© Distribution of this video is restricted by its owner
Transcript ×
Auto highlight
Font-size
00:00 begin. Okay, alright, so thing to discuss is more complicated.

00:09 the ability of argon data to assess disturbances to the system, um disturbances

00:18 can come about either by an episode reheating or by very slow cooling.

00:26 First let's just take the more simple of episodic loss, which we've already

00:30 about the episode of the Duluth for example. This is another

00:37 a similar sort of thing. As recall, these are samples someplace in

00:42 that had the same sort of a Luton intruding some other rocks.

00:47 are a variety of horn blends as recall, which are at various distances

00:54 the content. And again, we see that the closest to the contact

00:59 the most profound amount of loss. can see that some of these samples

01:03 up to, you know, Pennsylvanian here. But this one the maximum

01:09 it gets up to about 180. you go farther away from the uh

01:16 the disturbance, these will all be in these agents. But closer in

01:23 get this. So an episode of is pretty easy to come up just

01:29 understand, I think. But we to understand that similar age spectra in

01:36 in fell spars can look a little like they came either from slow cooling

01:41 reheating. You get an age spectrum this that goes from some value here

01:46 slowly goes down to some lower How did that happen. Well,

01:51 it, was it as we looked ? Some some age of 180 that

01:55 disturbed 100 million years ago maybe. quite often it is from another

02:02 It's from and and I alluded to last time and I'll get two more

02:08 a minute. But if we we that fell. Spars might not be

02:14 simple case of a single thing, they all have different sizes, which

02:18 the different domain sizes will lead to closure temperatures within the same crystal.

02:24 so if you cool slowly enough, one crystal Will record many different temperatures

02:32 a long period of time. It be that there are parts of the

02:36 that are retaining, there are gone 180 million years ago. That might

02:40 the temperature of say 300°. But other of the same crystal do not begin

02:46 retain their are gone until 100 million ago. That might be a temperature

02:51 like 180°. So it doesn't have to a single event that jams it out

02:58 gives us a situation like this. still look like this. There'll still

03:04 more gas in the middle than on outside. Well, that's the single

03:13 model. What we really have are domains. Some of these little domains

03:17 not be holding onto any gas until get cold enough and they're all mixing

03:22 . Um And let me come back that. We're gonna, we're gonna

03:27 and talk about this feldspar stuff straight and we'll come back. So I'm

03:31 ahead a few slides, I showed this this, this diagram yesterday where

03:36 have an Iranian diagram that originally kind goes nice and straight looks fine,

03:41 then falls away and and and we said that it might be because there's

03:47 variation in domain size, it's not single crystal that has just one size

03:54 we can we can vary, we we can model this strictly as a

04:00 of domains that have different sizes. don't have to change any of the

04:05 variables. We can just say there's big ones and some little ones and

04:08 we gotta do is vary the relative of the big ones and little ones

04:13 the relative volume fraction of all the sizes. Obviously if we have one

04:19 one, but it only makes up of the volume, that's different than

04:22 it made up something else. those are two sort of parameters we've

04:26 in our modeling opportunities, We've got domain sizes and basically, if we

04:33 that the small ones are one, know, the big ones are the

04:36 ones, 100 or 1000 or and then we have to vary the

04:42 fraction and I'm not, and that's a complicated endeavor. But this,

04:49 shows the results of and what what is is that, remember when we

04:54 a step heating experiment, we're heating different temperatures and going through there and

05:02 we're doing it as a diffusion well, we can. The nice

05:05 about feld's bars is that we get age experiment and a diffusion experiment at

05:10 same time. And this is because Argon 39 is uniformly distributed throughout the

05:17 because we just made it yesterday, artificially put in there. So it

05:22 a great condition to do a diffusion . We know the initial value and

05:27 can then as we heat up the , we can watch how much argon

05:31 out each one. Eventually we can know what F was. And that

05:35 us to calculate D not on a we get a diagram like this and

05:41 that We can make a mathematical And this mathematical model is based entirely

05:47 the diffusion of Argon 39. Argon is what we made in the laboratory

05:56 . From this, we build a model and we say, what

06:01 what would the various domains? Big , little ones. All luminous

06:06 unimportant ones. What would those various have to be in order for this

06:11 to look the way it is. the time and temperature heating schedule that

06:15 use, this is a this is result we not. We need to

06:21 up with a model that produced And remember this is produced in the

06:26 over the over the course of a days, perhaps this is a laboratory

06:33 . And so we say, we know how much argon came out

06:37 temperature at this time and so In the context of this multi diffusion

06:44 modeling. Can we match it? yeah, we can computer does some

06:50 and we end up showing that. , we think there are four different

06:53 there, of these different sizes of different volume fractions. And if we

06:57 that model and we apply the the heating schedule that was done in

07:01 laboratory, we can reproduce those And so now we have a model

07:05 the mineral for the mineralogy of the . Now we can take that and

07:11 and and we can then take a history, take some cooling history and

07:19 what that would produce over millions of . Because if we've got a mineralogical

07:26 that implies things about various closure The little domains are this big.

07:31 they retain gas at this temperature. big domains are this big and so

07:36 . You following me, that makes . So, if we have a

07:40 model that's based on our lab data , we can then try out some

07:46 histories such as this one here and and see what our mineralogical model will

07:53 for our age model, This age , remember not was produced as essentially

08:01 consequence of the cooling over millions of . So we've taken our mineralogical model

08:07 was produced because of heating over days using that to help us predict what

08:14 long term model, long term data look like if we applied various thermal

08:20 . And this shows that this This model works very well. This

08:24 the model in through here for this bar, and the red is the

08:29 , and the black is the is actual data. So we can say

08:33 from this single crystal or group of , this single sample, this single

08:41 , we can produce a model which that we can understand its continuous thermal

08:47 history from temperatures, it looks like 2 90 down to about 1

08:54 That's pretty great. That comes from one sample. A lot of modeling

08:58 into that, but just one Um And so you see that that's

09:04 that's not an episodic event, That's a boom, hit it with a

09:07 of heat and make it go out . That's just cooling slowly. That's

09:11 you see that we start and you can see variations in the

09:15 We can see that prior to about million years ago. This thing wasn't

09:21 and hardly off half a degree C a billion years, then then for

09:25 while it cools. And then from on it pulled out about four degrees

09:30 . And so um this this is from some basement rock. This is

09:35 granite or a nice or something like because otherwise it wouldn't cool this

09:39 But if you're trying to understand the range from when this from which this

09:44 um you're gonna be looking for events 68 million. You may also wish

09:51 if you're if you're doing a whole study, it may look they used

09:54 so why did it start to pull rapid? I do rock school,

10:06 went through this yesterday. Why do rocks cool? Small problem. Big

10:18 . Well whether you know, maybe to the surface, right erosion.

10:24 is trivial erosion is a big And understand the difference between weathering and

10:30 . No weathering is taking a feldspar make it into a play, erosion

10:37 taking the feldspar from colorado. And it in the gulf of Mexico,

10:44 corrosion is moving. Weathering is We're talking about erosion. And so

10:52 did, why did this sample start more rapidly, yep, it just

11:02 to the surface. How did it that? No, no,

11:07 This is a granite. You just erosion a minute ago. They were

11:14 were right on the answer and then ran away from it erosion.

11:19 why did it start to cool even rapidly because the erosion increased. Maybe

11:30 had a Techtronic event, Maybe the got higher or maybe that's a climate

11:35 thing. Something caused the erosion to . That's the reason why these things

11:41 their cooling rate 68 million years this rock was just sitting there.

11:46 wasn't moving. That's very very slow . Right? But then something happened

11:51 it starts to go trust the cool eight times faster than it was a

11:54 back, something happened. It's the above. It is now being pulled

12:01 so fast that this rock is fooling if you wanted to, if you

12:08 sure about this geochemical nonsense here, might you look for in the regional

12:13 to to cement that interpretation? If true, If there's increased erosion above

12:19 mountain in this mountain range, what should we expect to find? Where

12:29 this eroded? What happens to this means we're coming close to the

12:34 Right. So and here we're going 280° to let's just say, we

12:42 down 100 degrees in 15, let's say 100, Yeah. In the

12:54 millionaire, How much material is a When you, when a rock cools

13:04 How much closer is it to the ? About four kilometers. That's a

13:12 of stuff. Where does it go speaking, it goes into some basin

13:28 right now, we don't know if knew if this was the Himalayas,

13:33 might look for a basin in Northern or may have been the basin all

13:36 way out in the in the bay Bengal. If this was the Rocky

13:40 , we'd look for maybe some, know, we look for basis

13:43 The basins ultimately could be in the Mountains. It could be the gulf

13:46 Mexico. That's kind of hard to together. But sometimes you would want

13:50 least first look if there's a base nearby, what time period would you

13:55 rapid sedimentation to take place in that ? No, no, not

14:07 what time, when, when would ? This basin is right next to

14:13 mountain ring. If this mountain range being uplifted this basin, what's happening

14:22 ? What where does, where where does this sediment in the basin

14:28 from? Mountains maybe. So this us that this mountain is being

14:37 erupted, eroded kind of fast during interview here. This this pale Eocene

14:46 . We would expect to see some in strata with an equally impressive rate

14:52 change here. And if we could we could link the two together and

14:55 , aha, this is the We've got the source area, we've

14:58 the sink. This tells us what period to look for in the

15:04 This area was being eroded Before Not at all. From 68 to

15:10 quite rapidly. And that can come this modeling of these felt sparks kind

15:16 cool. Um what else do we to say? Well, let me

15:25 , okay, we're gonna do Do I want to do? I'm

15:29 , I'm gonna skip that. Let's go to a couple more examples of

15:33 we can use this in. Uh , we'll just use these two examples

15:38 then we'll be done how we can this in understanding the tectonics and the

15:44 area. No mountain building and you know, when and how much

15:50 a result from southern Tibet And what have here is a mountain and the

15:58 here doesn't really show it very but that's a big mountain. That's

16:02 that's an elevation gain of 1000, , 1000 m. Okay, Bottom

16:09 that hill is at 3600 m. top is at 40 600 m above

16:14 level. So What you see there five sample locations Each about 250 m

16:26 . And those sample locations are where , this is, this is all

16:30 big granite, the big granite here a grand jury. And we have

16:36 that go down here and these numbers represent the Argon 40, 39 bio

16:42 ages for this Pluto. They all from the same flu time. So

16:49 probably all similar. We can assume these bio types all have the same

16:54 temperature and that's all we really need interpret them. I don't really need

17:00 know what closure temperature it is. the same now in a minute.

17:05 pay attention to exactly what the closure is. But for the moment,

17:08 just say they're the same. What you notice about these numbers? They're

17:19 as you go? Well increasing as go up or decreasing as you go

17:22 . Yeah. Anything else? The the the distance that the time

17:32 between the two gets quite small as get to the bottom first, we

17:35 a distance of 3.5 million. And the last two have only 600,000.

17:42 these this is, let me tell another piece of information about this

17:47 This granite has been dated by uranium Zircon ages And that result was 42

17:59 . This this granite has been dated uranium lead on Zircon And the result

18:04 that that that method um brought back 42 million. These are all younger

18:11 that. Right? Remember what is what how do we interpret uranium lead

18:17 data on on a grant for granted happened 42 million years ago? I

18:30 hear you younger then. We got number 42. What does it

18:43 We spent a whole lot of time lunch talking about that that that that

18:47 , what the Zircon data needs? the closure temperature of lead in the

18:58 ? Something like that. Okay, when we date as your car with

19:03 closure temperature of 800°. How can we its geologic significance? What happened

19:10 That's the crystallization age? I spent minutes for lunch talking about the difference

19:16 the uranium lead Zircon age and the and the Argon age in the same

19:21 . They differ by 3000 years. ? Because one has a lower closure

19:27 . This is the same problem only lots slower. When we get a

19:35 million year old zircon out of this , what do we say happened 42

19:40 years ago? That's the age of granite crystals form. What does it

19:46 when we data bio tight out of same granite? Thanks same as

20:03 No, no, you don't. in general. What does it mean

20:06 we data bio type by the argon , does it mean the age of

20:11 ? Like the other thing we just , what's the opposite of crystallization?

20:33 , all of this always goes back knowing the closure temperature. That's why

20:40 put that slide up first thing in class. That's everything resolves around

20:44 What's the closure temperature of argon and type? Yeah, about three.

20:54 look at this, we have a pretty big mountain 1000 m takes all

21:01 to climb up there. Top of mountain has a via tight age of

21:09 . The bottom of the mountain has by tight age of 18. How

21:14 we explain that? If the if uranium lead Zircon ages 42, what's

21:23 closure temperature of argon and bio Okay, that's very important. How

21:29 we use that to interpret the geologic of this granite? What happened to

21:37 sample? 26.8 million years ago? . What happened to this sample 23.3

21:46 years ago. Same temperature. These different elevation by 250 m. Does

21:58 mean this is older, younger, , younger, younger. How did

22:06 , what's going on? You just what these things mean individually? What

22:16 that mean in concert? Where where is 300° down there? How

22:29 down is 300°. You got how what the geothermal gradient? You got to

22:35 that too? 25 into 300 is , right, that's all you gotta

22:43 . There's about 12 km down. we can say that that's that when

22:51 You rightly pointed out that 26.8 is time when that rock got to 300°.

22:57 would also say this time when that got to 12 km. Because I

23:02 , if we if we assume that a simple relationship of 25° Super

23:08 So Then what happened to the other of 23 million years? That would

23:16 the time when it got to 12 . This sample was 12 km below

23:21 surface. How far is this sample the surface? 12 km -250

23:32 So what do we have here? 12 km depth Is always 12 km

23:39 us. Now, let's assume that Earth's surface is this sort of

23:44 We are watching the passage of this of rock passed. This place is

23:50 , this this place where it's always and I saw a therm If you

23:56 here, if we got we got kilometer a rock down here. This

23:59 is now, if this thing is 12 km, this thing's at 13

24:03 , it's too hot to retain its down here. This one just get

24:09 that right temperature, starting to hold it a little time goes by now

24:14 guy is holding on to his heart then in seven in 8,000,007 million

24:21 whatever that is, nine million This sample gets to that place.

24:27 . You see that got it So we can see not only that

24:33 thing moved up, which of course did because Grant has always start down

24:37 at the surface today, we know happened, but with this we can

24:43 when and how fast And in we can say that this was an

24:48 uplift from 23 to 17. It getting faster. This took three million

24:52 . And you know, these samples strategically located to have 250 m between

24:57 all safe That 250 m took three years. That took three took to

25:06 one. We are seeing the acceleration tectonics in this region. Such that

25:12 really going crazy at about 18. a tectonic story. Just by seeing

25:21 thermal history off one side of one . And we could, we could

25:28 combine that, that these are the types, bio types with elevation.

25:33 also did the case bars and the fishing track data. And they have

25:38 closure temperatures. And you see that case bars are pretty much all the

25:43 . The case cars haven't closed your less than the five types. And

25:48 By the time we accelerated to go a temperature of 200°, we were going

25:53 fast at the top of the bottom about the same age. And then

25:59 , so we can put this all in terms of a exhumation ring,

26:04 rate of erosion Back here before 20 years ago. And we can

26:09 we can take this back all the to 42 million. And this thing

26:13 formed. Not much happened, but about 20 million years ago, this

26:19 starts rocketing up. It's going at a year for a very brief period

26:23 time, Then cooled off again. caught it just in the act of

26:28 it made most of the most of transit. This rock made from,

26:32 from it's uh formation, 13 km the surface to the surface. It

26:38 most of that Right here in this or 2 million year interval? That's

26:45 news. I mean, you so that's a story about the uplift

26:50 history of this for this area. can put all of that tectonic story

26:54 . And then, as I if you're in a region where you

26:57 link the provenance to the sink, you'll be looking for The sedimentary deposits

27:04 that age because that material had to . So we're talking about uplifting 10

27:09 of stock in a really short period time. You wouldn't think that's,

27:15 know, just looking at that all you can say is,

27:17 that's a that's a granite started down now. It's up here with this

27:21 chronology stuff. We can put timings . That's a tectonic story that will

27:26 be related to everything around us, the base information. Now, let's

27:34 this one to another granite just around corner. This is another granite nearby

27:39 50 kilometers or less away. And is the loss of granite and the

27:45 dark. But anyway, it's it's another big mountain and this time

27:50 have one and a quarter kilometers of here, But in this case the

27:55 tight ages are exactly the same 62.8 62.5 with uncertainties. We can't tell

28:03 difference between these two. How must interpret that? Is this another example

28:14 really rapid erosion or is it could if it is, I mean that

28:23 that is a one and a quarter kilometers in, Well, a very

28:28 amount in a 10th of a million . Let's say That is 12 mm

28:35 year. One millimeter per year is extremely fast. I mean, and

28:44 and that's the case for sedimentation or because I mean the erosion erosion is

28:49 you know, has to go And if, you know, if

28:52 ever sit down and look at basin , a basin that's filling up at

28:57 a millimeter per year is a really up basin. And if something is

29:01 up at half a millimeter somewhere else being eroded and half a million.

29:06 insisted on this being only because of . We would have to be talking

29:14 15 mm a year, 20 per . If you go to a

29:20 S. A. Or an A . U. Meeting and you give

29:22 talk about erosion in a mountain belt you say, I think this means

29:26 there is a time of two millimeters year erosion. A lot of

29:31 you know, they will stop, know looking at their program or imagining

29:34 they're going to go out to lunch they'll stop and say what two

29:38 I gotta pay attention to this. is either nonsense or extremely interesting,

29:45 ? But if they were to say mm per year, you're an idiot

29:51 that isn't happening. That's really very high. But is there an

29:58 ? How can we how can we these ages to be the same

30:03 1200 m cliff space? Not really cliff, but a mountain Over 1200

30:11 . How can we get those two tight ages to be essentially the

30:16 What what geologic story could we make would allow that? Mhm. It

30:24 with the closure temperature. What is closure temperature? Okay, This says

30:30 the top and the bottom of that , we're at 300 at the same

30:37 , we discussed the erosion possibility that thing, here's your 300 degree ISIS

30:43 and this this pile of of granite through here so fast that we can't

30:48 him. That's one. Is there else? Where do gran its form

31:02 there? Exactly how far down Are they all the same? What

31:11 it was shallow? Let me go to this one. At what depth

31:22 this granite form? What constraints can put on it? We know that's

31:33 million years old. We know that tight at the top. There is

31:37 million years old. What was the of the rocks surrounding this granite when

31:45 was formed? It was above 300°, . It wasn't above 300°. We would

31:52 cooled down to the surrounding temperature pretty . And in that case there by

31:58 age would be pretty much the same the zircon age. Right. This

32:02 us that this rock was intruded at depth, fairly exceeding 300 degrees.

32:09 we wouldn't have this much variation. waited around this was 42 million years

32:13 eventually this thing got up to 300 and then we saw the accelerating movement

32:18 300 degrees but 300 degrees here. was put down somewhere deeper. Is

32:26 the only option Down here? 300 here? It's another option. Is

32:35 possible? No, can't intruder granted less than 300°. How do we

32:42 How do we get volcanoes? Mount Helens. Where is that magma coming

32:49 ? What? Well under there How far down 12 kilometers? The

32:57 chamber for Mount ST Helens coming up kilometers in one burke. No Magma

33:07 Beneath Volcanoes might be only one or km down there. I mean the

33:12 chamber beneath Iceland is right right Right. And what if that?

33:21 what about this then? What if had a bunch of magma And here's

33:27 here's 300 again. What if we all this magma this? 700 800°

33:33 and we push it up to here at once. 300. It'll cool

33:41 below 300 pretty fast, no matter we are. And so that's what

33:46 think this one's telling us. It's , this is not an outrageously fast

33:52 of erosion. This is a shallower of intrusion. This is not a

33:59 tectonic story. This is a boring story. Pluto intruded relatively shallow,

34:07 all the time. And when it , the bio types are not interesting

34:13 a subsequent tectonic perspective because they're already in and telling us that that

34:19 it was cold when we started. may learn more information by going to

34:22 things that have a lower closure the helium, the fishing track,

34:26 we'll get to just now. But but this rock pool below 300°. Pretty

34:31 everywhere. Pretty much from the very , we don't have an app a

34:36 lead zircon image from this blue, we didn't think it was necessary.

34:43 if we did get it, what you predict it to be?

35:07 What number? Give me a What was the age you would predict

35:11 a uranium lead zircon anywhere in this . Okay. How much older?

35:22 80 million or 63 million. We've that this thing is Being intruded up

35:39 above 300°. Right? And then So means that this is not only that

35:45 is the cooling age. This is this rock got to 300°. But if

35:50 had to jam it up here above What condition was it is when you

35:54 it up here, it was at partially melted. Right? Solid things

35:58 pass up through the crust that So we had to get up here

36:03 . But then we had to cool quickly. This is the crystallization.

36:07 . So what would the zircon hb here? Hmm. That's the closure

36:20 . What age if you yes, that closure temperature. And given that

36:25 two things are 300, what would expect for any zircon here? He's

36:31 to be 63 because this is telling that the whole thing crystal cooled,

36:36 and then cooled. Mhm. The is below 300. Such that we

36:43 tell the difference. Remember, we're saying that this is a fantastic erosion

36:48 . We're saying this is just cooling the big bunch of stuff because we

36:52 it in a freezer Freezer being rocks are only nine km below the

37:03 So 63 for the Zircons, that sense. Neither one of your getting

37:11 . Okay, so, we've got closure temperature of these things are

37:16 Right, and go back to this , Same closure temperature. Why are

37:23 are these ages so different? Because took us a long time.

37:26 here's our special zone. Right? here we got 26 then 18.

37:32 took passed through here and we know passed here because we know it started

37:36 here, 42 million. That's the age of the rock. This rock

37:41 for 18 million, 18 million years this bio type cooled off sufficient to

37:48 onto the Argo. But we don't that's the case here here. What

37:54 know is that these the by tight the top and the bottom of this

37:58 big hill, give the same And the simplest way to do that

38:02 to take this granite and intruded at very shallow depth. Take some

38:08 make it down here. It's magma up here, becomes cold so cold

38:14 the bite types cooled so rapidly. these guys are cooling that rapidly,

38:20 everything else that has a higher closure must have also cooled rapid. That

38:27 leave any time for anything else. the other, all the other ages

38:30 we might get that have a higher temperature. They should also be

38:35 I would think 66 would be a big number for a situation like

38:41 It was the other way. Um was like, I didn't have files

38:51 wouldn't tell you anything. That would tell you that the crystallization age was

38:57 . What happened afterwards is still an question because we don't have those

39:02 You'd have, you know. But if you got a bio type,

39:05 was 63, then you say, , we cooled from 900 to 300

39:10 quarter million years. And that's either of cooling directly after emplacement or because

39:16 erosion. You have to decide which which in this case it would be

39:20 unbelievably fast. But let's go with intrusion. And particularly if we,

39:27 know, if we date something else here, like let's do appetite fishing

39:32 that has a cool at a temperature 100 degrees. If we, you

39:36 , we could get like 20 million there and they said, well,

39:39 cooled really fast to 300 degrees, then it took another 40 million years

39:43 cool to 100 degrees. That's the part, that's the erosion part.

39:47 is just the emplacement part. There's cooling because granite school down when they

39:55 , Where they stop, depends on fast they cool initially. If

39:59 if you intruder granite at 20 you know, it's still hot for

40:03 long time. But you can some are intruded at five km and that's

40:08 , that takes all those high temperature kilometers and starts taking them off.

40:11 get the economy and the corn blend the bio type. Maybe not the

40:16 spot, not the fishing tracks. get to those later. That's the

40:21 the erosion part, that's the tectonic . So we've got cooling because of

40:27 . We've got cooling because of erosion . And it's only the erosion tectonics

40:31 that's gonna show up in the I would think everywhere it's gonna be

40:41 , at the bottom and 63 at top. This is saying that it

40:46 mean I collected some samples here, we didn't bother to analyze them because

40:50 all we have no expectation of anything than 62. Okay, what else

40:59 got? Um Well let's just look this one. Then. Here's another

41:10 . And this gets back to our in central texas. We've seen rubidium

41:13 for these rocks. We've seen uranium for these rocks. Now let's look

41:16 our country, the same rocks. got an an football of 1076 plus

41:21 -8. You gotta buy tide of plus or -8. What does this

41:26 us about this? Granite? If recall the Zircon age was 1084 and

41:33 Rubidium strontium age was 1081. But have relatively high uncertainties plus or

41:42 What's the closure temperature of of argon Amfa Bill 500. What's the closure

41:51 in biotech. That's sweet. So are the same. These about the

41:57 . Right? What? Well, they're they're not just close. They're

42:03 . It's impossible to tell the Look at the look at the

42:07 They overlap so close might not be enough here. They are indistinguishable

42:15 that doesn't mean they're not truly It's just that our uncertainties are too

42:18 better. And you know this this analysis was done in 1999 if we

42:23 one of those fancy mass spectrometers, showed you before lunch where they

42:27 Nowadays, years later, these You know, you could get better

42:32 . But these these data are are for this for the educational value of

42:37 plot. What does this tell us the intrusion depth of this grant?

42:54 got 500 and 300. Right. yet the two systems can't tell the

43:03 . How is that possible? Same sarah. We have to have rapid

43:09 . It's unlikely that that rapid cooling because of tectonics or climate. An

43:14 way is to just have this granite at a depth less than what?

43:33 than 12 or 13 km. This 300 is the lowest one here,

43:39 ? 300 divided by 25 is add a few degrees for the surface

43:45 are we're at so we're at 12 12 inch kilometers. Let's say this

43:49 was intruded less than 12. That's hard to imagine lots of 12 kilometers

43:54 way down. So that's what this us. And some from that.

43:59 know, if we see a lot proteins that are intruded low. You

44:02 , if you see evidence for lots photons being intruded high up in the

44:07 that tells you something about the general structure. Maybe that'll tell you about

44:11 pattern. You know, it's just story that you put together big.

44:14 this tells us that this was not Luton that started way, way down

44:21 . Um All right now, let's it, but in uh you people

44:36 drill down and measure the temperature down . Well, 25°/km goes into,

44:48 into 300°12 times 25° means every 100 is four km. Follow me 25

45:01 four is 100. So that means go down 100° every four km.

45:07 300° is 12:00. That's all there to it. Every upset again.

45:22 , every volcano is Okay, I think we're gonna move on.

45:32 are going to stop sharing so that can get out of that. And

45:42 gonna open up another, what are going to? We're going to fish

45:47 tracks. Okay, new system, our chart of the Duke Clyde's all

46:09 potential atoms that we know about. proton number here, neutron number

46:15 They go up through here. You , here's here's height, then helium

46:20 here. Here's, here's uranium up . The colors here, black are

46:28 stable ones, those ones kind of the middle here. Um red and

46:33 are different kinds of decay, yellow and yellows and other kinds of

46:38 And then green ones. These green up here. These high ones here

46:41 new slides that have significant contributions of fission. Remember that's when something big

46:50 uranium 238 spontaneously breaks into two pieces are relatively large. Um you

46:58 here's the decay, we talked about before, a bunch of alpha decays

47:02 some beta decays. This is small compared to this. This is the

47:07 way in which something happens, uranium into two things. And this curve

47:12 here called the camel curve for obvious , shows and this is a note

47:18 this is a logarithmic scale over but it shows the yield in percent

47:23 the mass numbers that are produced by . So it's about 9% of the

47:30 have a mass of about 90 and 9% have a mass of about

47:36 And then the then the percentages drop as you go away from us.

47:40 you know, still you have, you have a 60 that means you

47:44 have a 1 60 or 100 that you have 100 and 30. They

47:49 to add up to 38 two And most often you get a piece

47:55 about 90 and a piece at about 40 for that would be 1 51

48:02 . Okay, it's two pieces. break into two pieces and they go

48:07 fishing and there's energy associated with that that they do go in some physical

48:15 has shown there. And these things they are not just alpha particles,

48:20 are, you know 40, 100 times bigger. Oh 2030 times

48:25 than an alpha particle. Their passage the crystal causes changes in the

48:31 significant changes in crystal um Before but we get to that, let's talk

48:37 the rate at which this happens the of alpha decay in uranium 2 38

48:43 has a half life of 4.5 billion . So we're making we're making lead

48:49 alpha decay at that rate. For probability, the rate of spontaneous fishing

48:56 a lot slower. Not 10 to ninth, it's 10 to 15

49:01 It's a two million times slower. that that happened very often. And

49:08 you know, that's an issue. that in this example here, you

49:12 , I showed you another example yesterday I was introducing the concept of spontaneous

49:17 . I think it was strontium and . I showed you here in this

49:21 , it's barium and krypton can be of different things, you know that

49:25 not this is this curve just shows possibility. Sometimes it's this and

49:33 You know, sometimes it's not important it is because we're not measuring those

49:39 ever again. We're not, we're we're not tallying up the barium or

49:43 and that's particularly because that's a big . They're not always the same.

49:47 know, we have to we have find all of these different things and

49:50 them up. It's impossible, but is always the same is these two

49:54 go like this and they change the . Um The minerals that we use

50:00 fishing track are the same minerals we for uranium lead, they are

50:05 they are zircon. They're sweet. Those are the only minerals that are

50:10 enough in many rocks and have enough to be worth doing. And they

50:15 have closure temperatures that are convenient for of geologic applications. So also seen

50:24 muscovite, maybe some glass. But all these cases, here's what's going

50:29 . This is a this is just model. But we have our original

50:35 would be sitting here sitting in some crystal lattice. Then this thing shoots

50:42 in two directions and the crystal lattice perturbed because of that, the distance

50:48 that would go is about 15 So it's a it's an observable distance

50:54 the medium, you know, the microscope. So here's another model

50:58 We've got this this uranium 238 and shoots that crystal for that new ion

51:05 the thing. And as it does it displaces displaces some of the um

51:11 of the lattice. There may be relaxation after this is over. But

51:15 still clearly a zone of of damage change. And that's a damage zone

51:22 we can observe. So every time occurs we make one of those that's

51:29 daughter product is the is the physical . The scratch if you will,

51:36 not gonna be measuring like before we some chemical thing, measure some

51:41 We measure some argon, measure some , we measure some astonishing both.

51:45 sort of geochemical thing. Right we don't do that. We observed

51:51 physical changes in the mineral with a . That's how we figure out

51:55 The daughter product is not a chemical , it is a lattice dislocation.

52:06 things to know about these tracks is they they are cylindrical as along the

52:13 section or their circular along that cross . And they um maybe that's all

52:21 need to say right now. Um they are made they might be um

52:30 says mm. That's wrong. That 10-20 mm. No, 10-20 microns

52:37 should be in. Let me just that. Where did it go

52:50 That's totally wrong. There we There are 6 to 10 nanometers in

53:01 . They're 10 to 20 micrometers in . Um Okay, Now being only

53:14 nm wide, they're pretty hard to with a microscope. But we can

53:21 that. We take these samples if take a sample like an appetite or

53:27 and we can enhance the visibility of things by polishing the sample. Getting

53:35 . Getting it nice and flat and putting some acid on that surface because

53:40 damage zones are are damaged zones. acid will attack them preferentially opening up

53:46 very thin lines to be lines that can now see with a microscope.

53:50 you can't do this without preparing the . But once you do, then

53:55 fishing tracks are observable. And and here's an example of one. Look

53:59 the scale bar down there, 2020 . So this is only like a

54:04 across here. No, this is half a millimeter across 10th of a

54:08 is a small little chip. But can see all these tracks all over

54:13 place. Each one of those tracks one of these fishes that took place

54:18 caused this to happen. Those are daughter products. Okay, um how

54:27 we use that then to date Well, the number of fishing tracks

54:31 going to be a function of And the uranium concentration. And we

54:37 then um this this fission tracks would density efficient tracks would be a

54:44 The time. Here we have we to keep track of the fact that

54:48 losing most of our uranium 2 38 alpha decay. But sometimes we do

54:52 like this. Okay, so that's like in that other instance where we

54:55 to decay constants. We have to at that ratio here is the amount

54:59 uranium and then here's the decay Um They are the number of fishing

55:07 is going to be a function of of uranium concentration. And one more

55:13 . Uh Well, we get to in a minute. And one more

55:16 , which is temperature because one thing haven't mentioned yet is what's very interesting

55:22 these fish and tracks is in all these minerals, you heat the mineral

55:28 the crystal, relax and goes back the way it used to be,

55:32 fishing tracks will disappear. And that's a kneeling and kneeling and they're

55:42 And the nice thing about this is the temperature at which that healing takes

55:47 is quite a low temperature In an . It's around 100°. What temperature do

55:55 make oil at? It's about 100°. why this this technique is known as

56:07 pretty well in oil companies because this you about whether a sand, you

56:12 , if you take a sandstone that's a bunch of appetite in it.

56:16 it's got lots and lots of fishing in it, you know, maybe

56:19 gives you a parent fishing tracks Let's say it's a say we got

56:25 Eocene sandstone. Well, let's say sandstone. What if all of the

56:31 track ages from the minerals in that stone give ages that are Kirby,

56:39 means the sandstone has never been above because none of the none of the

56:45 are younger than the deposition allayed. if a sample is a cretaceous sandstone

56:53 permian zircons in it. And then zircons get heated up enough. They

56:59 Maya scenes are calm. And the that you need to heat them too

57:05 about the same temperature that you're looking in a petroleum base. So that's

57:11 that they correspond because there's very little to decide other than, you

57:14 look at a look at a you get hot. You know,

57:17 can't tell by looking at the Sandstone 100°. It's not enough to start doing

57:21 lot of die genetic stuff. But know, if you look at the

57:25 tracks, they'll say, well, lot of these fission tracks have been

57:29 since deposition. That means the sample to get hot enough to enable those

57:34 . What temperature was it at about degrees. I'm getting ahead of myself

57:38 little bit. Um, well, come back again because the more efficient

57:47 more efficient will occur. But but those are brand new tracks. We

57:51 want it. This is not, not a, it's not a resurrection

57:54 the previous one. Um, You what, we're a little bit behind

58:00 and it really not too important how do this. I'm not gonna talk

58:04 the method because it's kind of We're just going to talk about the

58:11 . Let's go to interpretation. Um , let's just go here to the

58:22 basin. Well now let me let me just go back to

58:30 Let's just take this as a this is data, you need to

58:35 the closure temperature for various systems We've talked, we've got some

58:40 we've already talked about by a type argon case bar, potassium argon,

58:46 we haven't talked about yet, We're not talking about healing yet,

58:49 this shows the broad closure temperature for fishing track systems for fishing track,

58:57 on the cooling rate, you've got closure temperature. And let's just take

59:01 cooler. Right here, we have about 100 degrees For for Zircon.

59:07 more like 200°. That's good enough for . The temperature at which Fishing tracks

59:16 sensitive. Intercon about 200 In fishing . It's about 100. So,

59:28 us then use that to talk What's this. Uh Talk about the

59:33 basin basin is found in the south of Australia near uh near Melbourne.

59:44 is a series of samples that were in the drill hole. This is

59:49 that was collected. I think it's . I don't think it was cuttings

59:53 sure its core. And so we we're not talking about depth here

60:00 We're talking about temperature, which they in the well as well. And

60:05 can then plot um two things we talk about that. Okay, what's

60:11 ? Um we can plot two things versus temperature in this basin up here

60:18 the top of the well where the is Surface temperatures, the fishing track

60:24 are all about 120 million. But you get down to a temperature of

60:30 50 degrees, 60 degrees. You , you see a remarkably quick dimunition

60:36 the age of the fishing tracks. that when we get down to 100

60:40 20 degrees, the apparent fishing track is zero. There are no fishing

60:47 in this at the bottom of Well, The sample was collected,

60:51 core was collected at a depth of at the depth associated with 120° that's

60:58 , pretty deep. Well, That's gonna be what, 4,

61:02 km down there. But they, know, they make wells like that

61:08 the bottom of this. Well, got no fishing tracks and we pass

61:13 through here, they get older, their oldest most this zone right in

61:23 between about 80 and 100 degrees. known as the partial and healing zone

61:30 we've partially in yield or we've gotten of some of the fishing tracks,

61:34 not all of them. And this us to this diagram over here,

61:38 haven't talked much about. But in , another exciting piece of information we

61:42 get from fishing tracks is not only number of the fish tracks, but

61:46 length. Because a fishing track. it starts fishing takes place, a

61:52 track in an appetite is almost uniformly 15, 16 microns long. And

61:59 this heated up, a very nice happens is that they get smaller,

62:03 just in their length. They don't small. They're this big. They

62:07 this big throughout their whole thing. don't squeeze down like this. So

62:12 don't have to worry about something that's to go. You only see it

62:15 like this. And so over time track that's this big. If you

62:20 it up, will eventually that And it does that. And what if

62:27 , you know, and and if stops, if you take a thing

62:30 and you keep, it at say it will, you know, kind

62:35 go then if you take that thing you cool it off, you will

62:40 made a short shorts. That That fishing track is shorter forever.

62:44 only gets long once it can get again and again. So, if

62:50 look at the fishing track length of samples, the average track length can

62:56 safe samples stay up at the top the well here, it's 15

63:00 That's what we expect it to That's a natural that sufficient track.

63:05 . But as we go down in Well, by the time we get

63:08 not much data here, but by time we get down to 08,

63:11 have zero length right here, we've down about nine. This is just

63:16 average. We'll see that's even more than the average is the highest a

63:19 , showing the whole distribution of lengths are very important. Let's see do

63:25 have something? Okay we'll get two come on now. Yeah. All

63:29 . So let's that well is great . Let's just consider something a little

63:34 more generic. Let's talk now about about a well but about a

63:38 Same idea though. We are looking differences in depths or differences in

63:44 And so what we have for any track situation is we'll have up at

63:49 top where it's cold. Any fishing that is born up there will be

63:54 big long and stay long. Any track that's bored down here will be

64:02 um erased too hot. These guys the middle start out big and then

64:10 kind of do this because it's it's cold to immediately get rid of

64:14 But it's too hot for this thing stay forever. So the longer you

64:18 at I mean if you stood at you stayed at say 90° eventually it

64:24 disappear. But I have to do slow. If you were if you're

64:27 200° like that you're at 20° not . And so that's called the partial

64:34 healing zone or the P. Z. Let's imagine now a couple

64:41 scenarios about cooling through the partial healing and we're going to talk about cooling

64:47 hot to cold as an uplift. from the mountain belt later on.

64:52 can turn this around and imagine a rock going back down through the

64:57 But let's the simplest case would be down there. And imagine we have

65:02 simple cooling history in which we cool a linear rate through there were just

65:09 every million years we get a little . Well following this curve here,

65:16 fishing track that is formed here will look like this. It's gone now

65:22 because too hot. But once we into the P. A.

65:25 This crystal here is formed. It's big. But today it looks like

65:31 because it had to spend all of time passing through the partial and healing

65:35 and all that time through the partial healing zone. It's being shortened.

65:40 it gets up above the P. . Z. Then it stopped being

65:43 . But because this one was born here and spent all this time going

65:47 here, it's almost gone. And of these lesser. So this one

65:51 started here, it spent less So it's a little longer. This

65:54 a little longer still. And all these. Now, all of these

65:58 here are the maximum length. And if we were to plot up a

66:04 a gram of a bunch of fishing lengths, This kind of curve would

66:10 this sort of of distribution some short tracks forming those are the ones that

66:17 it down here and took a long to pass through um some full

66:22 Those are these ones and these ones the middle are all the ones that

66:26 over this. So this distribution indicates cooling through the partial and healing zone

66:36 the length of the distribution of tracks quite wide. We've got long

66:40 we've got short ones. You only that by going through the paling,

66:44 kneeling zone over a long period of . Um Here's a couple of other

66:53 . Let's let's start with number That's the simplest one. We basically

66:57 did number two. Let's go to one. Imagine we have some lickety

67:01 uplift goes through the pressure and healing . We would see track length distribution

67:06 looks like that number one. Nothing long tracks some 15, some some

67:14 not very many elevens because the passage here to there. I mean this

67:19 this one here, this one that up at 11, that was that

67:23 that was formed here. But compare with # two. A efficient track

67:29 was formed at this moment has all this time to be shortened and it

67:34 up looking like that. And so distribution is much wider out here because

67:40 district because the path is longer So not only can we tell the

67:46 of cooling, but by looking at distribution of the track lengths, we

67:50 talk about the rate of poof. thing. Number one attack distribution cooled

67:56 with the partial and healing zone. wide distribution pulled slowly. Imagine a

68:04 complicated scenario. No three, we up pretty fast, not as fast

68:09 one but slower but faster than But then we go back down and

68:16 we have a bi modal distribution because these things have to be shortened as

68:22 get pushed down the second time. that one that one's almost gone because

68:27 had all this time to be shortened then all this time to be

68:30 We don't have to very many of guys because this this back up

68:35 It's not enough to all these, of these ones that were produced on

68:38 period here didn't have enough time to short. So this this bi modal

68:44 considers is from an up and down of way. What do we got

68:50 ? Another example um this these are now of of starting from the surface

68:58 going down this would be the basin example. What if we had a

69:03 we and we're going to imagine that of these, all of these rocks

69:08 we're burying our simple, let's say are volcanic rocks. So they start

69:12 this remarkably tight distribution and they're continuing grow, You know, they're they're

69:20 fishing is always happening, But until get down to the partial and healing

69:25 , nothing happens to the fish and . And so a path down here

69:29 a that only gets down to about is gonna have a track length

69:34 That's really hard to tell the difference that and a real life because it

69:39 got to be hot enough, Good, not good. What does

69:48 look like? The track length distribution the highlight would be what why do

70:00 , why do tracks get smaller? get smaller because they're because they're they're

70:10 some condition that's hot enough to make go down. And that condition doesn't

70:14 until you get to about 75°. So you look at the highlight that never

70:19 buried, every single fishing track in real light would be as big as

70:23 ever got. You'd have a very distribution. They'd all be 14 or

70:27 microns law. Let's take that sample bury it. But let's only bury

70:33 to 70°. Nothing's going to happen to . It will be a highlight that

70:41 spent its entire life at the surface a highlight that has been buried to

70:48 C will be indistinguishable from their efficient length distribution Because nothing happens until you

70:57 down to a certain temperature, 68° not hotter. So that's the sensitivity

71:02 the system above 68°. Everything's the But between about 70 and about 1

71:11 we have these characteristics. Look look at B. Now if we

71:14 B. And we put it right in the middle of the P.

71:18 . Z. And let's say we it down there in a drill

71:23 we'll bring it up, it's going have this very broad distribution because all

71:27 the fish and tracks are being actively down at that temperature. If we

71:32 it down even further, where it's , it's up to 125 degrees.

71:36 really knocking these guys down. There that many fishing tracks, and some

71:40 them are long because they were created , but many of them were almost

71:44 . Two or three microns long. one's 15, this was four.

71:48 we got a very broad distribution. got to we've got three, we've

71:52 we've got 14, 15, because are pushing it down into high

71:57 If we were to take another path went down like this here, there

72:02 be no official trust whatsoever. let's look at these other ones,

72:10 little more complicated. D we go and back up. That's when we

72:14 the bimodal distribution. E we just , let's see e o if we

72:20 down further and are about the how they differ, depends on the

72:25 at which we do this, this going down faster. This is taking

72:29 , but they're pretty close, That would be, and in

72:32 the the track length distributions are not unique solution. Sometimes there's a lot

72:38 different, different thermal histories can give a very similar one. So it's

72:42 always a a real precise deal, F. And D. Have

72:48 you know, they're they're going down about the same temperature. E on

72:52 other hand, is going up and have this sort of tail here.

72:57 this is a cooling history and all these old these long ones here are

73:02 this part here. So the rate cooling through the partial and healing zone

73:11 the distribution of track lengths. And course it'll affect the number of tracks

73:16 have, the longer it's been since passed through the partial and healing

73:20 the more tracks you have. But you deal, if you if you

73:23 at it more closely, you'll see the distribution of their links tells you

73:27 lot about how you pass through this temperature zone. What else we

73:34 Um Okay then there's modeling, there's program that's called hefty, which is

73:41 to look at both helium and fishing , that's where the H.

73:44 F. T. Comes from. uh this is a program that will

73:48 some inputs, you know, your number, your track length distribution,

73:53 helium data, your fish and track . And it will then do monte

73:58 simulations are just randomly throwing thermal histories at it. It's not a very

74:03 program but it just does over and . What about this? What about

74:06 ? And then it it takes all all the thermal histories that matched the

74:11 close enough and plots it up like . So you can see these are

74:15 some examples of from one paper, can't remember where I got this,

74:19 it shows that the modeling bob, a paper model for example, that

74:25 all the way back to the but only part of the model that's really

74:32 is the stuff after 100 and 20 above 120 degrees. Well, it's

74:36 all hot. Right? So it have been could have been 200.

74:39 could have been at zero. But know that by about 100 and 20

74:44 years ago we got 200 degrees below degrees. We're fine above 100

74:50 All this all this stuff we don't . So it's and that's that should

74:57 not surprising if the closure temperature is degrees. We're not going to be

75:01 about what this system won't tell us what happened at 200 degrees. We

75:06 a different system for that. Just more examples of the modeling and then

75:16 why of course, you know, oil companies are keen to do this

75:20 because the The graph of oil hydrocarbon , you know, sort of maxes

75:27 at about three km depth or about And this is the partial and healing

75:35 here. This is the track length . Track fishing track age decreasing across

75:42 zone here. The zone in which tracks in appetite are most sensitive,

75:48 actively uh very closely corresponds to the in which china carbons are made most

75:55 efficient. So there's a good correspondence um We did that already. Um

76:05 again, here's the Otway basin. see the strata graphic age of the

76:10 . We go down in the basin once we get to a temperature of

76:13 60 degrees, everything starts to The ages go down to zero and

76:19 fish and track lengths go down to . And here's the distributions here.

76:24 the track length distribution, nice and and narrow, widening, widening,

76:30 widening. Next one, they're all . Um here's another example that's um

76:40 of a famous example that is used understand the structural history of a

76:44 This is near Red Lodge Montana, is where U. of H.

76:48 its field camp. Now, not that's important. And along this is

76:52 highway that takes this route us to which goes from pretty low elevation here

76:58 to about three km above sea up here in the near near

77:04 And so there are samples along this and then there are other samples in

77:09 subsurface. This Amoco bear tooth went one well. And so when we

77:14 at the data here, these are , the elevation goes from kilometers above

77:18 level zero up to almost three. samples here were sampled along the

77:26 And these samples here were sampled in , in the well. And what

77:31 have is an elevation, a really elevation because we've got, we've got

77:35 big cliff here on in the mountains then we're right next to quite a

77:39 well. And what we have here the appetite fishing track age plotted on

77:45 line. And the elevation here and associated with these graphs are the fish

77:50 track length distribution. And so what we got here is above three km

77:56 sea level. We have this very slight shallow slope in which we

78:01 from ages about 40 million pages. 280 million. But look what happens

78:08 we get below that elevation, we a whole bunch of agents that are

78:12 exactly the same from about 45 to million years old. Notice also that

78:19 we look at the ages that are , look at their track length

78:24 They're very wide. Sometimes they are model. Look at these track contributions

78:29 you go down the well, they're very narrow Averages of 30,

78:35 30, 30 12, 12, . All the ages are the

78:42 All the track links distributions are What does that tell us about the

78:48 of this place. And remember what's temperature at which these systems are most

78:58 . Well, the single temperature in middle of all, that would be

79:01 . The range from 70 to 1 something like that. So, let's

79:05 say 100 for now, How How did this happen? What?

79:14 too cold to talk about intrusions We don't have intrusions that that that

79:18 have intrusions that go down to 100°. furthermore, these rocks are are key

79:28 . I didn't tell you that of , but I mean, the thing

79:31 need to know, these are, are nice is these rocks are extremely

79:41 , yup. How'd that happen? is probably an example of remarkable

79:55 Now there's a big uh there's some thrust faults in here. Okay,

80:02 those. And that's how you can something cool rapidly by shoving up a

80:07 , right? And then have erosion top of that. The bare truth

80:13 system throughout here is real close And this is suggesting that there was

80:20 lot of activity on that fault system at 40 million years. So,

80:25 to push this up high mountain up enough and then that and deal with

80:30 and faulting is to you for a fault. You cool the rocks off

80:38 because you're bringing material off in a fault. You don't do it immediately

80:43 the rocks are still covered by whatever were covered by. But if you

80:46 a tall mountain that wasn't there before rains more on the mountain and you

80:50 more erosion and you cool them off afterwards, and that's probably what's happening

80:55 . You got a thrust fault takes flat surface and makes it into a

81:00 Rains more on the mountain, erosion over Whammo, we get erosion,

81:04 really we get, we get three of erosion here in a very short

81:12 of time. And so this is of those and this is a famous

81:16 . This example is famous because of the distance that we're talking about several

81:22 . That that that that thing I you from my PhD where we had

81:27 kilometer climb to the top and looked the by types. Well,

81:30 you know, that's a big This is three kilometers, but of

81:34 we have a drill hole to help out here, But because we have

81:37 of this information up and down three and they're all basically the same.

81:41 a pretty remarkable tectonic event and it's tectonic event. It's not an igneous

81:46 because we know these rocks are to , these closure temperatures are too slow

81:50 represent intrusion. We can't intrude a rock like this to one km,

81:55 moreover, these are mostly not igneous are mostly better for for rocks.

82:05 I look at this age elevation I've shown you two examples of it

82:09 is a classic way to understand the of uplift of places like this by

82:15 for the system. Now, note that this is what we see in

82:21 fishing track, David, what if were to look at the same

82:26 we get the same samples. And now do um the the argon bio

82:33 agents in these are key in We're not gonna see this this remarkable

82:42 , right? Because the only time rocks were cooling lickety split is when

82:47 were going through the temperature of 100 . Now, here's 100. So

82:53 100°. The only time you can record rapid nous is when they're passed.

83:06 you're looking at thermal chronometer that are in that sensitive zone bio tights closing

83:11 here. And so, you whatever is going on here, you

83:14 , it was the rapid nous was . So in that example from

83:19 it was the biotechs that found our uplift because that that occurred when the

83:23 moved from 3 52-50 really rapidly. the other changes that those rocks took

83:30 were more normal here. The odd is when these rocks are moving from

83:35 50 to 50 all the other changes back into normal. This is the

83:41 . You know, it's like if were trying to, I don't

83:46 trying to run home instead of walk , there's, you know, you

83:51 will be times when you could manage run real fast. But most of

83:53 time you're just gonna walk. And if the camera was on you when

83:57 were running, we find it. if it was something, it was

83:59 somewhere else, we'd only see you slowly. Most of the time,

84:02 have to have to be looking at right time, You have to be

84:05 at the right temperature, thermo thermo . This one's 100. That one

84:12 300. Other ones are different This is why you gotta know what

84:15 temperatures are. And then once you what they are, you can interpret

84:19 correctly or you can know which one apply to an unknown situation. Here's

84:29 mean track length of the same true thrust. And you see that

84:33 we go down the track length, out at 14 and they go down

84:36 11 in this very normal way, that the temperature is kneeling them.

84:45 , here's another example of the same of thing. This comes from Mount

84:50 in uh, in Alaska. And , because it's a big, big

84:54 , we have a lot of this all this is not a drill

84:57 this is, you know, this Mount Denali, one of the tallest

85:01 in North America. And we go 1500 m 6500 m. Some mountain

85:06 got these samples for the geologists And here again, we get this

85:12 zone, this shallow zone and then this and this cape in this here

85:18 us when something happens. And this here sometimes referred to as an Exume

85:25 and healing zone because I think back the to the basin where you have

85:30 part of the, part of the core, where the ages are kind

85:33 flat along their there, they're the at the top and then go down

85:36 zero over here and then make this . And that was in a modern

85:40 hole. Well, if we were take that that basin and lift it

85:44 , we would just add a certain of ages to everything. And that

85:47 range across here would be there. that's what we're seeing here in the

85:52 . This is these rocks were sitting 100 degrees until four million years

85:58 And the whole thing popped up, are all safe because they went through

86:02 partially kneeling zone rapidly. These quite them because they were sitting in that

86:07 healing zone for a long time and that shape. Yeah, okay.

86:18 . And so again, now, you were putting together a full regional

86:21 of this region, you'd say, , there's a basin somewhere that's filling

86:26 the sediment about four million years ago we we've pulled off 2.5 kilometers of

86:33 in about four million years. That's lot that went somewhere. Now,

86:39 course it doesn't have to have been right at the bottom of the mountain

86:42 it could be rivers could take it long distance. But, uh,

86:46 still some compliments somewhere where that material now. Um, yeah, this

86:54 the last one we'll do in fishing . Um, this guy, Glenn

86:59 , Andyg Leto was like the Pioneer fishing tracks in the 1980s and,

87:07 then afterwards. And this is, is the basis by the way,

87:11 here. And Andy Glencoe lived right in Melbourne. Um, and they

87:17 samples from all over Australia. You , Just because they lived in

87:23 And this is an interesting map, published a while back in 2002,

87:28 looking at the surface of the fishing age of rocks, at the surface

87:33 Australia. And you'll notice that a of them give ages between about 1

87:40 50 indicating that Australia has been a flat place for a long time.

87:48 not a lot of variation throughout all this area. You've got, you've

87:51 very little variation in the time since This is telling us that Australia at

87:59 aside from this part here, where get the young agents, aside from

88:03 most, the eastern alps of this tells us that Australia has been

88:09 tectonic lee quiet for hundreds of millions years because otherwise we see variations in

88:16 . We see faults and breaks in continuity, but pretty, you

88:21 There's a, there's 1000 km, km, 500 km diameter circle in

88:30 the vision track surface ages are all saved 500 km. That's uh,

88:37 that? In In American 500 km 100 km is 60 miles.

88:48 600 Last. Can you do Thank you. 500 km is 300

89:02 . That's about the distance from here Dallas or Oklahoma, the sides of

89:08 size of east texas. That has without variation for 250 million years.

89:18 a that's a bit of information that from this low temperature thermal chronology.

89:23 when we start looking at things of surface, we need really low closure

89:28 because the surface is up here Anyway, if we're looking at variations

89:32 the surface, we can't be, can't be enquiring about what the rocks

89:36 doing when they were down here. once you start talking about surface

89:43 you know, when the mountains were , what the rivers were doing,

89:46 they were cutting into the mountains. sort of tectonic geo morphology, you

89:51 really low closure temperatures because you're not to learn that by measuring the density

89:56 the helium a or the argon age the horn blend. That's gonna tell

90:00 when it was down here at 500 . And that's an important bit of

90:04 . But if you want to relate to what sort of landforms were going

90:09 that. Not that I won't tell anything but a variation of 100

90:14 That's beginning to say, look, rocks have all but been no deeper

90:19 three kilometers for 200 million years Three kilometers still a lot. But

90:24 mean the width of the variation is . Okay, that's the end of

90:32 Time, is it? It is 2.30. All right. Yes.

90:40 want to take extremely short break? . five minutes. Just in and

90:49 . Yeah. Okay. Our last that we're going to talk about takes

90:54 back to uranium. Actually started fishing . This is our third way of

90:58 about how uranium decays to make something using in a datable system. And

91:04 gonna call this the uranium thorium. marium helium dating system. Sometimes people

91:10 call it uranium helium. Um It indeed the oldest geo chronological system because

91:18 they first they first recognized that there things that were radioactive uranium decays to

91:24 plus helium, they said, let's take something, we'll use

91:29 And this was this was not long uh Kelvin had estimated the age of

91:39 earth based on the cooling, Did I tell you this story already

91:43 ? Yes. You know, So said the earth was 6 30 million

91:47 old and they went to africa. they used the principal position. They

91:51 some old rocks, they dated they got 30 million. They

91:56 oh no, you know, because the geologists thought the earth should be

91:59 than 30 million. The biologists thought earth would be older than 30

92:02 The first, the first actual date was ever calculated from isotopic geology,

92:08 30 million. And they were all . I said, oh no,

92:12 gotta revise everything, we know, kelp in was right. But then

92:15 used the same rocks and they dated by uranium lead And they got 2000

92:21 . And everybody's happy. Why did get such different numbers? Because of

92:25 closure temperature, closure temperature of helium is the lowest of anything we're going

92:30 talk about again, we can talk the decay of uranium 2 38 decays

92:38 a bunch of alpha particles, uranium 35 also alpha particles, thorium 2

92:43 bunch of alpha particles. And even 1 47 has one alpha particle.

92:52 Add up all these things. The of all these, you know,

92:55 got and here's 876, 1 depending the relative concentrations of these guys and

93:03 time you're gonna get a bunch of guys. And so that is then

93:09 to be a a another way to . We can then uh put all

93:16 things together and say the helium concentration going to be equal to all of

93:22 . Right, You've got the decay and the and the seven and the

93:26 and the six and the one depending how many you get in each one

93:29 these chains. Now obviously you can't that for tea because tea is in

93:34 single exponent there. Um But we that for times much less than the

93:41 constant of uranium 2 35 that is times of less than about 100 million

93:47 . The time can be estimated as helium concentration divided by the production

93:53 And the production rate can then be calculated uh here based on how much

93:59 makes the concentration of uranium thorium. Miriam that you have. So you

94:05 , you know, you measure that in some chemical way. Um And

94:14 for and you have to keep in of course that uh the half life

94:19 thorium and samaria were kind of long to the other. And sometimes it's

94:25 cumbersome to talk about um the combined of uranium thorium and sumerian. So

94:34 came up with this smart idea to about what they call the effective uranium

94:41 . Effective uranium concentration takes all of uranium and 23% of your thorium and

94:48 a percent of your. So marium that all up and says that's we're

94:52 going to say that's a number that be if we could have what we

94:57 in the sample or we could have this much if it was all your

95:01 , we're just taking this much thorium Samarian and pretending like it's uranium and

95:06 doing these by adding these factors here we've got to worry about the difference

95:11 care a and the difference in alpha . This even sit down. So

95:17 just talk about eu effective uranium rather having to deal with all this other

95:24 . So again, the minerals are same as before, appetites are common

95:29 or the Big East. There's you know, there's a bunch of

95:32 minerals that people have tried to work . I've worked on calcite, people

95:36 worked on go fight and Garnet and just going to pay attention to the

95:41 East right here and The alpha, amount of uranium and thorium in these

95:49 in appetite and blue dots. Generally got alpha per year from milligram as

95:57 as 10,000, as much as as million. So we got lots of

96:02 produced in these samples. Um and like all these other things, the

96:10 of the daughter products, which in case are these alpha particles is dependent

96:15 the uranium uranium and thorium concentration or effective uranium concentration and the thermal history

96:23 year Because helium is small. It leave the system even at low

96:29 the closure temperature for these various systems be as high and something like garnet

96:34 maybe 300° in appetite in calcite and other things, it's as low as

96:40 the lowest one we got. Um , unfortunately, we have a little

96:48 of a problem here in that because we have to measure uranium thorium

96:53 solarium and helium. We're back to problem of the parents and daughters being

96:59 different geo chemically, we cannot measure on the same machine. However,

97:04 do not have the same. We have as bad of a problem as

97:08 had with the potassium argon because in argon, in order to get all

97:12 the argon out of the sample, had to melt it and to destroy

97:16 sample for helium, we don't have melt it so we can get all

97:22 helium out of the sample without destroying . Then take that piece that we've

97:27 up and go measure the uranium thorium and the scenario. So it still

97:33 two machines, but it doesn't require split the sample into different pieces.

97:41 . Now, one thing that we to worry about in this system that

97:46 really don't have to worry about in , at least it's a much bigger

97:49 here is something called recoil or alpha , and that is because when just

98:00 with uh just as with uh fishing decay, when alpha decay happens,

98:10 alpha particle is thrown out some Perhaps as much as 20 Mike

98:17 It's now in a new place from it was These, these these two

98:20 and two neutrons. They were a of their uranium family. And now

98:25 over here And that's 20 microns. , why that's important is that what

98:31 the sample was only 17 microns from edge of the crystal? Sometimes not

98:38 time, but sometimes it here's the you're 17 microns from the crystal,

98:45 be this circle, you know, our sample will be a circle through

98:49 this is being ejected. If you the right direction, you will be

98:53 20 microns and out. And so that that helium that was due to

99:00 decay, it's not a part of crystal anymore. So we will we

99:04 sometimes have a distribution curve. Suppose a very small uh crystal, it's

99:11 100 microns across the alpha retention At edge of that crystal is gonna be

99:21 because a uranium Iranian Iranian pay to curse here, can go any

99:29 Half of those directions are within the , half of those directions take us

99:32 of the Christian and then that goes to about 90,% once we get a

99:38 20 microns away. But in in this 20 micron zone, we

99:43 are expecting through, not through thermal . You know, we talked about

99:48 before with the argon, we only that sort of thing because of some

99:53 perturb UNt's This is not thermal, is pure physics. This is just

99:58 alpha particles? Do not go there . No, we don't have to

100:03 with this in healing a potassium decays argon. This is our gun.

100:09 Well the whole thing changes right in . This is a little piece that

100:13 thrown away. So this is a that we can fix by noticing that

100:21 effective uh well sort of a fudge , the F. T. Value

100:28 is for a sphere, if we a spherical character of a sample,

100:35 bigger the sample, the less this an issue, right? Because the

100:39 the volume, the more material we away from the edge. And so

100:44 a sample of Radius 250 microns, can we can we can retain about

100:52 of our alien. Which means that we Analyzed sample that's 250 microns

101:00 We have to increase the age that get by about two or three because

101:08 want to understand the thermal significance of age and without, without the the

101:15 between This and 100 it's not It's just so we had that.

101:24 , if the crystal gets small only 50 microns across, we've got

101:28 much bigger uh correction factor. And course this is a model for a

101:34 with a pure sample isn't actually spherical have. How appropriate is this

101:40 So the lesson here is bigger crystals better. The bigger we get,

101:46 less we have to worry about are , did we measure the size

101:50 Are we applying the appropriate size So the teenager you get here,

101:56 mean, and of course the smaller you get the smaller closure temperature you

101:59 . So that's another issue. But if it had a big closure

102:04 you're gonna lose it all because of . And that's the problem. So

102:08 a thing bigger is better. That's that's the cylinder approach. That's so

102:15 just another way of looking at So shoot me. That was the

102:19 . That's effect. Number one is effect number two. I didn't really

102:24 . Number one is that can buy our helium measurement is so nation,

102:30 we assume that everywhere in the crystal the same concentration, if this is

102:35 map of concentration, this means 10 . This 50 PPm crystal like that

102:42 50 PPm. If we account for project, we account for recoil,

102:52 gonna get a crystal that really looks that. Or a crystal that looks

102:56 that because this is the recoil that lost out here. Right. And

103:01 can account for that by measuring the and adding it back in this stuff

103:05 not lost because of some thermal history . See, let me explain it

103:10 way, if we had these crystals the right light and we measured this

103:16 and the amount of uranium let's say got an age of 98 million years

103:23 . That's not when the thing was Because we're losing 2% of our of

103:28 helium. Not because it's been, know, anything's happened to it.

103:31 because it's just naturally goes away because was here. So we have to

103:35 that 98 million year old age to million. That's when the eruption

103:40 That's the geologically significant thing. And we do that every for every crystal

103:45 gets below about, you know, a millimeter. Once you get up

103:49 500 bronze, this isn't such a deal. But for small crystals,

103:53 is essential. Essential. But not big deal. If you if you

103:59 imagine that the concentration of uranium is the same. But if the concentration

104:04 uranium is more complicated and that's that's fine. But what if it's

104:08 complicated like that? What if you an appetite that grows with lots of

104:13 in the middle and not so much the outside? Well, you're gonna

104:17 ejecting only from the outside, but from the inside and you're going to

104:23 the age because you're going to have distribution that's different from what you

104:28 Or I suppose you have a really uh uranium rim, you're gonna overestimate

104:33 cage because you're gonna, you're gonna some here, but there's still gonna

104:37 a lot more out here than So we would like to try and

104:44 crystals that aren't zoned, but this an issue. Um and so with

104:50 concerns that I have broadly outlined here uh through experiments that I've described

104:56 we can come up with the broad temperatures of these things here, appetite

105:03 70 Zircon and seen about 180. so now we have these in our

105:13 . You're interested in closure temperature and interested in geologic events where the,

105:17 the temperatures are. These, these excellent choices. But now we've looked

105:23 systems that go as high as 800 as low as 70 and we've got

105:29 lot of choices in between, depending what what geologic problem you're looking at

105:34 the right system. Um Those are , here are some other helium

105:42 This is from compilation by Alexis Hall put this all together. The closure

105:48 of helium in the deli. I very high here is regarded floor right

105:53 is the other, there's appetite And so even in the, even

106:00 the helium system all by itself, got quite a range. Some of

106:05 are known better than others. The that are known really well, our

106:10 and, and sites not very many tight night. Um let's see now

106:20 thing I want to point out is there's this variation in appetite and here

106:27 . This big range and it's pretty understood. So I want to go

106:30 that in at least a little Um So let's go to here.

106:34 we have the clothes for for We have a graph of closure temperature

106:42 the the log of the concentration of four. The concentration of Helium four

106:49 a kind of proxy for how long system has been there. It's kind

106:55 a proxy for radiation jam. The helium four you have, the more

107:00 has been going on that, you , and you'll notice that the higher

107:07 concentration of helium, the higher the temperature, there's actually the longer this

107:14 sits around and gets helium. Um a crystal damage Proceeded to go up

107:24 temperatures as low as 50 or The temperatures as high as 120.

107:29 is that? Well, they think because and you can see it in

107:34 of changes in activation energy or do Well, just look at the

107:38 at the closure temperature broadly goes up here's why they think that happens.

107:44 you have a pristine appetite crystals, the activation energy of moving me liam

107:51 that thing should be always the same . But if you produce some radiation

107:59 , you produce basically what they think as kind of a well of activation

108:03 that it's easy to fall into this but that the activation I need to

108:07 out of this zone is really hot so you're producing kind of ins and

108:12 in terms of activation energy in the that that put little ramps figuratively

108:20 trying to get out. It's not to say there are places where because

108:24 transition from a damaged zone to a damage zone, it becomes harder.

108:30 in appetite, the more damage you , the higher the closure temperature And

108:36 a bunch of these little damage spaces funds these little places and each one

108:40 harder to get out of and the closure temperature goes up as much as

108:45 doubles from from 60 to 120. so that's something you need to pay

108:51 to and you can see it a of times. Uh what that means

108:55 the temperature of the helium. The temperature of helium in appetite will evolve

108:59 time. You can start out crystal have a close temperature of 70,

109:04 years later. Have closed temperature of and two samples from Detroit. All

109:11 from, from sand stones will have range of closure temperatures because they came

109:16 different places and have different concentrations. actually gonna be a good thing.

109:22 have a Sandstone which has all these kinds of appetites in them. That's

109:27 a bunch of different thermo chronometer. one tells you about 60. This

109:30 tells about 70. This one tells about 90. And so when you

109:34 a sandstone and you see a distribution ages that plots very closely with the

109:40 uranium. You can say something about history. Um I'm gonna skip

109:48 So that's all I want to say that basically the closure temperature of,

109:54 helium in appetite Can be as low 60° for a pristine appetite. But

110:02 the appetite damage, as the radiation increases, that closure temperature goes

110:07 Now, I should point out that we're gonna talk about Zircons. This

110:11 here is all about zircons. And thing to note about the difference between

110:16 and zircons is that Zircons have a more uranium and thorium in. You

110:22 , let me, let me go . I don't have to go back

110:24 far to this picture here. The blue dots, Alright, what do

110:34 do? The light blue dots are . And the dark blue dots are

110:42 . So you see that some appetite has have only one or two ppm

110:47 and similar amounts of thorium. But crystals can have 1000 5000 ppm uranium

110:54 an equal amount of thorium. you know, 10:00 200 times 1000

111:03 more. That's going to be an . And indeed what we're gonna see

111:11 uh radiation damage in zircon is that radiation damage will produce a situation that

111:23 like this. Is that here we , let's just go straight to closure

111:26 here's closure temperature versus something that they alpha dose in this case they tried

111:34 they tried to be more sophisticated in case rather than just saying the total

111:37 of uranium, they say, alpha dose is the amount of alpha

111:44 that have occurred since this. Since sample was at a certain temperature.

111:50 basically taking since the, since the track age of this sample fission

111:56 Once fission tracks start to feel the damage is growing at a hotter temperature

112:03 radiation goes away. So they take fishing track age in many of these

112:08 and they say how many alpha particles been produced since then. And what

112:13 see is that initially just like in just like an appetite. The closure

112:20 appears to go up from a low about 100 and 50 degrees to up

112:24 close to 200 degrees. But once get above an alpha dose of of

112:30 times 10 to the 16th alphas per . Look at what happens to the

112:35 tip, it starts to really fall . They're actually predicting closure temperature for

112:40 thing less than zero. And what saying is that radiation damage just tears

112:48 crystals apart. And at that level radiation damage, helium is just streaming

112:53 all the time. It's easy. you just you just give a stern

112:58 to the zircon and the helium Um this is a graph showing um

113:05 damage and and uranium thorium age I forget where this is from.

113:11 what we have is is here we've 2000 parts per million and we've got

113:17 very low age here. We've got parts per million and it's much

113:21 So once you get to some damage , you start losing helium like

113:28 Um and so that's that, you , zircons and appetites are a lot

113:34 complicated than we first thought. And should point out that this is one

113:37 the newest of these techniques. Um think it was at the 1995.

113:44 might seem a long time ago to . But at the 1995 Gs A

113:48 I went to in New Orleans. remember seeing these guys from caltech give

113:53 talk about Iranian helium dating the first that everybody, I've seen a big

113:58 was like, it's fantastic because they now showing about stuff that was low

114:02 stuff 9. 1995. There was paper in 1987 And then nothing.

114:10 had been 100 had been 80 years they had tried to date those rocks

114:14 Iranian helium. And they got, know, 30 million. And they

114:17 , Oh no, what are we ? The data to buy uranium

114:20 They got an old answer and then helium thing was just sets off until

114:24 had the better technology to assess closure . Now, we can say with

114:29 clarity That these things represent temperatures in range from 200 to 100 and it's

114:35 it's even and and the temperatures tend change with radiation damage. Um

114:42 I'm not gonna go through that. , I'm not gonna, I'm gonna

114:46 the calcite stuff too. Yeah. , I'll just show this this this

114:57 this is the last one or almost last one. Yeah, let's just

115:01 about this this paper here. Uh , that's not the paper. This

115:06 this is the results of the paper by house at all. This was

115:10 in nature and I think like 2000 like that. And what they were

115:15 is showing because of this low closure . What you can do in terms

115:20 landscape evolution, imagine you have some that have some periodic topography, eyes

115:30 lows. And the, the, model here is, suppose we're looking

115:35 the samples are going to come from sierra Nevada, in California, big

115:39 mountain range. And if you look the length of the mountain range.

115:43 way, rivers are coming out of mountain range at some level here.

115:47 so when the rivers come out, mountains are lower and on the ridges

115:51 the sides of the rivers, the are higher. Now, if we

115:54 at the mountain this way it it looks like this. But if

115:57 look at the mountain this way, looks like that. And that's what

116:01 saying here is to say, if we have these rivers cutting on

116:05 side of the mountain, they would valleys and ridges and that would produce

116:10 variation in the partial retention for for . We call it partial retention because

116:16 not a Neil, they're just Okay, it's the same concept top

116:21 bottom, just like the partial and zone for fishing trucks. So the

116:26 pretension zone will mimic uh photography because clothes because the temperature is so close

116:34 the actual surface, the partial attention for argon, it's also a

116:40 but it's way down here and it's going to mimic the topography unless you

116:44 mountains that are 10 kilometers high. just for these normal mountain size is

116:49 partial retention zone of appetite will in mimic the topography anything further down.

116:56 , no, But because it's so , we can start looking at bedrock

117:02 and tell us about the paleo elevation the surface topography because it's such a

117:08 top closure temperature. And so what did was imagine, you know where

117:13 , where the samples are below the . They should be older because it

117:17 longer to get out of here. below the valleys should be younger.

117:22 so then they did that they went down the sierra Nevada and they collected

117:27 from here, The red line shows elevation Going from 1 to 3 km

117:33 the blue line shows the way the think that's the running average of the

117:40 that are shown in the dots and the old samples are down here and

117:46 samples are down here, where the are near the ridges, the ages

117:51 younger, where the samples are near valleys, the ages are older and

117:55 what we predict from this year. what the show that look at these

118:01 . These agents are 70 million to million. This suggests that these

118:08 here's this is one river, this another river, another river coming out

118:13 us. And the pattern of these tell us that the these rivers have

118:20 there for tens of millions of digging down, making these patterns like

118:25 . So the western edge of North has had this edge on it.

118:31 sierra Nevada has been there for a long time, which has implications for

118:36 sorts of tectonics and geo morphology and picture stuff. Um And this can

118:42 done because the closure ship is so , Closing temperature of helium in appetite

118:51 somewhere between 70 and 120, depending radiation. The closure temperature of helium

118:58 zircon is somewhere between 200 and The planning on radiation. And that's

119:10 of that. So what time is ? 3:00. Okay, I want

119:16 move on to something else. At we can start this and we'll finish

119:25 next week. Okay, so the thing we're gonna do is not going

119:45 . Last thing we can do is doing geo chronology in sand stones.

119:54 we've been applying most of this to rocks, right? And and as

120:01 said, the best way to date sedimentary rock is to date an igneous

120:06 . But in many cases we don't that option. Uh Oh, I

120:14 one. Maybe I should, I think skipped one seven. This is

120:28 short one. Let's do this We're gonna go to thermo chronology and

120:35 talked about this some a little We can go through it pretty

120:40 Uh and then, and then we'll about sand stones. What?

120:46 I'm not sharing. Thank you. not. Okay. Uh hmm.

121:17 should I do that? Yeah, is thermo chronology and faulting. You

122:14 see it? Maybe I Oh maybe not. Let me open edit display

122:48 should be there. Uh Here, . What's what's your email there?

123:14 is. Okay. And yours. what way? Go ahead. Oh

123:28 , send it to Andy here looks man in the song. Yeah.

124:05 . Okay, I just email them you. Alright, so very

124:33 how can we use Thermo chronology to us about the timing of faulty.

124:38 mentioned this earlier that, you we've got ball thing is going to

124:44 topography and this is particularly, then gonna be able to use this to

124:49 us about vaulting, it's the lower closure temperature usually the better.

124:55 we already saw these things where we the elevation profiles and we go up

125:00 whether we see a change in slope those age versus elevation profiles, the

125:05 the plate, the point at that is the time in which tectonics change

125:09 rates. You change the rate of for some reason, that would

125:14 you know, something along here. , and in general, we don't

125:20 of the depth of closure to be this. Well, it depends on

125:24 we're talking about, but generally we're imagine the closure temperature will mimic the

125:29 for these low temperature things. And know, we can just start with

125:34 simple situation here, we have some erosion rate. DZ DT, We

125:40 look at an elevation profile like we've before and if it's a nice straight

125:45 , it's a simple erosion rate going same throughout and, and whatever this

125:52 temperature is, We passed through We've shown examples of this in two

125:56 examples already. Um, and the for fishing tracks, same thing.

126:04 know, if we pass through a A Z will get certain characteristics.

126:10 all of this was just sort of erosion. You can do the same

126:14 for a short meeting event. Something . But what about faulting in

126:22 If we have a situation like Normal faulting, if we have

126:26 what some people call tectonic degradation, this off, then samples up here

126:34 even this sample, but certainly samples here would immediately be cooled off.

126:39 when we look at a uh, we look at a sample and we

126:43 and we see that, you we've done a bunch of thermo chronology

126:46 we say it's hot, hot, and pulled off rapidly. Like we

126:50 when that feldspar changed. It's great it's in the it's in the foot

126:57 of a normal fall, that would a reason for it to cool off

127:01 when the false moves fault moves, sample gets cold immediately. And so

127:09 , you expect for fishing tracks, example, you'd expect them to be

127:13 cooling. A lot of narrow track . Conversely for us all things a

127:18 more complicated. You take a you trust it. You make the

127:23 are higher now, but immediately after rusty, you know, these happens

127:28 higher. The sample is still the distance it was from the surface,

127:33 that won't last very long because when make a mountain, you're just asking

127:37 more rain, right? Mountains are going to be eroded faster than than

127:42 they were not mounted. So this takes place pretty fast after you make

127:47 , you can't make a mountain forever . And so you make erosion here

127:53 the sink cools off. It cools probably slower than this. Um,

127:59 it could cool off pretty soon And so when we think of trying

128:05 understand faulting simply by changes in cooling , we will look for normal faults

128:12 cool immediately thrust samples in the foot of normal faults to cool immediately samples

128:20 the hanging wall of thrust faults to soon after. And this doesn't involve

128:27 of the crosscutting relationship kind of You know, we suppose we have

128:30 dike that crosscut default. Then we the dike by non thermal. Then

128:35 take the dike by the highest closure system we have available because we don't

128:41 to worry about thermal perturbations afterwards. want to know the crystallization age of

128:46 dyke. If there's a, that tells us the age of the ball

128:50 than this stuff. But we don't to date that fault with alien dating

128:55 that could be way too much Um, burial. Um, You

129:04 , was is much the same. have a sample, you bury

129:08 then you erode it back for the fishing track agent. You're gonna expect

129:12 to go back and forth like this you may have those sort of by

129:17 fishing track distribution. We talked I think that's all. One

129:23 Okay, well we've already talked about way, but this is the Taranaki

129:27 very similar to the Otway basin. think I'll skip that and new

129:33 New Zealand. Yeah, we can that part. So that's all I

129:37 to say about vaulting. Is that , thermal chronology by itself is good

129:43 the foot walls of normal falls. hanging walls of those forms. If

129:51 had more time we'd spend more time that. Okay, so now we

130:03 move on to looking at sedimentary rocks we will not finish this today,

130:10 we will start it and finish up rest on friday and then move on

130:14 some case studies. So, this is a talk I gave at

130:24 . S. A. And I've it to some other things and,

130:27 these are some of my former students postdocs that helped out with it.

130:34 , a couple of things we have worry about when we do sedimentary

130:37 The first is sampling bias. Are collecting samples from our sandstone and we're

130:42 to talk always about sand stones, you could do a shale but it's

130:46 fine. Great Sand stones. We to, are we doing a good

130:51 of sampling? I show this Uh, it's a famous thing from

130:56 presidential election of 1948. Uh, President Truman but, but, but

131:03 Chicago, daily tribune the night before said, well we know Dewey's gonna

131:07 . So just print up the Dewey defeats Truman the next day.

131:11 was clear that Dewey had not defeated . And the reason that the Chicago

131:16 got it wrong is they only called that had phones And in 1948,

131:25 was a very different group of people had phones than who didn't have

131:29 This was a phone survey. And learned that the people who had phones

131:34 going to vote for, do the people who didn't have phones were

131:38 to vote for Truman. And this a famously wrong headline. You

131:42 that's the winner. That's Dewey. mean, that's the Truman holding up

131:45 thing says, ha, ha, got it. Well, the reason

131:48 got it wrong is because they didn't , you know, it was,

131:51 know, telephones were a new thing 1948 and they said, well,

131:55 just call people up and see who gonna vote for. And most of

131:59 people who had telephone said, I like that. Do we got

132:02 was a sampling bias and we have worry about that when we consider this

132:07 geology. Um, how, what of sampling biases are we going to

132:11 in there? Well, we could provenance biases because the material that is

132:16 to our basin is going to be by the tectonics of the various

132:21 by the irritability. Are we eroding shales or gran, it's, it's

132:26 be by the climate over here, rains more or whatever, it

132:29 so that's gonna affect the grains that to us in our basis. So

132:35 gotta worry about that. We got worry about the as these grains are

132:39 liberated, how they come to They will be sorted by hydraulic

132:43 They may get broken up during sedimentary that's gonna, if they get broken

132:47 too small, we can't use Um The grain composition will matter.

132:53 talked about how radiation damage can change , uh whether they have the shape

132:58 matter. Um Then once you get just geologic considerations, then we get

133:04 into our laboratory, we've got to mineral separation. That that involves all

133:09 of choices we make in the Are we gonna do, we have

133:13 hand pick them? That means we this when we don't like this

133:16 we're gonna have to pick sizes and these things and then when we get

133:19 to the other laboratory were actually analyzing here I'm talking about Iranian lead analysis

133:25 it could be other things. We to, we are going to be

133:29 by how many grains we picked Where if we're doing these laser stuff

133:33 we put the shot point, is on the corners on the middle.

133:37 We are then maybe going to decide to throw away data and when to

133:41 it, How big was the spot ? Where was the spot size?

133:45 are all they're all akin to calling , only houses with telephone or maybe

133:52 doesn't, you know? Or maybe are not. We just got to

133:56 so of, of, of, all of this sort of related to

134:00 of this. And the first question really need to ask is how

134:05 how many grains is enough? We to recognize that we are dealing with

134:11 is guaranteed, well, almost guaranteed be a heterogeneous population. Right?

134:16 is a sandstone. And unless we're with remarkably narrow provenance, we're going

134:22 be dealing with sand stones that have source areas or the source the

134:28 The single sand deposit may have come lots of different places. And so

134:35 could be a problem depending on what doing this. And so how many

134:42 is enough? We gotta sandstone, a bunch of grain sand there.

134:46 many is enough? You have any of how many would be enough?

134:50 want to, let's say, let's restrict our right. Right now

134:54 gonna be talking about the grains that date. So we're talking about only

134:59 grains. We'll talk about, say , maybe we'll talk about sanity,

135:05 let's just consider those two possibilities a or zircon there. They exist in

135:13 sand or sandstone. Uh, but a in a rock this big,

135:18 know, there's probably a lot of cons, you know, could be

135:23 of them. How many do we to analyze to get a sense that

135:28 done a good job. Just what's gut feeling? What do you

135:36 That's enough? What do you Well, let's start with 20 more

135:50 less than 20. How Many More Enough? That's plenty. Okay.

136:00 , it probably isn't 20 twenties, way, not enough. 100 is

136:05 close. Um but before we get those details, we have to consider

136:11 enough to do. What do we to just have one representative of every

136:17 ? Suppose we're just looking at, know, we want to suppose that

136:20 got a we've got a cretaceous source a pre Cambrian source and a Jurassic

136:27 . What if all we need to is recognize that by saying, we

136:30 one of those, we found one those who found one of those,

136:33 a different question than we want to . A perfectly faithfully reproduce the distribution

136:39 Cameron's are much more important than the and then the cretaceous are a little

136:43 important. No, that's a that's curve rather than just, yep,

136:46 , yep. You know, if , if I want to know how

136:49 people at the University of Houston, , you know, uh were born

136:54 India as opposed to are there any born in India here at the University

137:00 Houston. Well, you know, told me you're from India. So

137:02 I know that at least one is group is taken care of, that's

137:06 way to look at or another way look at it is well, what's

137:10 , what's the proportion are you the one? Are there, are there

137:14 to answer that question? You clearly more more, you know? So

137:21 on what you want, depend depending where you're going, depends on how

137:24 grains is enough. But if we back to this first question of sampling

137:30 subgroup, we just basically want to , you know, in that

137:35 you know, here I would I would have taken care of the

137:37 of India and Louisiana just with you would be enough if those were the

137:43 two subgroups I was interested in, got lucky the first two I analyzed

137:47 those out. That's enough. That's though, but let's consider sampling the

137:53 first and then we'll talk about faithfully the entire distribution. These are two

137:59 that are fairly uh fairly similar. Vermes paper gets a lot more attention

138:07 because it was published 1st but what two guys did was go through some

138:12 of assumptions of this or that and and show a few things And

138:18 Anderson did in 2005, he Well, if I have a subgroup

138:28 represents a certain percentage like ST 2% will be the chances that I never

138:34 that group based on how many I And so he says that if you

138:39 a subgroup that represents 2% of your and you analyze 20 grains, you

138:47 a failure rate of about 65%. only find that 2% thing about one

138:52 three times. If you have ever 100 grains, your failure rate will

138:58 be 18% and if you analyze 300 , your failure rate will be essentially

139:05 . Now of course, if, it's too, if it's rather a

139:09 representation, then you only need to about 20 grains to have a failure

139:15 of of one or 2%. That's way to look at it. Um

139:21 never mind. Oh, so if wanted to have a failure rate of

139:25 20% of a 2% population, you to analyze at least 80 creeks.

139:34 here's a different way to look at . And both of these guys did

139:36 calculation, they said, well what the detection limit? If you have

139:45 90? If you want a 95% , how many grains do you need

139:50 analyze and what, what, what the one that's most famous is

139:55 here for Mish Mish said that If want a detection limit of five,

140:02 means that your subgroup represents 5% of total And you want to have a

140:08 confidence that you will find that you need to analyze 117 grains.

140:16 that's, that's the most famous number came out of this paper. People

140:20 about 117. It's like a magic I've talked with for me. He's

140:25 . That's all they got out of paper was the one number. But

140:29 a place to start. Anderson has has a much more optimistic view.

140:34 see, he uh, he would that you don't need 117. You

140:39 need 80. So these are these are different assumptions they make about

140:44 , but whether it's 60 or 100 17 depends. But a lot of

140:51 say that well you really should go 100. And the good news is

140:55 with all of these systems are You put all these zircons are all

140:58 valves bars into the system and you say analyze these guys and you can

141:03 it pretty quickly, you know, in, in, in the silicon

141:07 they can do 100 grains. So can do, they can do maybe

141:11 grains in a day. So you put it in there, you turn

141:15 on tomorrow morning, you've got 200 50 points on your graph.

141:23 now to reproduce the entire distribution. know, to really have a sense

141:28 what we're showing here is the same . That's a much more difficult thing

141:34 on what statistical criteria you choose. can get very, you know,

141:41 by, by some of these, you basically need to analyze all of

141:44 . The the end necessary is But even with the different um with

141:51 uh choices, You still need somewhere at least 300 to have a sense

142:00 Analyzing the total population. Not just we there are some people from Louisiana

142:05 they're in this group, it's about . That's a much more difficult thing

142:10 do. So that's 300 just to sure that you represented all the people

142:15 represent less 5% or more. That's about 100. Okay, so the

142:21 thing we can talk about when we about the triangle dating is the way

142:26 which the data are presented here is way in which you you plot the

142:31 versus the strata. Graphic age in case it's a cooling age because we're

142:36 about felt Spars but you could be about any age. You just plot

142:41 versus strata. Graphic age. And is, this is valuable when you're

142:45 at the variation over time. This the disadvantage of not showing the uncertainty

142:50 the individual points. Um but that's a huge deal. Sometimes the other

142:57 is to plot these things called probability plots which takes the individual Gaussian distributions

143:06 the samples. You know, this 100 and 20 plus or minus one

143:09 so forth. You take all those and you add them up in one

143:13 curve, that's what's called the probability . It's a kind of instagram but

143:19 better than a hissed a gram because includes uncertain. And so the samples

143:23 have a very narrow uncertainty will will up on the graph very high and

143:27 ones with the horror uncertainty will be . But you add them all up

143:30 you get some sort of cumulative look and you can say that you know

143:34 individuals all add upsets that there's a about 110, another peak at about

143:40 and so forth. So you can take that diagram and just add it

143:46 take the same data. These data on this diagram looks like this is

143:51 cumulative probability in which you say that oldest, the range of ages here

143:57 from 72 to 1 30 99% of the data are younger than 1 29

144:04 of all the data are younger than 10 and so forth. And we

144:09 see all three of these types of presentations made um in various situations.

144:18 more thing is this thing called the density estimate which tends to spread out

144:25 data and oh gosh I'm gonna have the k. Is some some value

144:31 the Colonel. And the h is thing called bandwidth. And what it

144:37 is it takes the individual data points it really ignores their individual uncertainty.

144:43 just says that they should be treated the same. And if you have

144:47 data that's okay. And so the line is the kernel density plot of

144:52 blue line which gives you a very sense. But some people are some

144:56 are keen on doing it this Um The problem people sometimes don't like

145:02 the probability distribution plots is they get spiky because the uncertainties can be so

145:07 . If you transform A if this the true distribution And you sample it

145:17 times there's that number again um You get a probability distribution that might look

145:23 this and but the kernel distribution it make it look real more like the

145:30 data. And if you analyze 10,000 then you don't really have to worry

145:36 . But for analyzing only 117 with these good uncertainties it looks fight here

145:42 it really is. This is the distribution. This is the sample

145:46 But if we use this kernel we up looking like that that looks more

145:50 that. But if we we we the luxury of analyzing it 10,000 times

145:56 they all look the same. Um other way that's often done is either

146:05 the kernel density or the probability density you will show different samples and along

146:12 line and then you'll just knock them one on top of each other and

146:16 maybe strata graphically from lowest to something like this. And so you

146:20 , you can visually inspect the differences these sand stones as you look up

146:25 strategic city like that. Um that's . Okay, so why to try

146:35 thermo chronology? There's four reasons why might go to a sandstone and interrogate

146:41 grains with ice topic investigation such as the first and one that gets a

146:48 of play these days, especially with zircons is for understanding of paleo geography

146:55 we're going to describe the provenance in good way here, we're going to

146:59 these zircons, let's say and say well the terrain that were being eroded

147:04 this base and included zircons of these ages and we know provenance is up

147:11 that have these ages. So we say, oh these grains came from

147:14 and these grains came from someplace Another reason to do, to try

147:19 thermo chronology is to understand the tectonics the provenance. This is more done

147:26 sort of argon dating because the argon , you know, if you're looking

147:30 the mountain range up there, you , the grains came from that mountain

147:35 as they come up through the mountain , they will go through their closure

147:38 and then they'll come out here into basin, the difference in time between

147:42 they were in their closure age, they're in the base and tells you

147:45 about the vigorous nous of tectonics, talk about these things in in detail

147:51 , the third thing to talk about that this maximum deposition all age,

147:55 we don't have any clue about how the sandstone is. Of course we

147:58 to know uh we can just measure bunch of grains and then the youngest

148:03 is the oldest that the sandstone could be. That's another thing. And

148:09 the fourth thing is to understand the de positional thermal history of a

148:15 And we're interested in the basin sandstone is deposited, it gets buried

148:19 conceded up. How did it change we did that? Now you can

148:25 do if you're doing one of the three, you're not gonna have much

148:30 about the fourth one. And if doing the fourth one, you're not

148:35 to learn much about the first Because if you're if you've gotten to

148:38 point where post deposition, all thermal is something you learn that means you're

148:44 information from the provenance. But if got information from the provenance, you've

148:49 haven't got hot enough to tell us deposition, all post deposition, all

148:53 history. So They're sort of you're doing one of these first three

148:58 the 4th 1. But you can't all four at the same time.

149:04 , good. Okay. Once we've got these minerals to choose from

149:09 same group of minerals. We've got high temperature ones. Medium temperature low

149:12 ones. The ones that are used often are the highest ones and the

149:16 use uranium Lanzer Con because it doesn't changed and tells us about the

149:22 And then the other ones down you look at argon in the cell

149:27 , fishing, tracking the appetites and and everything. So you're either interested

149:37 the when you when you're doing the temperature stuff like Zircon and oh,

149:44 will also be used a lot and that will be used a lot and

149:46 that will be used a lot and maybe that those are the ones that

149:49 most of. Um When you're interested in paleo geography, we're gonna be

149:56 to hire closure temperature one. When interested in maximum deposition allayed, you

150:01 do almost anyone except the really, low ones When you're interested in tectonics

150:06 the provenance, you want to stay from the really high temperature ones.

150:10 when you're interested in post definitional thermal , you're definitely down on the low

150:15 in. Okay, we did this . So I'm gonna skip that.

150:31 And here's a bunch of Sir cons their and their measurement pits. But

150:35 kind of talked about that too So now let's move into talking about

150:42 use of the tribal thermo chronology to us about paleo geography description of the

150:49 . Um Now the first thing you , well we could just investigate the

150:53 area directly. Wouldn't that be easier trying to understand the source area

150:58 from some sandstone? And the answer that is well, um you make

151:04 problem with, with investigating the it'll minerals, the disadvantages the spatial

151:11 . You got a sandstone. All can say is this Circon came from

151:16 up there. Right. If you to look at the paleo geography,

151:21 know, if you went and analyze mountain range, this mountain,

151:23 You know more details about the paleo , but you have to go to

151:27 mountain range and figure it out so the disadvantage of spatial evolution, The

151:33 of course is much faster than visiting mountain range. Um It's also the

151:38 that maybe that mountain range doesn't exist . If you're looking at a

151:42 the provenance may have been eroded You know, the Himalayas will be

151:46 long before the Bengal fan is We will know, we know about

151:51 , there's a place called the ancestral mountains. You're familiar with this

151:56 It's a bunch of mountains that that were found in colorado. They're

151:59 the Central Rockies because they're in the place as today's rockies, but they

152:03 there in the permian at the pennsylvania we know about them in large

152:10 Not entirely, but in large part looking at this place called the Anadarko

152:15 which is in Oklahoma and Kansas and and colorado. It's a basin full

152:23 sand that came from that way. know that there was a place called

152:29 the Iraqi ancestral Rocky mountains were There had to have been a mountain

152:34 to produce this all this pile of . That's the Anadarko basin. That

152:39 be the geologist of 100 million years now will have the same problem when

152:43 look at the Bengal fan, where the big mountain range? The Himalayas

152:48 be gone by then, but the fan may not. So that's an

152:53 in that if the source doesn't exist , we're just left with looking at

152:57 sand and still we can learn a that way. Um here's an example

153:04 looking at samples from western United States the uh in uh Mesozoic. And

153:14 , yeah, so you can look samples from the mesozoic and we know

153:19 rivers in this time when all the from the Western Appalachians all the way

153:24 the Western coast of North America. know that because when you look at

153:27 sand stones, places like Arizona and , they have all of these 400

153:33 year olds are constant, which is dominant signal you expect from the

153:39 They do not have a lot of that come from up here which would

153:42 like two billion years. So we track the, the fact there must

153:47 been rivers going across the whole Um, here's another example, maybe

153:53 little more detail. This goes for rocks. We've got paleo seen rocks

153:58 with the orange outcrops and ceremony in in in green. And these are

154:05 where there are sand stones and people taken to try to cons and looked

154:09 them and based on the here's the we got cretaceous rocks, the center

154:15 and police seen rocks. We can them in the cretaceous based on these

154:20 . We've got samples here along the coast based on these samples in the

154:26 coast. Rivers were going north because don't have any of those are key

154:31 grains that coming from Wyoming in the main ian rocks, they don't

154:37 We only get grains here from the Appalachian rocks. But when, by

154:42 time you get to the paley So this is santa mania. That's

154:45 million paley scene. That's 50 million the 40 million years that this this

154:51 tracks, we switch and now we evidence for these river systems going up

154:56 Wyoming and up into all of all of these protozoa rocks of West

155:00 States. Now these zircons in in the orange outcrops not far away

155:06 the green outcrops give a very different because we look at the distribution of

155:13 and we say, aha, there's , there's a representative of the arche

155:17 that came from Wyoming, here's a of the proto resort that came from

155:21 . Now we don't, excuse Arizona, it could have come from

155:25 lot of places, but it came Arizona or New Mexico or some,

155:28 know along there, it did not from the Appalachians. And when we

155:34 these samples down here in the all they got Is this 400 million

155:38 old grains. They are not the billion year old grades you get Wyoming

155:42 1700 million grains you get from They don't exist. And so by

155:48 analyzing some sand stones here in we can understand broadly speaking the paleo

155:55 of north America at two important So that's the thing to do.

156:02 here's, here's a here's a similar on a much smaller scale. This

156:06 some work that my student Marie de Santos did for her undergraduate thesis a

156:13 years ago and we were looking at place in New Mexico where we had

156:17 out to outcrops to two sections of of this rock that was called the

156:23 formation in the florida mountains, that thinner, it was finer grained and

156:29 interpreted that as lancastrian or alluvial sedimentation 40 km away in the Victorian

156:37 it was thicker, it was we interpret that as alluvial fan sedimentation

156:42 when we put that together it was , well they're both been called the

156:46 formation, their flu viel and alluvial . Alluvial fan over here flew viel

156:52 , moving into the rivers, moving the swamps and lakes here. And

156:56 we said this is all part of big basin, this is the bottom

156:59 the base and this is the side a perfectly good um sediment, a

157:05 interpretation. Then we looked at the zircons we took we took sand stones

157:11 each one of these sections and analyzed bunch of their construct and here they

157:16 in red and the blue. They're anything like each other. The Victorian

157:22 has this huge spike at 1400 The Florida Mountains have a very big

157:27 at 1000 million. And you know we've analyzed almost 100 grains in

157:33 That's getting close to a good Does this strategy, does this does

157:39 paleo geographic model make sense now? , probably not right because this suggests

157:47 all of this material, this is a course version coming off some mountain

157:52 . This is the fourth version. is the fine version. But the

157:57 that the ages and their cons shouldn't so different. They're all coming more

158:01 less from the same place and all seeing is variation in grain size.

158:07 we have to to reject that So either they were either these two

158:14 which now exist 44 km apart from other or never a part of the

158:18 basin or maybe you could you could away with it by saying that this

158:23 of this basin was receiving receiving sediment the axis of the base may be

158:29 up and down here. Whereas these are coming off the side, this

158:32 the middle of the basin was coming and down. You know, that

158:36 that works. But just the simple of this is the course side and

158:39 is the fine side and they're all of one big deal doesn't work.

158:44 was a total surprise, you good thing we bothered here that that

158:48 is wrong or at least it needs significant tweaking. So whether it's whether

158:54 the whole continent or just a little part of new Mexico were able to

159:01 potential drainage basins by saying these are . And of course this is the

159:07 of stuff you can can't do with photography. You know, you've looked

159:13 sand stones or you have that Maybe you didn't learn about the history

159:18 looking at the courts, feldspar, fragments and it tells you tells you

159:23 about de positional environment or provenance or , but all courts kind of looks

159:30 same. And so all failed sparks of look the same and that you

159:33 plot them on diagrams like that. they can sell plot in the same

159:37 and still have very different. This an example of that. These these

159:42 these these these sequences of sand stones silt stones, people called them the

159:48 formations global formation and we thought they made sense. But the say

159:55 Um So you can you can you also if you have some known source

160:04 you have a zircon sample already have tribal sample. If you have a

160:09 you have a no if you have measured here and you have some known

160:14 , maybe you you know you can , you know there are ways in

160:18 you can you can mix up them you can say well this this black

160:23 is the result of 10% of And 80% of b. And 10%

160:26 c. And that's a nice that's paleo geography. Now that still may

160:33 the result of some of those biases talked about where it's raining more on

160:37 source seed than it is on A but it still tells you how

160:40 were being delivered to the base. And you can do it strata

160:47 there's a study in in Peru where looked at all of these places which

160:52 strata graphically going from 16 to 10 . And they looked at these different

160:57 in here and they were able to at these all these potential sources and

161:01 that for this sample. You know were the relative contributions. This this

161:05 became really important here at 30 million important here. So this is giving

161:09 again a uh understanding of the relative of places in the paleo geography.

161:18 that's um that's paleo geography, whether a big picture or a little picture

161:26 looking at the variation comparing them. they different? How are they mixing

161:30 ? Um That's a very helpful Um We'll try one more and then

161:39 stop tectonics and the provenance. Another to do this, as I

161:46 you can look at the courts feldspar diagram and it'll tell you some things

161:51 there are tectonic signals you can get of this, but you get even

161:57 if you can understand the ages of things and again, we could just

162:02 the source directly. Why don't we with that? Well, again,

162:06 , their spatial resolution. Um this faster both in the field and the

162:10 . We even if, you even if we were going to

162:14 we can analyze all these parts of mountains or we can just look at

162:18 sand in the river. Now, in the river has the low spatial

162:22 . It's a lot easier to scoop the sand and go home and just

162:26 it there in the lab and not in the case of the

162:30 but again, the same thing goes the source isn't there anymore. Now

162:36 tectonics and the provenance, we're gonna on material with lower closure temperatures.

162:42 uranium lead. Zircon is not very here because it doesn't have a very

162:46 tectonic signal. But nothing like argon and feldspar or helium and

162:53 They would do because look at this here, we've got, if we

162:58 a situation here, we've got a down there and it's it's below its

163:02 temperature. It's hot, let's just it and when it's down there,

163:06 isotopes are diffused out of that surrounding . But as we get up

163:11 you know, these isotopes. Once get to, once we get to

163:14 about this point, the daughters start be retained, their carried along with

163:19 and then eventually the erosion goes and it up there. And now this

163:24 , The amount of time the material below the closure temperature changes sides top

163:29 . And so it's better if this temperature is not very high, it's

163:33 down here. Then passage through this may have nothing to do with the

163:38 when we finally get up here. we want to get this pretty

163:41 low closure temperatures better. And so we're then going to see is since

163:48 talking about the triple dating, we're be looking at these samples that are

163:52 here in the basin. And what going to be comparing in many cases

163:56 the age of cooling to the age deposition. And the cooling occurs as

164:04 move through the mountain range and then is out here in the basin.

164:08 the difference between those two times is the lag time. That's a tectonic

164:14 . The shorter the lag time, more vigorous the tectonics. Um an

164:22 of this um comes from the Bengal where samples were collected in the ocean

164:31 program down here leg 1 16 and , you know, they drilled down

164:37 deep. They had to go all way out here, 2000 km away

164:42 the mountains in order to get Enough . If they drilled up there by

164:49 , you'd only get very young rocks , because it's the sedimentation rates too

164:53 . They can't drill down 15 So they go out to the edge

164:57 the, of the Bengal fan. they get more more age age.

165:02 they and what we have here are that went all the way down in

165:05 lower mice. If they had done up here, then it just got

165:09 last million years, let's just say . Alright, so we plot this

165:18 from my PhD and we plotted um mineral, this is for fell spars

165:27 the Bengal famine Argon 40 39 ages felts bars. And we actually did

165:33 heating on these with the laser just of course, turn it up,

165:36 up, turn it up and what plotting here is the minimum age,

165:40 know, we may have, we sometimes you only did three or four

165:44 . We didn't have enough gas in in the thing to measure more than

165:47 or four steps. And it was crude. But sometimes we would get

165:50 young of those three or four steps plotting the youngest one here because the

165:55 one is the most important. And what do we got here?

166:00 this, this is a log this is a linear scale. So

166:04 red line is the 1-1 line. what you'll find is that every place

166:10 look going from the ages of about million to about 16 million. We

166:15 able to find grains that essentially plotted that 1-1 line. That's the lag

166:21 . We're talking about lag time when go back back here to lag

166:25 when we say cooling age to deposition . Well, I guess you can

166:28 that for every single grain. But most important lag time is for the

166:33 grain because it is. And so this case the lag time throughout the

166:41 that's available to us here is always . What does zero lag time

166:47 That's really something because this is, is felt spars, these are coming

166:52 the Himalayas. We're going to say closure temperature of argon and feldspar is

166:58 degrees. That's a pretty good guess . Where is 200° down there?

167:14 , something like that. I mean could be six, Not less than

167:20 , several kilometers. Right. And . And these sedimentation ages come from

167:28 . Okay. So there you there's some uncertainty there too. But

167:32 the uncertainty of the of the Argon and certainty of the fossils, in

167:37 case we found ages that we really tell the difference between the two of

167:42 . What do we got then? got ages that telling us that this

167:46 , this this sample here Came from km below the surface, up to

167:52 surface and out 2000 km to the of the Bengal fan. Such that

167:57 can't tell the difference, Meaning that probably occurred within a quarter of a

168:02 years. That whole thing six km the top out here Couple 100,000

168:09 That's really something yeah, we know really something tectonic because Himalayas don't have

168:23 . If there were volcanoes around, always gonna be some some ages that

168:27 the same age as the as the . Right. If we were to

168:31 this exercise off the coast of Costa , we find the same thing,

168:38 always find some grains that were the age as sedimentation because they were thrown

168:42 into the air by a volcano and down at the bar. That's not

168:46 lee interesting. But there are no in the Himalayas because it's a different

168:50 setting. So that means this is of this is tectonic. And so

168:57 we've got here and and but but only do we have done grades,

169:00 we have all grades in all of other places. Just as a little

169:04 when we were doing this, when was doing this for my PhD,

169:07 one was the first grade we analyzed it was like, you know,

169:12 is what we were wondering, can find zero age grains found this

169:16 the first one, wow, this the second one he analyzed and then

169:23 the others are in between. This the second one is the first

169:25 Well that's not important. It's just . Um but the fact that we

169:30 , we've got this long range We got big variation here. What's

169:34 variation? Tell us? This tells that there's a slot coming up like

169:40 , right? But the variation and continued variation tells us that it's not

169:45 whole mountain range coming up like crazy if it was the whole matching range

169:50 up at three or four a we'd eventually get rid of all our

169:54 samples. There'd be no old stuff . We still are getting old grains

169:59 into the Bay of Bengal at three years. Same as when we were

170:03 in 16 million. So what this telling us is that although there are

170:07 in the Himalayas that have come up a rocket during this whole time,

170:11 must be very small place because if was the whole mountain range we'd eventually

170:16 away. All the old stuff would be pulling up young stuff, the

170:20 thing would be brand new. So can say that it comes up like

170:24 rocket but only in little bitty So again, where are those little

170:29 spots are? Well we recognize one them in that place. In in

170:34 to bet that was one place, are other places that people are showing

170:39 you know how many there are where are that's going to be a lot

170:42 work. But we were able to this by just analyzing a few dust

170:46 By the way. I should point this was done a long time ago

170:50 this was considered a lot of data then. Remember I said you should

170:54 do 11717, even close to I think the one place we got

171:00 was maybe 20. But the good is is that because these these young

171:07 seem to be as prevalent as they , we didn't have, we didn't

171:10 to analyze 1000 grains to get We found what we were looking for

171:14 single time. If we could find zero age grain by only doing 20

171:20 , it must be, it must be 1% of the total. It

171:23 be a lot of the total. nowadays this would be considered a remarkably

171:29 amount of data. Um And so mentioned, so we've got this problem

171:38 volcano. The lag time is proportional tectonics. The smaller the lag

171:42 the more the exceptional the tectonic But of course this is only for

171:47 volcanic grains. As I mentioned, are we going to deal with the

171:51 of volcanic grains being in our system looking exciting. But being boring,

171:56 really boring. It's just a But if you don't know, it's

172:00 now you sound like that. Like study in the Himalayas where they're going

172:03 like crazy, how can we tell difference? Well, first option would

172:08 to work in the Himalayas. you know, but if you

172:11 But what if you what if you're something in the Rocky Mountains or you're

172:15 in an active zone which is also a subduction zone? Well you do

172:19 thing called double dating. Here's an from uh the alps where we've got

172:26 bunch of fishing track ages from the basin and the fishing track ages are

172:34 pretty near the age of deposition. What is that? So, but

172:41 we can double date something that means date the same sample by both Iranian

172:46 and fishing track. Then the samples have the same value for those two

172:51 are volcanic, right? Because volcanic cool rapidly. And if you get

172:57 same answer for efficient track and as from the same zircon that has to

173:03 a volcanic zircon, see that if different, then you can ignore the

173:12 age and look at the second age a cooling age. But if they're

173:15 same, they must be volcanic and volcanic is not what we're interested

173:21 So if you have different systems and same age it's volcanic. Um And

173:27 here's some examples of some zircons. see what's that doing in there I

173:34 . Yeah, let's just look at . Um These were this is a

173:40 from Western United States where they looked all these samples. Here's the cooling

173:44 positional age. When you start talking lag times and stuff. But this

173:48 a bunch of data. This is the appetites. These are the non

173:53 genic psychogenic appetites. We had to some of these appetites because they had

173:57 same age as their fishing track and ages fishing track and their lead agent

174:03 the state. So once we get of that, then we can start

174:07 about tectonics lag time, all that . Um I think I'm gonna stop

174:15 and we'll pick up the rest. Starting on friday we'll finish this uh

174:21 the first part of friday. And I'll go through some exercises that which

174:26 basically be practice test will just give a bunch of scenarios and ask you

174:32 to sort the amount, The key answering all these questions is to know

174:37 the closure temperature of every system And once you know the closure temperature

174:43 know have to start thinking about why why do rocks change temperature change temperature

174:48 they will be very cooling off because some kind of erosion, whether it's

174:52 know, normal rain erosion or maybe tectonic erosion. They're heating up because

174:58 they've been next to a futon that nearby or because they're being buried.

175:02 can be buried because of sedimentation or of thrust faulting. Uh they can

175:09 off if they're in an igneous, they're in a granite, they can

175:12 off because they have been eroded away because they've been intruded into rocks that

175:17 much colder than them. Ah metamorphic don't move around like that. So

175:24 a metamorphic basin and they're cooling it's just because of erosion, um

175:31 rocks cool off very fast and we always decide if a rock is volcanic

175:38 how fast it's eroding, you so when we see a or or

175:44 eroding how fast it's cooling And we apply that same understanding of fast cooling

175:51 um active tectonic regions where you have big fault moving really rapidly and cooling

175:56 a whole bunch like when we saw in Denali that mountain range goes up

176:00 20,000 ft and a lot of, all the same age. So it's

176:04 surprising that one of the tallest mountains the world has experienced a lot of

176:09 since four million years ago. so those are all the different ways

176:17 which rocks cool off. And all these then are valuable in assessing the

176:25 history, the detailed structural history, subsequent erosion and deposition, all history

176:33 various places. And by choosing the thermal chronometer, we can work on

176:41 problems. If you want to know this sandstone came from, uranium lead

176:45 , because that's gonna tell us about ultimate age of these places where it

176:49 from. But if you don't care where it came from, you just

176:53 to know what happened since it's been , You probably want to look at

176:57 in appetite. That's the 70°. So on your question, depends on your

177:07 . So have a review of all things. Um because we can go

177:12 them on next friday, we'll have sense of how to use them.

177:17 you know, come come with a , you know, giving you let's

177:21 go back to uh is it, it in this diagram? I

177:27 No, it's not here. But , I've given you that diagram several

177:31 where you've got the whole range of chronometer uranium lead Argon fishing track

177:39 you've got a list of maybe, don't know, 15 of them.

177:45 your temperature of argon and close your of fission tracks and appetite. Close

177:50 temperature of lead in zircon. These all things you need to know.

177:56 gone through how it is, we these things and some of the particulars

178:00 how we make these measurements. Some the particulars of how we need to

178:03 about system. Any questions Okay? have a question during the week.

178:15 me an email. Um, otherwise can pack it up for today and

178:23 , start again on friday with the of this and then the exercises.

178:29 .

-
+