And you can it. But but that's not until
88:51 melt it, you know, you For for for some minerals if you
88:55 to put it at say seven or Um it would take forever to get
89:01 the argon out of it. So just go ahead and melt it and
89:03 it gets out easier. Um But is why um you can't measure the
89:11 and potassium on the same bit because you melt the sample, the potassium
89:16 volatile ized. And it's not a melted bit that you've got the argon
89:20 , it's no longer a viable choice measure the potassium. You've changed the
89:25 too much, you've got the argon . That's good. But now if
89:28 changed it in other ways that you trust, so you could do the
89:35 thing as before. We did this rubidium. Well obviously there's more potassium
89:40 these rocks than than rubidium. Use same minerals do the same thing.
89:45 know, here we here in you know, the only thing that's
89:48 is we're done still parent normalizing daughter normal, this is the case
89:55 this blah blah blah. Great. , we can do this. Um
90:04 the problem is that the closure temperature these various minerals tends to be kind
90:10 different. Some some some cases quite bit different and this might be okay
90:17 a volcanic rock, but this won't for a granite or a granite or
90:24 shift because those things slow cool And if you're clocks don't start keeping
90:33 at the same time, they won't this assumption of having the same initial
90:39 . This this works because these things all broken all set in at the
90:42 time. But if the closure temperature these systems is very different and by
90:47 , we can mean, you some systems for are gone the closure
90:50 500, some it might be as as 150. And so if you're
90:55 cooling just like that, then they begin telling time at the same
91:00 And that's an assumption we make in ice upon approach. Probably not true
91:05 in almost any granite. That won't true. So you won't see potassium
91:11 ice icons very much in plutonic Um and so to get around
91:19 they chose this approach which was phenomenal in the in the frequency with which
91:26 kind of worked. But they did things, which really can be thought
91:31 one point Cron's. They said, , we're gonna measure our sample
91:36 we've got a point on the darker what we're gonna do with that.
91:39 , we're going to assume that the value of our system is equal to
91:44 initial equal to the atmosphere. We're right now, That ratio of
91:50 30 to 6 is about to And so they said, well,
91:56 just going to run the line through point and that point we got a
92:00 and it produced a series of data many laboratories over many years, that
92:07 sense other than it had to be on this ridiculous assumption. These
92:13 you know, we're looking at a type from a granite, it crystallized
92:17 there 100 million years ago. Why the composition of modern atmosphere be valuable
92:23 this? Well, they didn't have other choice because they can't use the
92:29 a Cron approach generally because we know closure temperatures are so different. But
92:33 all basically ran it through this one to this one point and it seemed
92:37 work. And that's all you can is it, it worked. The
92:42 news is we're soon gonna stop, know, I'm giving you this merely
92:46 historical context. If this sounds don't worry, we don't use
92:50 we don't do this anymore. But they did it, you know,
92:55 found some data that was okay. even back when they were doing
93:01 they began to understand that there was issue with, with ages being different
93:05 this is why and this is the understanding that I would have, you
93:10 about closure temperature of argon. These the Big Four minerals. There are
93:15 minerals that we can talk about. may mention a couple of other examples
93:18 or there, but these are the East horn blend, high closure temperature
93:24 500 muscovites closer to 400 by type bales. Park has quite a range
93:30 can be as low as one high street, all of these um
93:34 to be depending, you know, It's not 300, always 300 can
93:41 with lots of things. We talked the reasons it can vary because of
93:44 rate. It can also vary because composition. The iron magnesium ratio of
93:48 biotech matters a little bit. But , this is a broad understanding that
93:53 like you to start with. And we can illustrate that with what I
94:01 is a real classic paper of potassium dating. And it shows that even
94:07 potassium argon dating you can get some results and and prove that, you
94:12 , and all of these data were with that remarkable assumption that the uh
94:17 the one point ice akron was was by having the second point the modern
94:24 . And what this is what this is is a block of data at
94:45 at and away from the contact between Duluth Gay bro. Up in,
94:55 in Minnesota, the gay bro, is intruding some metamorphic rocks and these
95:03 not from the Gay bro, but the country rocks away from the context
95:08 these are all potassium argon ages. this paper was published in 1967,
95:13 really old. But again it's fine idea and what we see are different
95:19 plotted reference to the content is a gap bro. So those are
95:26 big thing, there's a lot of energy that's attached to the to the
95:30 rough. And if we look at data in some combination, you
95:35 we see the harm blends are plotted here in the circles and we plot
95:41 like that. We have a pattern that. The muscovites are in the
95:46 , the baia tights are in the square and the cape belts bars are
95:49 the red square. So you see we look at them with the lines
95:53 on there, we see a really pattern right at the contact. All
95:59 tell us that they were reheated about billion years ago and that is in
96:03 the age of the he'll lose But as we move away from the
96:13 we see variations for the horn We see that we move just a
96:18 100 just a few meters away from contact, we start to get older
96:23 and by the time we get up about a kilometer and a half away
96:26 the contact, we've got agents that close to three billion years old.
96:32 the other systems not quite so much muscovites, we don't have as many
96:38 , but the muscovites, If you to three km away from the
96:43 there is still not up to there are more like 2.3 And the
96:49 . And if you notice the bio we have going back here two km
96:53 , we still have agents that are much the same agency intrusion eight
96:57 But then they rapidly move up here they get to these old ages
97:01 But not until you get to be km away from the from the
97:07 And then the Cape Belts bars, stay very young even five km
97:12 They've only jumped from one billion to and a quarter or one and a
97:16 billion. And so this illustrates very with this sort of graphic approach how
97:23 is. These systems behave differently as country rocks were disturbed by this big
97:30 of thermal energy. Remember Gabe rose a high high temperature, big a
97:36 of energy goes in here. And but the horn blends except for the
97:40 blends right next to the contact. didn't really feel it for given ages
97:45 are, you know, the age you'll see throughout the region. Whereas
97:50 K. fell as far as you five km away and they're still profoundly
97:54 by this. What can we, can we conclude about these systems based
98:01 this uh, on this pattern? nice thing about these geologic experiments is
98:16 we can say that this whole all these systems have have experienced the same
98:21 event. Right. And whatever it and it's not like, you
98:26 I talked yesterday about how we we our we want our experiment to
98:30 you know, not like my oven home. We want the oven to
98:32 straight up and straight down. that would be nice, but that's
98:37 what happened here in geology land. the good news is whatever happened,
98:41 happened to all the samples the So, given that why are these
98:49 displaying such a different pattern in age distance? Yeah, that's because of
99:01 . Because of this, they have different sensitivity to being reheated.
99:06 the harm blends have to be heated hot in order for the you know
99:11 we do. What we don't have is a graph of temperature versus
99:15 Right? But in a sense, do because if we know the closure
99:21 of these minerals, we can say the rocks one kilometer away from this
99:26 probably never got heated above 400 degrees the horn blends seem to be undisturbed
99:32 them and the muscovites only slightly Whereas way out near five kilometers
99:37 we can tell that those rocks probably heated in excess of 200 degrees if
99:43 believe the closure temperatures that I gave earlier, These rocks were heated up
99:48 200° or more way out here somewhere 200. Yes, but below
99:55 perhaps because these minerals here are So this is just a quantitative description
100:02 the variation in these mineral systems, corresponds to the call. I said
100:09 , This is a qualitative approach Which to the quantitative numbers I gave you
100:15 ? Those numbers come from more sophisticated that weren't done in the 1960s.
100:20 this is the best sort of graphical distance one that I like to
100:26 Um Okay, now it turns out because of a lot of reasons nobody
100:34 potassium argon anymore. There's a couple labs in the world that exists for
100:38 special reason, which we'll get But we're gonna move on now.
100:43 for the rest of any kind of argon stuff, we're gonna be talking
100:48 this thing that's called the Argon 40 technique which completely supersedes potassium argon.
100:55 it has many advantages which will contrast a minute. It first started to
101:03 used in the 1960s when it was . And I think it was kind
101:07 discovered by accident. Although they might thought theoretically this was possible, but
101:12 was discovered by a guy in That if you put some potassium samples
101:19 a nuclear reactor in the right part the nuclear reactor, neutrons will be
101:26 out in that. And neutrons will in and some of those high energy
101:31 or the right kind of energy will the uh potassium 39 atoms and be
101:39 into Argon 39 atoms. And this the n key reaction I wrote on
101:46 board yesterday, neutron goes in proton out now. Why is that a
101:51 thing. Haven't slide here. I don't. Why is that
101:56 Because remember what's the problem with what's of the biggest problem with potassium argon
102:02 is you can't measure the potassium and on the same machine. Now,
102:09 have made a thing. We have a an Argon 39, which doesn't
102:16 in nature because it has a half of 269 years. That's a
102:21 That's a really short half life for , but it's plenty long for our
102:26 . 369 years. We're gonna, know, we've got plenty of we
102:29 have to worry about it going You know, if if potassium argon
102:34 had a half life of 10 the rest of this lecture wouldn't be
102:40 . We have to have this hanging , it's perfect. Um but why
102:46 it? What's so good about Because now, what we have done
102:49 we have made something that didn't exist , Which is a consequence of potassium
102:57 . It's a proxy for the potassium now. But the potassium 39 is
103:02 really the parent, you say That's because the ratio of potassium 39 potassium
103:08 isn't fixed amount. So with But with this technique we can figure
103:13 how much potassium 39 is in our by measuring the Argon 39. And
103:18 we know how much potassium 39 is the sample, we can figure out
103:22 much potassium 40 in the sample That's just a big ratio in
103:27 So now what we've done is produced by sending the sample a nuclear
103:31 We have tricked some of the potassium becoming argon and we can measure now
103:35 argon 39 which is a proxy for parent on the same machine that we
103:41 the argon 40 which is the daughter . So now we just need one
103:51 . Is that your happy face? bit what? Well, it's a
103:58 bit complicated, but I mean, understand the problem right? We've got
104:04 geochemistry. Can't measure them on the machine. But if we could trick
104:09 of the potassium into becoming argument now can measure two kinds of organ on
104:14 machine. So what we by sending to the nuclear reactor, we take
104:19 of the Argon 30 some of the 39 make argon 30. There wasn't
104:25 Argon 39 in our sample before. so. And what I haven't gotten
104:31 is how we can measure how we the amount of argon 39 that we
104:35 to the amount of argon potassium And in our sample There's there's some
104:41 more coming on that. But if can if we have if we understand
104:45 ratio of produced Argon 39 to targeted 39. We understand how that
104:53 Well then argon 39 is telling us three 1,039 is not the parent,
105:04 It's perfect. We know exactly what ratio between 39 and 40 is in
105:08 sample. Right? Let's just go to make sure I Oh right.
105:22 why this isn't red here. Because have a way of learning about this
105:26 immediately tells us about this because this is a fixed value. That'd be
105:32 comfortable. Now. That's the key . Is that learning about this?
105:36 it the parent? No, it's . But all you gotta do is
105:40 by this ratio. And that tells the concentration of the parents. And
105:45 with that we have now a wonderful deal with, in which we have
105:50 capacity to measure the parents and the on the same machine. And we
105:56 that's just the beginning of the Um But because we're measuring on the
106:01 machine, we a don't need to and be remember I said that these
106:06 are good at measuring ratios, but at measuring uh concentrations. We no
106:12 have to measure concentrations. We can stick with what we're good with
106:16 So we're way ahead. And then third advantage that we'll get to is
106:22 we don't actually have to measure all gas all at one time. We
106:25 to that in a minute. Um here's the here's the equation for Argon
106:33 production. The amount of Argon 39 we make from potassium is going to
106:39 related to the concentration of potassium 39 present. It's going to be related
106:47 how long you send this to the reactor. You know, do you
106:51 it in there for an hour or a week? Obviously, the longer
106:54 shoot the neutrons are, the more these you'll make. And then it's
106:58 to be related to these two which is the the flux of energy
107:03 these electrons are coming out at. that's some function of the uh of
107:08 nuclear reactor. And then it's also to be what's called the neutron capture
107:13 section, which is the probability of N. P. Reaction actually
107:20 And he throws some neutrons out of uh new nucleus. They don't always
107:28 . Okay, well um the problem is that these things are kind of
107:36 to know. Really hard though and don't they're not constant values. The
107:41 reactor can change its power settings and up and down and that's a real
107:45 issue. So we're not there But what we can do is we've
107:51 this, we've got this Argon 39 and we've got the, excuse
107:58 we've got the oh I did. just divide that equation by, we
108:02 the age equation by that equation and get this and you know, we're
108:08 not there yet. But what do have now is the radio genic Argon
108:14 provided by the Argon 39. That produced by potassium by this ratio which
108:21 that ratio of potassium isotopes in That's a constant. And this is
108:26 ratio of the total of the cake the art dot com. That's a
108:30 a constant. And then you've got of this stuff here from the last
108:35 which is difficult. We still don't what that is. And then we
108:37 got your standard one stuff. So gotten something this this we measure this
108:47 measure the constant. It's a That's a constant. That's the time
108:51 looking for. And this is a difficult problem. Good, how are
108:57 gonna deal with that the way we're to deal with that is that we
109:02 all of these things we don't And instead of calling it delta times
109:07 integration of lambda and v delta We just call it J. And
109:13 we've got one equation that that gets of all this stuff and we just
109:17 it J. And of course that's problem but it doesn't fix anything because
109:22 we're just calling it J. Um do we bother to do that?
109:27 the reason we do that is because get a simpler equation. Okay,
109:31 fine. But now we could take equation and we can solve it for
109:37 t the time they were interested in we can solve it for this
109:40 Thing. Why do we do Well the reason we do that is
109:47 Art on 40 39 technique requires us have a standard mineral to compare to
109:54 we're gonna do. When we send samples to the nuclear reactor, we
109:59 our unknowns, which we don't know . We also send some standards.
110:05 do know t from that, we're to calculate J. J. Is
110:11 this parameter of the radiation? And we know J. Put it in
110:20 . And so the 40 39 technique standards of known age. And this
110:25 why we still have some potassium argon because they can actually give you the
110:31 argon age of the sample and we use that as the known value.
110:36 are other ways to get the known and that's not always done these
110:40 But anyway, you have to have accepted value and you'll find. And
110:44 every time you do this you have , you know, you publish a
110:48 , you have to say the monitor we used was this, no,
110:53 mineral has got a name. You , we used G a 15 50
110:57 type. That's what we used fish tough sanity. You know,
111:02 and we assume when we used fish stuff tough sanity, we applied an
111:08 of 27.7 million years And you you report that because if it comes
111:14 pass later on that people decided that age of fish Canyon tough sanity is
111:20 than is not 27.7 it's actually Well all you gotta do is recalculate
111:26 old asians. Uh huh and anything that's fine. As long as you
111:33 what you did. So you take age, you take a standard that
111:39 think you know that would think, know the the age of and this
111:44 one fashion this, it can be other ways but but graphically this is
111:48 to show you will, you would a bunch of samples to the nuclear
111:55 and maybe in a fashion like this you've got like a two full of
111:59 minerals. These minerals might be wrapped in some kind of foil or you
112:03 , some sort of capsule or some sort of capsule that the neutrons
112:08 go through and you arrange them like with standards and unknowns and you might
112:13 to put these standards every centimeter or because the variation of neutron flux can
112:19 that much over a relatively short amount space. You can't just throw in
112:23 standard for the whole thing, which more work. But that's the way
112:27 is. So you put all these in here and it comes back from
112:32 nuclear reactor and we're gonna analyze all these. And the thing we do
112:37 is that, you know, because we know that this this this neutron
112:42 varies with position. We're gonna keep of where these were in position.
112:48 then we're going to plot out a that that comes from these unknowns.
112:55 standard calculate J. For each one these because we haven't accepted a 27.7
113:02 years or whatever it is. Go to that, go back to
113:08 You know, we know the time this, we calculate the J.
113:14 we plot the J. On a like this and it may have,
113:18 know, some variation like this, been a typical J. Curve and
113:23 and then we can then use that to figure out what the J.
113:29 for all our own donuts. all of that stuff that was hard
113:35 know is packaged into this one factor . And we just plug that in
113:40 we never worry about what the what what the, you know, the
113:44 cross section function looks like. It's all jammed into this one thing and
113:50 all, but it all depends on having a good sense of the age
113:53 the standard, the standards 27 million old. We're gonna get one age
113:57 the standard is 37 million years we're going to get a different.
114:01 you have to have some confidence in your standard ages. So a lot
114:05 work goes into that? And you know, fish canyon Tufts a
114:08 example in which people fuss for many about, you know, they started
114:11 27.7 and they've moved up to 28 oh five or something like that.
114:16 a little bit older than they used think. Um And so if you
114:20 at a paper that's published, you , a few years before this other
114:23 , you can't compare them unless you the same. And they and they
114:27 both using fish Canyon tough. If use fish canyon tough 27.7 and they
114:32 fish canyon tough 29.0. Well, agents are gonna be different just because
114:37 that. So if you want to them, you have to decide which
114:40 age to use for both of them . But this this is the foundation
114:47 the 40 39 technique. We can measure argon 40 40 and 39 on
114:54 same sample, That's better. But we have this problem of having to
115:00 what is the relationship between produced Argon And the amount of our potassium 39
115:09 your sample, which then tells you the amount of potassium 40 in your
115:14 . Because that that understanding is We have to send samples to which
115:19 know something about already. We know age. We can calculate the
115:25 So it's always relative to your But once you do that you're you're
115:30 and why? So why do we this? There's lots of advantages.
115:33 first would be speed. You you analyze it just in one you know
115:38 go. Although the speed is mitigated the fact that you have to send
115:42 sample to the nuclear reactor. And actually quite a bit of a
115:47 Because when you send your sample to nuclear actor you it would be nice
115:51 we could just shoot neutrons and hit potassium 39 atoms. But all sorts
115:56 other nasty nuclear things are made when shoot especially on on isotopes of iron
116:02 scanned iem and they make all sorts really radioactive stuff. And so your
116:08 nuclear radiation safety officer will be very about that. It's dangerous stuff.
116:14 good news about really dangerous radio activity is that they're they're dangerous because they're
116:19 active. But if they're highly active means they go away fast. So
116:25 on the size, depending on the of your radiation, um you may
116:30 to wait two or three weeks before will be before it's legal to put
116:34 in a Fedex bounce and send it to you. So the speed of
116:39 analysis is much better because it's just better thing. But it's that
116:44 Only happens after two or three weeks waiting. Uh The other advantages this
116:51 as I said, isotope ratios are more precise. And then the third
116:56 thing we haven't talked about yet is in the and the potassium argon
117:02 you have to measure all the argon all the potassium all at once and
117:06 get that ratio. We're gonna have do that here. And so what
117:11 gonna have is this technique called step . Where we heat the sample up
117:15 little bit and let some of the come out. We measure the
117:20 then we heat that sample up a hotter and we measure that ratio is
117:23 incremental step heating and oh I've got paragraphs on each one of these,
117:31 daughter and the parent are measured Yeah, so I said all this
117:37 but mitigated by the need to send sample of new brack. Um This
117:44 allows smaller samples to be dated. never have you know whatever you did
117:49 . The smallest thing you had had be split up and put apart.
117:55 I mentioned this already, the data with much more precise than the ratio
117:59 two concentrations. And then this did say this already the most significant advantage
118:05 is that this step heating which will into a lot more detail but just
118:09 is good. Um However, we to pay attention to the fact that
118:17 addition to the reactions on iron, just make the sample physically dangerous.
118:23 are some other reactions on potassium and that make the analysis of this stuff
118:31 . Uh and here's a couple of here. We have a neutron comes
118:37 to a potassium 39 nucleus and a is shot out. Great news.
118:43 made some Argon 39. Argon 39 a half life of 269 years.
118:49 perfectly usable. However, we're shooting to this whole thing. Right?
118:53 don't we don't have a neutron uh , you know, aiming gun that
118:58 just only shoot at the potassium 39 stuff. If we could just do
119:02 , that'd be wonderful. But the are going everywhere. And sometimes a
119:06 might hit a calcium 42 nucleus. in that case a neutron comes in
119:12 an alpha particle comes out. the consequence of that Is making Argon
119:22 . That's clearly bad. Right? now we're making Oregon 39 out of
119:26 different things. We only wanted to it out of potassium 39. But
119:30 made some out of calcium problem. are we gonna deal with that?
119:37 , in fact, that's not the one. Here are some other interfering
119:40 . I just mentioned this one, meant to just mentioned. Uh
119:44 here's the good one. Here's the one. I just mentioned this
119:50 We've also got this one from our calcium 40. We have another
119:55 This is an N. And alpha neutron goes in. Another neutron comes
119:59 and an alpha particle comes out. make Argon 36 which we haven't talked
120:04 much like yet. But that's also issue. Um There's also a reaction
120:10 potassium 40. You put a neutron approach on it comes out and make
120:14 40. Oh, it's getting All of that is taken care of
120:21 this other one thing. Look at . We also get an an alfa
120:25 on tests on calcium 40 neutron goes alpha comes out. We make Argon
120:34 . The good news is Argon 37 a half life of 36 days.
120:39 means the only argon wherever only Argon were ever going to measure this stuff
120:44 made in the nuclear reactor. Why that valuable? Because I'm gonna come
120:50 to that. Oh, I guess So why is that valuable? Because
120:57 we take our sample that comes back the nuclear actor, in addition to
121:03 off these santa, Dean's or bio of known age, we also have
121:08 put a couple other bits of standard in there. We put a like
121:13 bit of calcium sulfate or some calcium thing that has no potassium in
121:19 No argon. And in fact we make sure it has no argon in
121:23 by putting in the oven for a to get rid of all the So
121:27 maybe calcium sulfate or some other. then there's another thing you also put
121:32 potassium rich glasses which have no organ them. These are this material is
121:38 in potassium and has no calcium. material is rich in calcium and has
121:42 potassium. We send them to the and when we we get back we
121:48 the argon isotopes that came out of guys. There was no argon isotopes
121:52 begin with. And so we measure . But but now they've got argon
121:57 because this stuff We've made argon in samples because we've shot neutrons at the
122:02 and calcium in these things. But we know they didn't have any argon
122:06 begin with, uh we can measure ratio of 37-39 for example, in
122:15 that special calcium stuff. And that's because that gives us the the the
122:21 production ratio in our nuclear reactor for this much 37 we get this much
122:28 . And that tells us how much to subtract away because that's the calcium
122:33 39. But because we know that came from nothing else. We can
122:39 37 in our bio tied or in unknowns. We measure 37 And that
122:47 and then we measure 32nd and 39 these special samples. That gives us
122:52 correction factor. We take that and it by our 37 in our real
122:57 that tells us how much to subtract , understand. Got it.
123:03 we're fixing our problems now. Um we get the same thing by measuring
123:08 measure the 37-36 ratio, 37, . Both of those are problem
123:15 But because we have the 37 that comes from this and then there's a
123:20 approach we make for our for our rich material is we see how much
123:25 is made uh just on the P. reaction and that allows us
123:30 fix these problems. Um There's Oh and and no, but 11
123:37 then. I know that. I earlier that 39 has a half
123:41 269 years. That's nice. asking her. Uh Should be 39
123:49 a half life. 269. Here around 37 is a little less convenient
123:54 life, 36 days. What kind constraints does that place on a
124:04 40 39 laboratory. How Long is Argon? 37. Gonna be
124:24 Mhm. No, no. Half . This is the half life for
124:32 37. Half life. 37 Oh yes. Okay, well,
124:39 don't worry about 39. It's So what's the problem with 37 Something
124:52 a half life of 37 days. , it's not too short. It's
124:58 , but Well, it's it puts constraint on us. How fast do
125:02 have to analyze this stuff? You go a little more than one half
125:08 because I mean after 36 days, much is left? Yeah that's still
125:14 . After 75 days. How much left? Still might be enough.
125:20 about after 90 days? We're down about a quarter now. We're getting
125:27 be a little worried now we're a worried. So we Have to make
125:34 analyses when there's still enough argon 37 measure. And so that's gonna be
125:42 more than 3.5 4 half lives. you can't send this stuff the nuclear
125:50 and then wait a year to do analysis by then there's no 37
125:56 And so you can't make you can't multiply the 37 by some super correction
126:02 . Take away the bad 39. away the bad 36. Get an
126:05 . We've got an unknown now. only way that you can get away
126:09 that is if maybe what the sample sent to the nuclear actor didn't have
126:13 calcium. If it's a really high Ortho Claes has no calcium in
126:21 Well okay but if it's a horn it's got lots of calcium. You
126:27 analyze this in about a couple of because otherwise you got a problem.
126:32 and remember you have to wait three before it even comes back to
126:37 So unfortunately these these react these you the reactor will charge you for the
126:44 . You know you send your they're gonna want some money for
126:47 Um You can't take advantage of because they charge you by the radiation not
126:52 the size of the radiation. They say what service cost $1,000.
126:58 you can't economize on this service by them 100 samples Because then it's only
127:04 know, it's only $10 per The problem is you can't analyze 100
127:10 in in three months. You just there's just not enough time. So
127:17 active Argon lab is going to have have, if you want to analyze
127:20 every day of the year, you're have to be sending samples to the
127:24 every three or four months. Which that you know your radiation costs are
127:29 to be four times as much as you could send just one big
127:32 I got 1000 samples. I'll send all at once. Just $1000 per
127:36 for $1000.01 dollar per sample and I'll analyze them when I get around to
127:41 . You can do it. So a problem. That's just a quirk
127:50 the system. Right. You get Zircon, you know, you can
127:53 it whenever you feel like you know a sabbatical, come back two years
127:57 , it's still sitting there ready to analyzed. Not these guys, you
128:01 something to the nuclear actor, you be ready to analyze it when it
128:04 back. And I've heard stories of they send they send the sample
128:08 In the meantime there system breaks. got a problem with a pump or
128:12 got a problem with this is a with that. Those samples just eventually
128:16 stale, can't use them. So the thing that's absolutely Calcium 37 Calcium
128:30 Argon 37. That's our limiting thing . And and you know, the
128:35 news is is because it has a half life. We know that the
128:39 of the 37 we measure is from reactions only. And we can use
128:44 value and use that to say that we measure the 37-39 ratio in our
128:50 stuff in our standards. That gives the correction factor That we can because
128:55 the correction factor, you know, back to this problem here. We're
128:58 argon 39 out of the thing we plus out of something else that
129:03 Not at all to our situation, have to have a good way of
129:07 how to subtract this stuff away from stuff. Have a good Just so
129:12 as we analyze the samples before all 37 disappears. And that's gonna
129:18 that means analyzing the samples within about months, three months maximum, the
129:24 the more I mean, so that's issue. Um Here's another nuclear,
129:31 interfering reaction. You might have to about that. You don't really,
129:36 another on chlorine. If you have bunch of chlorine in your sample and
129:40 the only samples we're gonna have a of chlorine in would be the Micah's
129:45 the anthem balls where you have that . H. C. L.
129:49 . Part at the end You have lot of chlorine. You could have
129:53 end gamma reaction which produces chlorine 36 is itself radioactive two. And it's
130:02 that producers are gone 36. The news is that the half life of
130:08 , is 30,000 years. So it's slow. So the amount of bad
130:13 you're making for chlorine this is not issue because remember we have to analyze
130:19 samples in three months. So in months this doesn't change. Um We
130:26 produce a bunch of argon 38 from 38 37. We make up argon
130:35 this has a half life of 37 . So we get rid of all
130:38 right away. So we produce a of argon 38. That wouldn't have
130:42 there. If you have chlorine in sample, all that tells you is
130:47 sample had a lot of chlorine in . It doesn't it doesn't take part
130:50 any of the equations for calculating So that's a that's a reaction that's
130:55 really interfering reaction. In fact, know, it gives you it gives
130:58 more information. Oh gosh, this has a lot of chlorine in it's
131:03 really important, but you learn So this one is theoretically a
131:08 But if you've got a lot of in your sample, you probably also
131:12 a lot of calcium in your which means, you know, if
131:16 waited long enough for this to be problem, you have much bigger problems
131:19 worry. So I said that and I so I mentioned this already,
131:27 this is just the words we send potassium rich in calcium rich zero age
131:33 included with all the radiation packages. things my death himself, a calcium
131:41 glasses, whatever. One key thing that this must be done each
131:47 Some lazy labs will just take the factors and they say, oh,
131:51 nuclear reactor in michigan has this ratio , you know, and I've done
131:58 many times when I would be reviewing paper as a as a reviewer.
132:03 know, these people say, well just use that, we use the
132:06 factors as published by Dalrymple and and Fear 1975 You know, this is
132:13 30 years ago and I get out my PhD dissertation which used one reactor
132:21 three years and those, those correction . We, because we're, you
132:25 , my PhD had a lot of and we we analyzed a lot of
132:29 we had. I think during that my PhD used like 12 different radiations
132:34 those four years And those 12 different , some of these things would be
132:39 by a factor of 50%. So can't just say we're going to use
132:43 correction factors of Lamphere and downward from . Uh because that just gives you
132:49 vague note. And especially if you're at young stuff, all of these
132:57 , most of the things, most the problems that we're talking about this
133:00 one, we'll talk about another. of this really matters. If you're
133:04 at a, you know, protozoa Because all of these are little tweaks
133:11 on the ratio of the 40-39. if you've got gobs and gobs and
133:17 of 40, Not subtracting away a bit of 39 won't change that ratio
133:22 much. But if you're trying to something that's 10 million years old,
133:28 can make a big difference. You , you can take something that's 10
133:30 make it look nice. That could significant. Make it make it look
133:37 . Whereas if it's two billion and you use the decay factor, the
133:43 factors from Dalrymple and Land Fair, instead of what you really should have
133:49 , your two billion year old sample a 1999 million year old sample,
133:55 not different. But you know, the difference. That's one out of
134:00 is a lot different from one out nine. So the younger your sample
134:05 , the more all of these issues to be taken care of very
134:12 And so the actual equation for the 40 39 ratio, for example,
134:19 like this with all of these corrections uh you have to correct and when
134:25 measure this, we have to then for the decay of Argon 37.
134:29 measure it today, but there was as much to fix that. That's
134:33 this is about. So there's scouts changes. There's the amount that
134:37 you know, you got to work all this. So, but all
134:40 that comes from measuring the right correction . And then measuring the right things
134:44 our sample. We subtract this all way we get the right the thing
134:49 we wanted to begin initial that's the the ratio that we have in the
134:55 sample. So measure M stands for eyes. For initial calcium is the
135:02 factor based on calcium and land up a correction factor based on time.
135:10 . So, um This describes the of things we can worry about.
135:19 and sources of uncertainty if we look the uncertainty of our of our of
135:26 our work here. We've got uncertainty measurements of Argon 36. Argon
135:32 Argon 40. And for a young rich sample. Most of our uncertainty
135:39 from measurement Argon 36 I haven't described 36. Yes, yes. So
135:51 got, we talked about radio Janet gonna be um we are going to
136:05 that some of the gas we are is not radio. Probably in this
136:10 the assumption that it is atmosphere. better assumption. When we put our
136:14 into the machine, we have to it down, get rid of all
136:20 . We don't give him the ball we do. But we're going to
136:25 that the amount of part of is 40 that we measured. Hi,
136:33 , 95.5 times the amount of 36 that's the ratio 40 36 ratio in
136:48 hair. Now It's 2 95. we're gonna assume that there's some of
136:55 atmospheric, excuse me, some of atmospheric here, it's in our
137:02 in our system. And that's why need to measure our And again,
137:08 younger the sample is, if this a young sample, most of the
137:13 comes from being able to measure this very precisely because we're going to leverage
137:18 value by a factor of 300. this number is a very small number
137:24 we get this wrong, we've made wrong. This is a very old
137:30 . We don't care. Right. is this is a huge number and
137:34 also a bit. They won't apply by 300. But if this
137:40 so this is another example. This a little correction for how much
137:45 believe in our system. Oh, also the patient that there's some argon
137:52 maybe in the Crystal women's board and wouldn't be making compounds. But interesting
138:00 the interstitial spaces of the prison. our some of that might be,
138:06 know, and now we're gonna go the simple idea that we don't have
138:10 pay attention to how much organic anything compounds that knows no dramatic place where
138:16 are going to be. You can find explain little places in between.
138:21 , at some level, who But at the level of a single
138:27 of sanity is only two million years . And sometimes we can just say
138:34 there's extreme 95.5 things what we're but we have the ability by the
138:40 that I know how to discuss actually what that value not happened yet.
138:49 48 for a younger potassium four the argon, 36 becomes even more
138:56 . This this is a pie diagram the magnitude of the of the uncertainty
139:02 individual management. And you can see for an old sample that's potassium
139:09 36 becomes less important than getting the is pretty important because 39 is telling
139:15 how much potassium And for for a poor sample that's really old, 36
139:22 important. So 36 is the most measurement. Generally speaking, it's the
139:29 one and it has some leverage especially on young samples. Um So
139:37 are we? Oh yeah. So I said before, as I said
139:45 , we have a relative dating Known age influences in there. That's
139:55 . But we can take this a further Because in 40 39 analysis organs
140:01 by hitting the sample up, but don't have to do all that heating
140:05 at one time. You can have a little bit needed, some
140:09 get some more. And this allows to assess two important assumptions of,
140:15 potassium argon dating. The first is argon is uniformly distributed in the
140:22 That may not be the case. know, we just, we have
140:26 other, we have no other way worrying about the argon distribution. When
140:31 analyze all the argon over here and the potassium over here, we get
140:34 ratio. Well, we don't actually assume it's uniformly distributed. But all
140:39 get is an understanding of the this is the average amount of argon
140:43 the average amount of potassium. You have a distribution, we're looking at
140:48 distribution business distance and this is you can have something that looks like
140:55 or you have something that looks like . The potassium argon system can't tell
141:00 difference, but the consequences of the looking like this and this are quite
141:04 . If you go back to our of the Duluth bro, you've got
141:15 moving out and you can understand that you heat a sample up. If
141:19 heat it up a lot, you're to get rid of all the
141:22 you're gonna start with a nice flat . But if you just heat it
141:25 a little bit, you know, gonna affect the outside more than the
141:29 and you're gonna get a distribution that like that. You know, you
141:34 a blowtorch to that banana for two . It would be different if you
141:38 it in the oven for a right, But you might, if
141:43 only analyze the total thing, it be hard sometimes to tell the
141:47 So how are we gonna learn these ? Okay, so we'll get to
141:54 , I guess before we get to , let me just want to point
141:57 that I mentioned potassium feldspar by its , muscovite, horn blend. There's
142:02 a wide range of things in which 39 has been applied, uh,
142:10 , methylene are minerals that are high potassium that are found in volcanic rocks
142:15 low silica. You know, you know what all these other things are
142:22 temperature, We know pretty well for four, uh the problem with some
142:27 the potentially useful ones are generally pretty in potassium for salt ground mass.
142:33 maybe some examples of how this works well. Um And this works pretty
142:38 again because if we just make the really big, we can fix
142:42 Oh thank you. Um Anyway um people have even done plays to date
142:52 . And as I said here the problem is getting enough argon to
142:55 But we can do that. so now back to this testing these
143:01 , we've got two things we need worry about and we're gonna get around
143:06 by this step heating approach and we're display these data on a two on
143:14 diagrams that are sort of Um particular the Argon 40 39 approach. The
143:22 is this diagram called the age spectrum , in which on the Y axis
143:28 show the apparent age of the step the width of the box. There
143:34 the uncertainty in the age. And the Y axis shows the cumulative amount
143:40 argon 39 in the steps. And the width of that box is how
143:46 argon 39 was into that individual And then we we normalize all of
143:51 boxes to the total amount of argon that was measured in the sample.
143:57 why do we use argon 39 as cumulative measure? Because remember our ground
144:04 was produced in the nuclear reactor. it was produced predominantly on a reaction
144:11 the task in 39 Tax him 39 a mineral like K feldspar or but
144:18 abide. Or even in even in even in the salt glass, it's
144:25 uniformly distributed. There's no reason why , especially in a mineral, especially
144:32 potassium rich mineral, that's just that's , potassium is in the lattice
144:37 so it's uniformly distributed. So when shoot some neutrons at it, the
144:43 the argon 39 that is produced from , potassium 39 will itself be uniformly
144:51 . Wait, Remember we're trying to the idea whether the Argon 40 is
144:57 distributed. We don't know it might might be this, But the good
145:02 is is we have a pretty good that Oregon 39 was uniformly distributed.
145:08 we need variations in the ratio of 39. Well, because it changes
145:13 40, not 30 and so there's eight spectrum diagram. And if we
145:21 a diagram that looks like this, all the steps are about the same
145:25 , we can say, well, is a sample that's had a very
145:28 history. There's no variation in just a concentration with just with the
145:35 Um And remember we're generally don't, doesn't have to be this way,
145:41 we are generally going to heat the starting at lower temperatures and then eventually
145:46 up to melting. So the samples the left side of this diagram are
145:50 first ones and they move towards that and not only that, they're always
145:55 , we always plot the samples in order in which they are obtained
146:00 That will be in a mon atomically temperature approach that one's cooler,
146:06 hotter, hotter, hotter. And when we take a crystal or a
146:14 , if we heat it up at temperature, we should expect to get
146:18 argon which is on the edge of crystals and later on we'll get the
146:24 that's coming out. It had further go. We had to heat it
146:27 longer, heated up hotter. And this this gas is the gas that
146:32 resided in the interior of our This is the gas that resided at
146:37 edge of the crystal. It was to get down. It came up
146:41 because it was on the edge only up a little bit of temperature,
146:44 got it out easier. And so it's a nice flat diagram like
146:50 And because we assume that the Argon is uniformly distributed, this tells us
146:55 the Argon 40 is also uniformly I said all this, this is
147:03 describing the diagram. Um and so some people go a little too crazy
147:09 this, but they talk about these called plateaus where they see a bunch
147:13 bunch of ages that are all the and they think of this hope a
147:18 and they say we've got a plateau and we can treat them all the
147:22 . We'll talk about how plateaus might be the greatest idea, but that's
147:26 thing in some literature. Um But look at this a little bit
147:34 we'll see what time is it? 11 Sing. Yeah. Why don't
147:49 take a short break? Because this a good spot? Um It's
147:57 Let's come back at 11:15. so here's a little bit more about
148:05 age spectrum diagram. We're gonna be at presentations on the top here of
148:14 versus distance going from the rim to center to the rim. And on
148:19 bottom, the corresponding age spectrum diagrams we would get from heating these things
148:25 . Um If we imagine in the case we start out with a situation
148:31 which the argon 40 and the potassium be, Yes, the argon 40
148:36 the argon 39 are uniformly distributed in crystal. We would heat them up
148:43 we would get the ratio of 40-39 change. And so the apparent age
148:49 be the same across. And we a nice flat aspect. This might
148:52 the kind of age spectrum you'd expect get from a volcanic, well,
148:57 in which had a very simple history get, he he cooled rapidly.
149:01 got heated up. Let's now imagine sample which we um take from this
149:12 and we heat it up Recently. take it and we we we heated
149:18 enough such that the Argon 40 that at the edges of the crystal is
149:26 but not so much that we get of all the Argon 40. There's
149:29 some in the center. Now I'm the Argon 39 as a flat line
149:35 the Argon 39 wasn't there during this disturbance that only gets added later.
149:41 ? So don't confuse the fact that managed to move the Argon 39
149:46 Why did 40? We moved around some sort of, let's let's say
149:50 is a contact metamorphic ism event. come the Argon 39 didn't move around
149:56 gone because it didn't exist until we it to the nuclear reactor. This
150:01 the this is the thing that we to the nuclear reactor. This is
150:04 thing we made in the nuclear And if we were to analyze this
150:09 , it would end up looking like with the first step being at or
150:13 zero and then rising up to some very close to what we got
150:19 And um but if we took that and then we let it sit around
150:26 a while and then analyzed at some of years later, we would get
150:31 an eight spectrum diagram that looked like with the first step not being zero
150:37 the disturbance wasn't yesterday, it was time ago and then this going up
150:41 some value up here, which which is more than our initial value
150:46 we let it sit with more And so there's the advantage of the
150:52 spectrum diagram is that we can see things. And you know, for
150:56 first one straightforward, we would we have gotten the same answer from potassium
151:01 in the first example, but we got something very different in these two
151:07 examples. If we'd have done the one, we would have gotten a
151:10 argon age, that might have been in the middle. Let's let's just
151:15 convenience, let's say that the age this sample over here is 100
151:21 This then goes from an age you know, one or two million
151:25 to 100 million. The reason I show this as zero is because it's
151:29 very difficult to get gas from the edge without getting some in the further
151:33 go in, the more the average up to be non-0. But here
151:39 going from age is close to zero close to 100. But if we
151:43 a potassium argon approach, we would aged somewhere in here is like an
151:46 of like 60, right? What to the sample 60 million years
151:52 Nothing. We've got two events. is the eruption of this royal light
151:58 100 million years ago. The next is that the reheating of this real
152:02 yesterday. The average of those two end up looking like 60. That's
152:07 potassium argon would tell you. It 60. Nothing happened at 60.
152:12 why this is so much better because can look at this age spectrum and
152:16 ah ha we got some interesting things here. One is quite recent
152:19 maybe 100. Now, let's try figure that out. Nothing happened at
152:24 million. And that's the same problem were talking about here when we were
152:28 old and young ages in Missouri Cons , take this average, take this
152:33 and this value, get some intermediate . It has no geologic significant.
152:38 then this one is just the same we would get instead of instead of
152:42 60 million year old average here, would get 100 and 10 million year
152:46 average. But again, nothing happened 100 and 10 in this example.
152:50 relevant times are what, that would like 150 this would be 40 or
152:58 . Let me show you some Real data here. Um and
153:05 I ignore these ones on the We're not going to talk about recoil
153:08 much, but here's here's what an spectrum. You know that it's nice
153:12 simple might look like all of these about to say all these are about
153:16 same. That's fine. Um but think a little bit more about what's
153:23 on. As we heat something up , we've got a initial concentration.
153:30 , say He's 39. The dotted . Yeah, let's say 39 is
153:37 dotted line, and 40 is the the dark solid line. These are
153:43 concentration gradients. And in this case got X over R. R.
153:48 the total radius excess the individual So this is the this is the
153:53 of the crystal. This is the of the crystal. And if we
153:58 , this would be our age spectrum that we're going to produce over
154:01 Over here, we start heating the and we produce immediately. A variation
154:09 the argon 40 distribution because we start out the argon, excuse me,
154:15 . This is 39. That changes . The argon 40 has also sucked
154:20 a little bit, but it already a profile. We heat it up
154:23 little bit longer and we get this that's well along in the in the
154:27 the heating. And we're dropping everything and we're changing the slope as it
154:32 out. But the slope is not important is the absolute slope. But
154:37 relative slope of these two isotopes as coming out. And you see the
154:41 is very different when we get slope becomes the same because we are pushing
154:48 from here to here. And So the here's how the slopes would
154:55 . The two slopes of the flocks 40 and 39 are quite different when
155:00 start out, Then they become to more of the same and then they're
155:04 much the same on and that's why part is slapped. So an age
155:09 diagram then is a, is a of the relative flux is of 39
155:14 40. As they leave the they should approach a constant ratio if
155:19 go far enough back and see that their initial ratios were not changed.
155:24 initial concentrations were relatively constant with with . Um We can actually even determine
155:33 temperature throughout here if we work at , but we won't go into that
155:37 detail. But here's some more real of simple systems that we would
155:43 Here's a wry light from Ethiopia. is something I did in my
155:48 Um and it just shows two interesting the red data for the first time
155:53 analyzed it and I wasn't really happy the quality of the data.
156:00 a little bit bouncy there. And what we did is we sent out
156:03 second radiation with a much bigger amount sample instead of five mg. We
156:09 15, which meant that we were everything better, you know, remember
156:14 Argon 36 is hard to measure if if you make the argon 36 more
156:19 the uncertainty in Oregon. 36. down the uncertainty and everything goes
156:23 This is the same sample better analysis second time just because we had more
156:28 . But in both cases you get you expect out of a Riley.
156:32 flat spectrum good. Um you can in a really light, gives us
156:39 like this is spelled for them. should have potassium in it. Good
156:46 . But as I said, you , we could sometimes do other
156:49 And here's an example of again, was from my lab where we analyzed
156:54 assaults. These were from Argentina and knew the assaults were going to be
157:02 . So we were analyzing a lot I just, the amount of material
157:08 for this is not big in modern . If you have fewer, Like
157:13 muscovites from the Himalayas say Muscovite has lot of potassium in it. Himalayas
157:19 going to be about 10 or 20 years old. You can get by
157:23 doing a step heating experiment on as as say five mg of material.
157:28 some sprinkles of Muscovite will be We didn't use five mg here because
157:35 knew that was the salt and the doesn't have much potassium in it.
157:38 we needed more sample, I can't how much, but it was a
157:41 more, maybe 100 mg. The and again, if this was a
157:47 from the moon would have would be limited, but you know, people
157:51 people in Argentina, they knew how sample to collect that bring home a
157:54 like this, that's fine. What shows is however, is that if
158:00 have enough you can get perfectly good . This was a very big
158:03 which is why we ran it Because this was a study. This
158:08 was mostly dealing with with rocks which we thought they were going to be
158:14 million years old. We got a of seven or eight million year old
158:18 . This one turned out to be , noticed this this is the parent
158:23 in thousands of years, not millions years. We got ages that were
158:28 200,000 years old. And because you this was a surprise uh because all
158:35 other samples that were submitted in this were three or four million years
158:40 My first concern was, did we this sample mixed up with something
158:44 How come? So we so we the same bottle that said T.
158:47 . R. Two on it, it to the reactor a second
158:51 Got the same answer. So with sample you can get good results on
158:57 assaults that are quite young. So know the weighted mean of this rock
159:00 100 and 75,000 years. So this another example of, you have a
159:06 enough sample, you can go Uh you know, people have gone
159:11 than this, but this is it's pretty young. Um So that's
159:17 age spectrum diagram showing how it works well when we do with volcanic
159:25 Next problem we have to worry about this issue of, so I should
159:30 all of these steps on all of age spectrum diagrams are calculated with
159:37 with the with this the correction for argon that I've drawn over here where
159:45 multiply the amount of argon 36 measured 2 95.5 and you subtract that away
159:52 the argon 40 measured along with all other concerns, we make What if
159:58 the wrong number 2 95.5. We learn that by using this other
160:03 which is called the Argon 40 39 a Cron diagram. There are two
160:08 Akane diagrams sometimes get used once called because it was used first and the
160:14 one is inverse because somebody said, , wait a second, we should
160:17 this one. Nobody uses conventional I don't think, I mean,
160:21 don't, I don't know why they anyway. The conventional would have the
160:26 36 ratio and the 39 36 which is very it's conventional because it's
160:32 to like the rubidium stretch there. have a parent, there was the
160:38 daughter, the daughter, the normalizing . Same for start out like this
160:43 up this, people like to use X. Cron diagram because it takes
160:48 the problem that was also seen in Terror Wasser burg diagram while some people
160:53 that better is that we are using worst isotope less often here we have
160:58 36 in both axes. And remember 36 is the hardest thing to measure
161:03 . It's the smallest one. It more error to the system than anything
161:08 . Well, if we get a tricky here and instead of using um
161:14 36 39 36 if we use 36 and 39 40 we get an ice
161:21 diagram that looks like this. But only got 36. Used once instead
161:26 twice. When we when we turn around here, we have a different
161:31 point. We start with data. zero example is horizontal like before but
161:37 we rotate clockwise instead of counterclockwise. we're gonna be using this one for
161:43 for the rest of the thing. so if that's the case, what
161:48 should expect for some a simple like a like a highlight. We
161:54 expect some initial value in our sample some radio genic component and any gas
162:00 out of our sample should be a between the two 0.00338 is the reciprocal
162:07 2 95.5 Because this is 36 40 that's 40 36. So if everything
162:15 to plan, you should have a that goes through 00338. Uh But
162:23 if you have a line that's mixing some other value and your radio genic
162:30 ? If you just likely went ahead said we're going to use to 95.5
162:35 00338, you would calculate each one those points this way, you
162:40 you know, because we're basically what doing here is we got this 01
162:45 ice across which we're forcing through a intercept to 95.5338. So if we
162:53 our ice accretion of all these points go through these through that one point
162:57 there, we're gonna get different points this axis and this axis is what
163:02 read the H from. But really we should be doing in this case
163:06 say, well no, the mixing not between this 2 95.5. Between
163:11 other value. The radio genic component out here and all these points go
163:15 the same point. If we make line correct. So This diagram allows
163:22 to say, well wait a let's just run it through whatever it
163:26 through. Not forcing it to go to 95.5. Here's an example.
163:33 is a real life. Again, situation in which the data are mostly
163:40 along this line. These last few fall off of it but we're going
163:43 ignore them. But the data falls all this, you know, I've
163:46 a lot of data down here, some of these early points, these
163:49 early and in general when you're heating a sample like this, it tends
163:53 get more and more radia genic as move on. Because the non radia
163:58 gas is not found in normal parts the, of the crystal. It's
164:04 in the interstices where it's just got in and it's easy to get it
164:08 back out. So the first gas usually the least radio genic. So
164:13 would be those points over there. that would be the first one in
164:16 second and the third one in all down here. And if you draw
164:19 line through all that, you get 40 36 ratio, that is when
164:24 flip this value over, you get ratio of 3 61. That's very
164:28 from 2 95 But there's no reason it should be 295, that's just
164:32 air we're breathing right now. This point this this this sample says we
164:37 be using uh 361. And when do that, we run a line
164:43 all those points, we get a genic component down here, which corresponds
164:47 an age of 25.6. Now, we plotted that sample on the ace
164:53 diagram with just assuming the regular 2 , the first few steps up here
165:01 gonna be too old Because we're running , here's 2 95. Here's
165:09 I got that, I got it . Here's the sample. If you
165:15 you look at that, you it looks terrible, right? But
165:18 you look at it on the Akane dragon, you got a nice
165:20 ride. But this is assuming right? But just that little difference
165:29 . So in this point at this leverages this said, if you
165:34 you draw at this point here, get this and you see how much
165:40 we have on this sample. Because and excuse me, this is a
165:44 sample than this one. This samples sample. But that graph up there
165:50 the percent radio genic which is basically when you when you subtract away your
165:56 radio genetic component, how much did change by? Because it changed
166:00 You know, if you if you all of it away, that's non
166:04 jet. But of course, Whether radio genic or non energetic depends on
166:09 much you should tracked away, which on whether you're using to 95 or
166:13 other number. This graph says percent genic assuming to % 95.5. And you
166:19 , we start out at only uh or 2% rate identical points up
166:25 So those points, it's hugely Whether we multiply by 295 or 97
166:33 306. This is 306. So graph assumes 2 95 makes a graph
166:40 looks like that. But and you say, well why does it have
166:43 funky shape? Why does it get ? And then young? It's because
166:46 ages are calculated incorrectly. If we we look at this diagram, If
166:53 we change it instead of and we that line through, we get we
166:57 an age of 306. And but we had done it the other way
167:01 get this spectrum of ages from 3:00 shooting at an age of 12.8,
167:08 this is an age by 35. so if we if we were to
167:14 this, hey, I have, haven't done it redone it. If
167:17 will redo it instead of using 2 which which we did for this.
167:22 we used 306. We get a spectrum here at 12.8. Ah So
167:29 age spectrum diagram has an assumption in . The Cochran diagram does not.
167:34 when we see funky stuff like we we we we reference the ISIS
167:40 to say is that funky behavior of crystal or just an inappropriate mathematics that
167:45 chosen? In this case. It's the wrong algebra Because we fix
167:50 we get a nice, perfectly This is 12 million year old
167:57 Um What is this? Oh this shows the ability of really modern good
168:07 . I forgot about this why this shows how good you can do some
168:14 nowadays there's thousands of years. This this is moving from, This is
168:20 of an advertisement for the August six spectrometer. These are the same
168:25 you do one sample with this older spectrometer, the M. A.
168:28 . 2 15 50 or the G. 3600. Those are like
168:34 year old machines. These these are six are brand new machines. And
168:38 see this is an example of how machine matters And that lab I showed
168:44 the picture of from New Mexico, got one of these machines now why
168:47 it better because the electronics more Everything's cleaner. It's just now we
168:53 get here are ages now. Here's age of 6.25 million years plus or
169:00 . So that's one part in There's an uncertainty. And oh shoot
169:08 . That was for the M. . P. That's one part in
169:11 . They don't have the, well actually say that for this sample is
169:18 because you've got you've got a Um You know if you if you
169:23 you make the uncertainty really big it looks like the same thing. But
169:26 you get the uncertainty down here you say discord here for the M.
169:32 . P. We've got three parts 700 here, we do it with
169:37 newer machine, we get one part increase the uncertainty factor three, same
169:43 . Here we go from A factor 50 To a factor of 60
169:50 Uh so this is just an illustration how good the data can be now
169:57 to machines of just 10 years So this has the capacity to
170:01 And so if you're using, if interested in strata, graphic constraints.
170:08 you've got an inter bedded volcanic which is the, you know,
170:11 standard for figuring out how old things . This is a really good technique
170:15 use. It's reasonably fast, it's cheap and you can see with these
170:20 modern machines, you can get an of two parts in 202 parts in
170:26 for a rock that's only 300,000 years . Um That's for age spectrum.
170:37 another way people will date volcanic And again, this can be a
170:41 important concern if you're in some oil and you've got a bunch of of
170:47 rocks without fossils, but you really to know some age control. Maybe
170:51 drilling and you're running across something and did that once for an oil
170:56 they were drilling and they didn't know they had repeated the section or
171:00 Was is this a fault or we through the same rocks again, they
171:04 sure but they found a volcanic They sent it to me and they
171:09 tell us whether this is you know older or the upper, you know
171:12 this repeated or is this a much ? Um And so we dated it
171:20 this is a bit of an I'll get to this fit in a
171:22 . But it just shows you, know when you're dealing with somebody that
171:26 care about the money. Um This about the fast as you can deal
171:31 in argon dating. You know you to wait for the sample to come
171:37 and but you but you and one the reasons you have to wait is
171:40 you want to try and balance You know, they charge you,
171:43 think now it's up to like $3 $4,000 for radiation. That's how much
171:48 costs. So you know you want you want to make it as economical
171:51 possible. You want to put about months worth of samples in there.
171:54 can't put in six months because they'll stale but you put in about three
171:59 worth of samples so you get a cost per sample. But what if
172:05 got, you know Exxon willing to more because they're really concerned they drilled
172:12 hole and they want to know so were like how fast can you do
172:15 for us? So we said well can you get us the sample
172:20 They hand delivered it to us We did the mineral separation, we
172:24 the grains out, we sent it to the nuclear reactor the next
172:28 We just had one sample it because they were paying for it. The
172:34 and the good news is when you a very small mass. Well there's
172:38 a lot of you know Fedex doesn't so much right because you can put
172:42 in this big box and tiny bits radio activity. You know, they
172:47 able to ship it back to us about four or five days Because the
172:51 the radiation is dependent on the time also on the mass. If you
172:56 out 40 samples there's going to be sorts of nasty stuff going all the
173:00 they send it in one of these cardboard boxes. But anyway, so
173:04 smaller the smaller the mass of the less radiation there is, the
173:10 they can legally send it back to . But then you get into the
173:13 of cost per sample. But Exxon paying. They didn't care. So
173:18 at that time, you know, would charge them like We would charge
173:21 university for this sort of work maybe or $400. We charged X on
173:27 . Thank take care. All Um But and we were charging them
173:34 this was super rush service and we able to send it to the nuclear
173:38 and you know because we knew these were going to be tertiary. Um
173:45 said we don't need a lot of . Oh I didn't mention I'll get
173:49 get you that in a minute. don't need a lot of neutrons.
173:54 And they sent it back to us four or five days later we we
173:58 that the top priority of the lab we stopped everything we're doing and put
174:02 in the system and were able to them the report and send it to
174:08 . Um I think the total from time they handed us to the sample
174:11 the time we sent them the report about 16 days. And I told
174:16 of my colleagues about they go wow really fast. That's that's about.
174:20 when you're talking about this stuff you're you know if you're working for all
174:24 and you really need to know um is a great technique but you can't
174:29 about it in less than two Now if you could find a zircon
174:33 of the same rock and find somebody go you know do it in your
174:38 your I. C. P. . S. Lab you could probably
174:41 it in one day. The precision be as good. So anyway.
174:48 Another concern when you're dating these things doing this sort of thing. These
174:53 all be volcanic rocks, nice, flat age spectrum but flat just means
174:59 all the same. And this is is a concern that that you
175:03 we haven't talked about this much but you understand already that precision in accuracy
175:08 not the same thing. Right, is just how well, you
175:13 if I said I am absolutely confident are 1000 people in this room.
175:17 not 990 that's not 999. There's 1000 and one. It's 1000.
175:22 it. That's a statement of great . It is in fact raw.
175:28 , so they're not related. If said there's about 12 people in this
175:32 . Could be one, could be . That's a true state. Not
175:37 precise. If I needed to know many donuts to buy, it wouldn't
175:41 very useful. But it would be . Here's an issue is a nice
175:47 number. Is it? It's more than that data. What if what
175:52 looking at especially the sanity sanity is here. What if you're mixing.
175:59 if this is not a single crystal a group of crystals? Because we
176:03 more than one crystal to analyze to enough organ to measure. Well a
176:10 Riley is particularly prone to this If you have a very eruptive explosive
176:16 of eruption, you're going to bring all the crystals from the magma of
176:20 , but this big eruption, you , think of Mount ST Helens,
176:25 know, you've seen those pictures Surely in that cloud there's pieces from
176:31 the magma from the country rocks, is explosion blew the whole top off
176:37 mountain, some of the little bits that cloud could have been felt.
176:41 not from the magma, but from were just hanging around and then if
176:46 gather them together in your sample and carefully get them, listen felt
176:50 we need about 100 feldspar, put in there, look at the precision
176:55 in 300, I'm very proud. if you tossed in those felt
177:02 a few of the country rock felt that were older instead of getting an
177:07 300,000 plus or -40000 You might get age of 400,000 plus, your -40000
177:15 just tossing in 13 Cambrian. You'd tell by this because if all the
177:23 stars give off their gas in about same way, the average of everything
177:27 not going to change very much over course of this experiment. So the
177:31 is not going to be the true , it's going to be the true
177:34 plus this contaminant called Zeno kristic crystal chris not belonging and the age spectrum
177:43 is a difficult way to invest in recognize that. And so a lot
177:49 times in for real, I should to what's single crystal dating and here
177:55 abandoned the ability to do an ice Cron, we're just gonna use 2
178:02 because what we're gonna do is we're take this crystal and we're gonna heat
178:06 up all at once. Get rid all the gas right now. And
178:11 is often done with a different heating . I haven't talked about how we
178:15 things, but in the, in other system, the system in which
178:18 modulate temperature, it's in some sort oven, it's a furnace of some
178:23 where we have temperature control and we heat it up to 506 107
178:27 But in order to heat up your , you have to heat up the
178:32 , you have to heat up the of the furnace and the crucible of
178:35 material and that has little bits of sticking on it. And when you
178:40 it up, so that that's, what's called you, you have you
178:43 heard of something called the blank of geochemical system? You know what that
178:47 ? Blank means when we run through entire analysis without a sample, We
178:54 do it when we heat up to see what we get, There's no
178:58 down there, but we heat it , we can still measure some argon
179:02 remember we're breathing 1% argon, it's hard to get rid of all the
179:07 inside that sample, even if you for a week. Um, so
179:13 you heat up the sample and then rest of the apparatus, you'll get
179:19 argon you don't want. But with single crystal dating, we use a
179:24 beam to heat the sample. And that, you know, we are
179:27 heating the crucible and the whole thing , we're just using, we're heating
179:31 sample. It's just the same. so there's very low blank of that
179:36 . That's why we like single crystal . The disadvantages, we lose the
179:41 to do a nicer con because just point. So we're back to just
179:44 it through to 95.5. Why we that is because what we're really concerned
179:49 here in these explosive volcanic eruptions is scenic Chris stuff that might throw in
179:54 to throw it all off. And here we have an example of look
179:58 all these things that are right here about 87.9. Look at this
180:02 Look at that one, it's 92 years old. It's off here by
180:06 . We're just gonna ignore that Right? And of course we're gonna
180:09 any of the ones that are way . You get, you know,
180:12 get an age of 30 30 30 75 30 30 30 65 30 30
180:21 . If you were to put all together, you wouldn't have gotten 30
180:24 you, You've got like 30.2 or and that and that the level of
180:30 that we've got here, we can down to precision. Here's here's an
180:33 of 0.2. You can see. last example I gave you is
180:36 Here's the precision of points. Uh was it? Well, this is
180:43 of plus -2000 years. But if just tossed in, you know,
180:48 60 million year old grain in this year old rock, you know,
180:53 gonna vastly overwhelm your precision, your but now you're way wrong. You
180:58 from, it went from 26 to . So let's just say 296 could
181:05 significant for your studies. You'd never it. So that's why we do
181:10 crystals. No, you just have subtract away what you think the blank
181:22 . That's another correction we make for all this. And so what we
181:25 is we run through the run through analysis with no sample in there and
181:29 the blank. That's how much is that gets subtracted away from everything.
181:36 , it is the is the amount argon that's inherent in the system.
181:41 but then and that actually goes down time because we put these samples into
181:46 system and we start pumping the system it gets better and better and better
181:51 better because we're pumping longer. And actually the first sample that you run
181:56 should should be one of the older you're gonna work with because older it
182:00 , the less this matters. And might take if you put in a
182:05 of samples into a system like it could take two or three weeks
182:08 analyze them all by the end of three weeks you've been pumping during this
182:12 time or at least different parts of time and eventually you get down to
182:16 is probably your best blank of all to, you know, almost no
182:19 left. And that last sample you got really good blank and then of
182:25 , what do you have to Open it up and let all that
182:28 back in because you're done so, , but a blank is just another
182:32 of these corrections and the younger your is more important that blank is and
182:37 why we like to use the laser because the blank goes way down because
182:41 we're doing is heating the sample and just a little bit of a of
182:44 metal underneath it. If you're using furnace, you're heating the sample and
182:48 this furnace apparatus. The trade off the laser has glow blank. The
182:55 has temperature control. When we turn laser on, it's almost a sort
183:00 on off switch, you know, to modulate, you know, using
183:03 laser to say, I just want eat that at 300°. We don't know
183:08 . And it's hard to control. might be 300, and then you
183:12 up, there might be 500, 600 terrible temperature control. Really wonderful
183:18 . So that's why we have both in mini lab. You've got a
183:22 for when you don't care so much temperature control. You've got a furnace
183:26 when you do. Um, here's example of better a better illustration of
183:32 variety you might get from from looking single crystals in a volcanic rock.
183:38 is from the bishop tough famous eruption California. And these are the spread
183:44 crystal agents where you've got ages, agents here that start as low as
183:51 , these thousands of years, whatever really seeing the victim, very fine
183:56 here. This is about 762. of them come along here at about
184:02 . There's some older ones that he's to 778. And so you have
184:09 choose then what group of these you're use. Here's, you know,
184:12 these are different met uh, statistical . Method one gets away to be
184:18 a low different filters on here. you can get either six 6765.4,
184:30 or 7 64.8. You know, some level, you know.
184:36 these these guys overlap. This one overlap, but it's only overlapping missing
184:41 few few 100 years here, but just shows that clearly you don't want
184:46 guys, these are something else is on. What's going on here?
184:51 these are crystals that sat around uh are probably crystals that fell into the
184:57 chamber. Now, if they were the magma chamber for 1000 years,
185:00 would have lost all their arms, they fell into magma chamber, they
185:04 have an additional 10,000 years worth of . We don't want that. This
185:12 the age of eruption is about not 780, which is what this
185:18 looks like. So when you're dealing really precise concerns about this, you
185:25 , you will trade off this, concern about 2 95.5 for this business
185:32 . Um Here's another um here. that is, that was that was
185:38 study they did with argon shoes. these guys Then they did the same
185:45 , but they did it by zircons this is really pushing it for the
185:49 for using uranium left that, you , modern techniques you can get
185:53 they got for the zircons, they gone, they got 765 for the
185:59 , they got 767. Um and guess these are all different. What
186:05 this, I forget. Doesn't say will ignore that for now. Um
186:13 if there's a problem is the zircons 3000 years older. It could be
186:24 sir, cons in the magma chamber retain their lead closure temperature of organ
186:30 bladder con might be Might be 900°. magma chamber might not be 900°. And
186:38 as soon as this Circon is it starts to retain its lead.
186:43 magma chamber sitting down there for 3000 is quite reasonable. So the difference
186:49 the argon age and the zircon age be the difference between the crystallization age
186:54 the eruption age. Why doesn't the give the crystallization? You understand the
187:13 between the crystallization age and the which are two different events. We
187:17 crystallize some crystals. We went from to partly solid and then sometime later
187:24 mixture of crystals and liquid gets tossed in the air. There's a time
187:28 there. That gap might be these numbers here between 767 and 765,
187:36 or 3000 years. Why do we these two ice topic systems not agreeing
187:41 a volcanic rock, They should be same. What's the closure temperature of
187:54 in Zircon said it a bunch of About eight or 900°. What's the closure
188:12 of argon in Salzburg? Well, would be good enough, but it's
188:19 than that. It's probably about So crystals can form feldspar crystals
188:26 What what temperature do they form at 700° 600°. What if you've got a
188:38 in crystal that sits in this magma 3000 years, how much argon will
188:43 retained in those crystals during that They're they're in a magma close the
188:56 of the magma is at least 650 . Right, What's the closure temperature
189:02 this system? 200 degrees. So we keep a sample that's 400° above
189:10 closure temperature at that temperature for 3000 , how much argon will be in
189:16 crystals? We can do better than lot less. First of all.
189:24 lot less than what. But I want I want a number, simple
189:32 . The simplest number There should be . We're way above the closure
189:42 We're at 600° when these crystals are in Magma. But the closure temperature
189:47 our system is 200°. We don't start argon in a false bar until the
189:53 gets below 200°. So if that's not case for a for a zircon
190:02 it's hard to move out of the . We got a magma at 700
190:07 . Probably the arc. The lad said, okay fine, I can
190:11 out here at 700 degrees. Not problem. Every tiny little bit of
190:16 that's produced in that feldspar during that has gotten out of media, it's
190:23 . And so this is a measure crystallization time. This is not because
190:29 longer those crystals sit in the the longer there is a difference between
190:33 time of crystallization one physical process and process of eruption, a different physical
190:40 . This only tells us Whether it's it's a volcanic rock or a shift
190:46 whatever. When we date a rock argon in fells bar, we're learning
190:50 the last time the rock was in 200°. Now for this rock we know
190:56 can interpret that very simply. When that feldspar lasted 200°? No, it
191:06 at 700°. Crystallization does not occur at When was this crystal then at 200°
191:17 most recently volcanic rocks are straightforward and . Excuse me? Not before.
191:36 this isn't the lava. But I mean basically the answer is the
191:40 of the eruption. When this this is down there, we you've seen
191:47 of Mount ST Helens, right? think of that the day before the
191:51 we had magma down that magma is which means it's very hot. And
191:59 on the morning of May 18, happened to that material, They called
192:06 that very morning. Okay. But that it was at 700° and when
192:13 at 700° there ain't no argon in felt spot, there is lead in
192:21 zircon. So we're doing thermo chronology , even on volcanic rocks, if
192:29 have this level of precision Now, the uncertainty on these things is
192:34 you know, then of course they , You know, we expect the
192:37 age and the argon age to be same. But not when you have
192:40 sort of really outstanding precision. And is the kind of thing you might
192:45 from ice topic systems or volcanic systems or minus a volcanic system can sit
192:52 for three or 300,003,000 years. I really expect this volcano to be a
193:00 opportunity for for for thermo chronology, but this is probably the easiest way
193:06 think about thermo chronology because we know the thermal history is. We go
193:11 a magma through a bunch of hot to cold In a day or
193:17 Right? You walk up to that from Mount ST Helens a week
193:21 it was cold. So but yet so so we know the day that
193:29 ST Helens ash cooled off. If go to get the zircons from and
193:33 and so if we were able to . But if you were able to
193:36 the Mount ST helens stuff, we get an age of uh 42
193:42 Right? Because that's the day it 42 years ago. But if we
193:50 at the zircons from that very same , we might get zircons that looked
193:54 three or 4000 years old because that's crystals were forming The sanity and crystals
194:03 4000 years old. That the argon just give us 42 years because that's
194:09 long they've been cold. But the don't care about cole Zircons will tell
194:16 the moment they crystallize, they might an age of three or 4000 years
194:20 if you add 760,000 years would look this. understand that's closure temperature in
194:31 nutshell right here it gets it gets pronounced and more complicated when the cooling
194:36 slower. If we now we now this concept and apply it to a
194:43 range. Himalayas are coming up in Himalayas are coming up fast but they're
194:48 coming up fast compared to this. , Himalayas are eroding at a one
194:52 two millimeters per year which is so . That means they're cooling down in
194:57 few, you know, tens of per million years. This is thousands
195:02 degrees per year. Right, So but the concepts are exactly the
195:06 . And here we see this variation of these outstanding. This is a
195:12 published just a couple of years ago they're showing the top notch precision that
195:17 can now put on some of these . We couldn't we couldn't deal with
195:19 questions before. Now we can actually well how long was the magma chamber
195:23 there. Well look at the difference the argon ages and the zircon ages
195:28 3000 years. That could be the in which the magnets it's done.
195:33 it's longer anyway, that's what we here. And and we would never
195:39 that. We've analyzed all of these these are Xena Kristic. Perhaps these
195:47 the old ones that we want to . If we analyze all of
195:50 we get an answer of 767 and have no idea how long the magma
195:55 was down. We couldn't even get answer older than 767. I wonder
195:59 come that's the older one because we in some bad crystals because they were
196:04 a crystal. Um so that's you know, goes goes back to
196:13 . Different systems have different closure temperatures they're gonna show up like that.
196:20 It's now 12 o'clock gonna stop for now. Yeah, we've got a
196:29 bit more argon to do but I it's better to, I don't want
196:32 go until we just have lunch at . That'd be silly. So um
196:38 take about an hour for lunch or . Alright, let's meet back here
196:43 1:00
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