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The Room Problem (and how Geochronology solves it) · PlanetGeo: The Geology Podcast

• 0:02
  SPEAKER_00: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:13
  SPEAKER_00: Dr.
• 0:13
  SPEAKER_00: Mike Aggerson.
• 0:14
  SPEAKER_00: Welcome back.
• 0:15
  SPEAKER_01: How are you doing?
• 0:16
  SPEAKER_00: I'm great, man.
• 0:17
  SPEAKER_00: I'm great.
• 0:17
  SPEAKER_00: I get to talk about granites with you.
• 0:19
  SPEAKER_00: This is so fun.
• 0:20
  SPEAKER_00: Uh, what could be wrong in the world?
• 0:22
  SPEAKER_01: We left off your subscriber numbers.
• 0:27
  SPEAKER_01: Plummeting.
• 0:28
  SPEAKER_00: Just straight plummeting.
• 0:30
  SPEAKER_00: People, people, you know, click on it and like, oh, it's Mike and Granites.
• 0:35
  SPEAKER_00: Now I'm out.
• 0:35
  SPEAKER_00: Granite.
• 0:36
  SPEAKER_01: Great.
• 0:37
  SPEAKER_01: Okay.
• 0:37
  SPEAKER_00: That would be interesting to look at the uh listener.
• 0:40
  SPEAKER_00: I don't actually know if we get that.
• 0:42
  SPEAKER_00: Like, I wonder if we can find there must be a way to find off when listeners drop off.
• 0:45
  SPEAKER_00: And as soon as we talk about granites, more granites.
• 0:50
  SPEAKER_00: Second 29 people are jumping off.
• 0:53
  SPEAKER_00: Well, uh, we have, in all seriousness, we have been doing uh a series and we've got much more of a series to go on granites.
• 0:59
  SPEAKER_00: We're kind of taking a deep dive.
• 1:01
  SPEAKER_00: This is, I would say this is like upper level undergrad.
• 1:05
  SPEAKER_00: Maybe depending on where you go to undergrad, it might be grad early grad student level uh igneous petrology, but definitely upper level undergrad in most places.
• 1:14
  SPEAKER_00: So that's the kind of the level we're pitching here.
• 1:16
  SPEAKER_00: If you want to get some of the basics, like hey, what is a granite?
• 1:19
  SPEAKER_00: What are these guys talking about?
• 1:20
  SPEAKER_00: Feldspar, Zircon, what are they talking about?
• 1:22
  SPEAKER_00: Are those minerals?
• 1:23
  SPEAKER_00: Are those rocks?
• 1:23
  SPEAKER_00: What's the deal?
• 1:24
  SPEAKER_00: You can go to our Camp Geo Mobile app, first link in the show notes there.
• 1:27
  SPEAKER_00: We have a bunch of introductory stuff there.
• 1:29
  SPEAKER_00: Um, but we left off last time, Mike.
• 1:32
  SPEAKER_00: You gave this beautiful history of the field of granite petrology and many of the debates and arguments, and how those are really debates about the founding of the field of geology in many ways.
• 1:43
  SPEAKER_00: And that was really fun discussion.
• 1:46
  SPEAKER_00: We left off there talking about the room problem or the space problem.
• 1:51
  SPEAKER_00: And uh maybe it'd be good if you just give us a recap of of that problem.
• 1:56
  SPEAKER_01: Sure, yeah.
• 1:56
  SPEAKER_01: The uh the the cliffhanger from our last conversation was this idea of the room problem, right?
• 2:01
  SPEAKER_01: So to bring people up to speed, the room problem is the idea that if you are uh taking a magma, creating a magma somewhere in the crust or the upper mantle or something, and bringing it up into the upper crust, you need to create space for that magma in the crust.
• 2:18
  SPEAKER_01: And really this came from the last kind of big scientific shift you and I were talking about, which was granitizers versus magmatists.
• 2:27
  SPEAKER_01: So people that thought that granitic rocks can be produced through uh water flowing through the system, which are the granitizers versus magmatists, which are the people like Norman Bowen who made the observations that you could produce uh granites through magmatism and crystal fractional crystallization.
• 2:46
  SPEAKER_01: So the magmatic perspective took hold in the mid-early to mid-20th century, and that then brings with it the room problem again.
• 2:56
  SPEAKER_01: So the granitizers thought they had the room problem solved, but turns out most granites are igneous rocks in some form.
• 3:04
  SPEAKER_01: And so then you've got the room problem again.
• 3:06
  SPEAKER_01: So if granites are magmas, how do you create room for a magma in the upper crust, which is where we know a lot of the granites reside?
• 3:14
  SPEAKER_00: Yeah.
• 3:15
  SPEAKER_00: And so to kind of paint the visual of this is uh let's let's imagine, you know, Yosemite National Park.
• 3:21
  SPEAKER_00: This is the rocks under your feet in Yosemite National Park.
• 3:24
  SPEAKER_00: You can, I mean, most people I think can picture Yosemite National Park.
• 3:26
  SPEAKER_00: It's Half Dome, uh, these beautiful I think it was one of the Mac computer, you're not a Mac guy, but one of the original Macs, oh the default background was Yosemite National Park.
• 3:37
  SPEAKER_00: Um that shipped in like you know the 20 2010s or something like that.
• 3:41
  SPEAKER_00: Um but you you picture Half-Dome, beautiful U-shaped valley, you know, basically mountain cut in half by a glacier, Yosemite National Park.
• 3:48
  SPEAKER_00: The rocks underfoot are the Tawalumni Intrusive Suite, which you have worked a lot on, Mike.
• 3:55
  SPEAKER_00: Um and is a big, it's a very famous, it's like I mean, it's like the template for one of the templates.
• 4:03
  SPEAKER_00: It's one of these areas that people have studied for a long, long, long, long time.
• 4:06
  SPEAKER_00: It's like the fruit fly.
• 4:08
  SPEAKER_00: We use this analogy when we were talking about the bishop tough, as the bishop tuff is the fruit fly of uh volcanism, and the Tuami Trusa Suite is like the fruit fly of igneous petrology, right?
• 4:18
  SPEAKER_00: Like it's valuable because so many people have studied it before, it becomes increasingly more valuable for every study that's done on it.
• 4:24
  SPEAKER_00: But if you're walking across, okay, you're walking across this entire national park is like granite, and you walk across it, it all looks the same.
• 4:31
  SPEAKER_00: It's easy to imagine.
• 4:32
  SPEAKER_00: If this is all magma all at one time, it's a huge amount of space, and that's the room problem.
• 4:39
  SPEAKER_00: Like, how do you get room in the crust for liquid, a big liquid vat of magma that's tens of kilometers wide, a couple kilometers thick?
• 4:48
  SPEAKER_00: It's a huge batch of magma.
• 4:50
  SPEAKER_01: Huge batch of magma.
• 4:52
  SPEAKER_00: Yeah, if it intrudes all at one time, and if it's all magma, we have a room problem.
• 4:58
  SPEAKER_00: And what we talked about before was like the the knob of if it's all if it's magma, right?
• 5:04
  SPEAKER_00: The granitizers are saying it's not magma actually, the magma just saying it is magma actually, and and that was the knob that we were discussing.
• 5:11
  SPEAKER_00: But but time is a part of this, right?
• 5:16
  SPEAKER_01: Sure, sure.
• 5:17
  SPEAKER_01: It's absolutely part of it.
• 5:18
  SPEAKER_01: And you know, the Tuolie me intrusive suite that that we're talking about, much like other similar rocks in the Sierra Nevada of California or other places across the globe, you can actually walk around the edges of these rocks and you can see where the granites are interacting with the pre-existing wall rock and other granites.
• 5:39
  SPEAKER_01: So you could, yeah, boots on the ground, you could walk around and trace the outline of the Tuyumi Intrusive Suite.
• 5:45
  SPEAKER_00: Okay, I want to maybe you're going here, but I want to ask the question why the name Tuamni Intrusive Suite?
• 5:49
  SPEAKER_00: What does that mean?
• 5:51
  SPEAKER_00: The suite part of it.
• 5:52
  SPEAKER_01: Yeah, so you're like a student who asks the question that's coming up on the next slide.
• 5:57
  SPEAKER_00: Sorry.
• 5:57
  SPEAKER_00: Really annoying, extremely irritating.
• 6:00
  SPEAKER_00: Yeah.
• 6:00
  SPEAKER_01: No, not at all.
• 6:02
  SPEAKER_01: Um so you know, if you go out to Tuanyi, you can walk around, you can see the perimeter, you can define the perimeter of the entire intrusive suite.
• 6:12
  SPEAKER_00: What's around the intrusive suite?
• 6:14
  SPEAKER_01: In some places, it's older igneous rocks, older granites.
• 6:18
  SPEAKER_01: Like uh the L Cap granite, which is also in Yosemite Valley, is older than the Tuolumee intrusive suite.
• 6:25
  SPEAKER_01: Uh-huh.
• 6:25
  SPEAKER_01: So you can actually kind of map the contact between the Tuolumene and the L-Cap granite.
• 6:31
  SPEAKER_01: On the eastern side, the northeastern side of the Tuolumni intrusive suite, you can actually walk uh the contacts on this amazing place like called Saddlebag Lake, where you can hike and you can see where the igneous rocks are intruding into uh sedimentary, meta-sedimentary rocks and things.
• 6:47
  SPEAKER_01: It's just that's a beautiful, but you can actually uh see these contacts and you can walk the contacts through these rocks.
• 6:53
  SPEAKER_00: Amazing.
• 6:53
  SPEAKER_00: Uh-uh.
• 6:53
  SPEAKER_01: And it is, yeah, like you're saying, it's tens of kilometers.
• 6:56
  SPEAKER_01: It's it's a massive uh scale of this plutonic system.
• 6:59
  SPEAKER_01: But why it's called a suite, or some people call it a complex, uh, is because if you walk from the outside of the Tawang Me to the inside, the compositions of the granites change.
• 7:10
  SPEAKER_01: They are uh have more dark material or mafic material on the outside, and they as you go interior, the rocks uh get lighter or more felsic.
• 7:23
  SPEAKER_01: So more mafic, darker on the outside, more felsic, uh lighter and the interior.
• 7:29
  SPEAKER_01: And so you can see this over this broad period that you go from darker rocks to lighter rocks.
• 7:34
  SPEAKER_01: So this came with it then this idea from a lot of geologists.
• 7:38
  SPEAKER_01: One of the more famous papers is from these guys, Bateman and Chapel, a couple of legends in the field, where they basically said, Okay, well, you had these uh older magmas, like a huge batch of magma intruding into the upper crust that was uh older and more mafic, and then younger and more felsic stuff was intruded to the interior.
• 7:59
  SPEAKER_01: But we're talking huge batches of magma on the scale of something like a bishop tough or larger.
• 8:04
  SPEAKER_01: So huge volumes of magma intruded into the upper crust, kind of one Pluton, one magma chamber gets injected, another magma chamber gets injected, and that's how you have like three or four or five pulses of like magma coming in to form these huge bodies.
• 8:22
  SPEAKER_01: So, like most people, I don't think, ever really thought that the whole twailum was at one point like one huge magma chamber that was injected at once, but still huge magma chambers injected over a couple of pulses.
• 8:37
  SPEAKER_00: So the individual units here, and there's five dominant units that that we'll talk about, I mean, broadly that are that have been broken apart.
• 8:44
  SPEAKER_00: There's subdivisions in there as well, but but broadly a couple of different units here in this suite.
• 8:50
  SPEAKER_00: And this is a very common feature uh amongst granite complexes.
• 8:54
  SPEAKER_00: These are what's called zoned plutonic complexes, they occur all over the world.
• 8:59
  SPEAKER_00: Uh they're very common where you have sort of more mafic on the outside, then intermediate in the in the middle, and very felsic in the interior.
• 9:06
  SPEAKER_00: And this has led to, I think, at the very intro level, like the the intro level textbook version, is that yeah, this makes sense, right?
• 9:15
  SPEAKER_00: Like high temperature magmas are more mafic, typically.
• 9:20
  SPEAKER_00: And so the first batch of magma is mafic, and then maybe there's some magma source down below, a big magma chamber down at depth that is the source of this.
• 9:28
  SPEAKER_00: And as that cools and fractionally crystallizes, things go from mafic to intermediate to felsic, and these zoned plutonic complexes are kind of aggregating, you know, they're they're recording pulses of that different magmatic system down below.
• 9:41
  SPEAKER_00: So maybe the magmatic, the fractional crystallization is not happening in the upper magma chamber, but the upper magma chamber is recording processes that are happening down below, and that would be basically fractional crystallization.
• 9:54
  SPEAKER_00: A high temperature thing, a mafic magma intrudes at depth, punches a little bit of mafic stuff up, then it cools down, forms more andesitic intermediate composition stuff, punches that back up to the crust, that cools down.
• 10:06
  SPEAKER_00: Basically, what you're left with when you've got, I don't know, 20% of the magma left, you're left with something felsic, and you that stuff can also move up into the upper part of the crust.
• 10:15
  SPEAKER_00: And these are really common complexes globally, and we often see this is a very common feature, these kind of ringed, you know, mayfic on the outside, felsic on the inside, intermediate in between.
• 10:25
  SPEAKER_00: That's that's pretty common.
• 10:26
  SPEAKER_00: So, anyway, the the point here that we've we talked about previously that the magmatists and the granitizers and the you know, they were debating about whether it was magma or not.
• 10:39
  SPEAKER_00: But another like escape hatch for the room problem is time because it's easy.
• 10:44
  SPEAKER_00: And for me, this was all the way till I think I referenced this.
• 10:46
  SPEAKER_00: Like this was maybe year two in graduate school when I was like, wait, magmas or igneous rocks don't crystallize all at effectively the same time, you know?
• 10:58
  SPEAKER_00: Like it's not a single pulse of obviously it takes a bit of time, but I was always like, Oh, it happens basically geologically instantaneously.
• 11:04
  SPEAKER_00: No, it takes a long time, right?
• 11:07
  SPEAKER_00: And it takes actually an incredibly long time, and that's what we're gonna focus our discussion on here.
• 11:11
  SPEAKER_00: The way out of the room problem is really time, it takes a long time to aggregate these things, and that's how we get around the room problem.
• 11:20
  SPEAKER_00: We're kind of giving up the the point of this episode here, but I think it's an interesting story in how we got there, right?
• 11:25
  SPEAKER_00: Like we're gonna center our discussion on a key paper.
• 11:28
  SPEAKER_00: What's the key paper?
• 11:29
  SPEAKER_00: It's near and dear to your heart, I know, Mike.
• 11:32
  SPEAKER_01: Oh, yes.
• 11:32
  SPEAKER_01: Uh Coleman uh et al.
• 11:35
  SPEAKER_01: 2004 from the journal Geology, I believe.
• 11:38
  SPEAKER_00: Yeah, it's in it's in geology.
• 11:40
  SPEAKER_00: Um, this was kind of one of the framework papers or the initial papers that kind of documented time in this particular igneous complex Tatuami intrusive suite.
• 11:51
  SPEAKER_00: If you'll allow me, uh Mike, I think there's an interesting story here in the history of geochronology, in a way, of how we got the ability to measure time in a magmatic system.
• 12:02
  SPEAKER_00: Like it took a lot of technique development, and this was not possible.
• 12:05
  SPEAKER_00: So you're the paper we're referencing is a 2004 paper.
• 12:09
  SPEAKER_00: This type of science was probably not possible in 1970 or 1960.
• 12:16
  SPEAKER_00: The techniques did not allow it.
• 12:18
  SPEAKER_00: And you made a reference uh last episode about like how as our techniques get better, we get basically we get to ask better questions um because the techniques allow it, and this is a great example of that.
• 12:29
  SPEAKER_00: Like geochronology, so I I I mean, measuring time in magma chambers is uh a hot field.
• 12:37
  SPEAKER_00: I mean, it's something we do very frequently, lots of different labs are doing it now, but it used to be really hard, and I think that was part of the problem here.
• 12:45
  SPEAKER_01: Yeah, it is, it is.
• 12:47
  SPEAKER_01: I I will uh if you if you'll allow me to ask you not to call them magma chambers.
• 12:52
  SPEAKER_00: Okay, what should I call them?
• 12:53
  SPEAKER_01: Magmatic systems or something.
• 12:54
  SPEAKER_01: You know, I I'm just I get a little, you know, because a magma chamber, magma chamber evokes the same thing that, you know, eventually I think we're gonna be talking about is maybe not what is really happening.
• 13:05
  SPEAKER_01: Okay, so oh mag magma chamber evokes churning vat of magma.
• 13:09
  SPEAKER_00: All right, all right.
• 13:10
  SPEAKER_01: It's a magnetic system.
• 13:12
  SPEAKER_00: The word magma chamber evokes the room problem, and we shouldn't be talking about that.
• 13:16
  SPEAKER_00: Okay, perfect.
• 13:16
  SPEAKER_00: That sounds good.
• 13:17
  SPEAKER_00: Let's come back to that point.
• 13:18
  SPEAKER_00: I won't call it a magma chamber again.
• 13:20
  SPEAKER_00: I'll call it a magmatic system.
• 13:21
  SPEAKER_01: I call it a magma chamber like this.
• 13:23
  SPEAKER_01: You know, like the future marks out here.
• 13:25
  SPEAKER_01: For for the podcast listeners, quotation marks your own magma chamber.
• 13:29
  SPEAKER_00: Okay, we'll do that.
• 13:30
  SPEAKER_00: All right.
• 13:30
  SPEAKER_00: Um, I won't do that, and we'll come back to like what are what we should visualize when we think of a magmatic system, um, and what this data and some other field data uh has told us Coleman 2004, and that kind of kicked off a boom in geochronology uh in this field.
• 13:47
  SPEAKER_00: So the the history of geochronology, I'm gonna give some dates, Mike, if you're okay with that.
• 13:51
  SPEAKER_01: Oh, yeah, please.
• 13:52
  SPEAKER_01: This is your bailiwick right here.
• 13:55
  SPEAKER_00: I I like this stuff.
• 13:56
  SPEAKER_00: It's fun to talk about, and it's I don't know, the techniques involved are amazing, I find.
• 14:01
  SPEAKER_00: Oh, incredible.
• 14:02
  SPEAKER_00: Geochronology is a very old field, and it was kind of came into being.
• 14:07
  SPEAKER_00: The first radiometric dates were actually from uh Ernest Rutherford, Ernie, in 1904, very soon after the discovery of radioactivity.
• 14:15
  SPEAKER_00: Um, with these were helium-uranium dates, and so helium was determined to be a product of the decay of uranium-238 at that time, and so the obvious thing was hey, we can measure helium and uranium, and that's like a chronometer.
• 14:28
  SPEAKER_00: Um, they didn't really know the uranium-238 decay rate very well.
• 14:32
  SPEAKER_00: Helium is produced a whole bunch of times throughout the decay, but they didn't know that, so they're really rough numbers.
• 14:37
  SPEAKER_00: But they're 1904, that's really impressive.
• 14:39
  SPEAKER_00: A full century before this magma chamber paper.
• 14:42
  SPEAKER_01: Oh, uh, I didn't know this is you know, this is this is all new information for me because I'm not a geochronologist, but like the fact that they were immediately thinking about how you could use decay to measure time is really cool.
• 14:56
  SPEAKER_00: Like it's really cool.
• 14:58
  SPEAKER_01: They're thinking about radioactivity, right?
• 14:59
  SPEAKER_01: They're like, wow, this something's happening.
• 15:01
  SPEAKER_01: You know, we're discovering alpha particles, and immediately we're like, okay, how can you use this to make to measure time?
• 15:07
  SPEAKER_01: Like, that's super cool.
• 15:08
  SPEAKER_00: This was in the time period where you know Lord Kelvin was trying to calculate the age of the earth from cooling rates, just simple cooling models, and they didn't appreciate that Earth was generating its own heat by radioactivity.
• 15:20
  SPEAKER_00: Um, and other people were trying to calculate the age of the earth by using the saltiness of the ocean and comparing the salinity of rivers to the saltiness of the ocean and saying, like, that's the age of the earth, right?
• 15:31
  SPEAKER_00: So this was this was a wide open question.
• 15:34
  SPEAKER_00: So any amount of data would would have been useful in this scientific discussion.
• 15:38
  SPEAKER_00: Um, very soon after, uranium lead was used to date uranium ores, so uranium-rich ore deposits.
• 15:45
  SPEAKER_00: You could measure uranium and lead from those, just chemistry.
• 15:48
  SPEAKER_00: The age of the earth in 1913, Arthur Holmes started to actually develop some techniques that allowed for the measurement of zircon and feldspar, individual, not individual mineral grains, but those minerals in particular, not minerals that are loaded with uranium.
• 16:03
  SPEAKER_00: And then we really get into mass spectrometry improvements.
• 16:06
  SPEAKER_00: In 1919, Aston builds a mass spectrograph in which you can measure individual.
• 16:11
  SPEAKER_01: Sorry, can I just ask a question there just to get myself on track?
• 16:14
  SPEAKER_01: So you're saying all these things up to this point, they're just bulk measuring the elements.
• 16:19
  SPEAKER_00: So the element, it's chemistry.
• 16:20
  SPEAKER_01: Uranium and lead concentrations in a big old vat of zircons that they dissolved or something.
• 16:25
  SPEAKER_00: This is all just chemistry.
• 16:27
  SPEAKER_00: So it's like separate uranium and separate lead from the minerals of interest and measure the concentrations of each.
• 16:33
  SPEAKER_00: It's not anything to do with isotope geochemistry, which is using the specific isotopes.
• 16:39
  SPEAKER_00: That comes around in 1919 when Aston builds the this mass spectrograph, which um could separate the different isotopes from one another.
• 16:49
  SPEAKER_00: And very soon after that, in the 1920s, analyzed lead isotopes, and this led to the discovery of the 235 uranium isotope because they're looking at lead and said, Oh, wait, there's something called 207 lead here.
• 17:00
  SPEAKER_00: There's a 207 lead mass.
• 17:02
  SPEAKER_00: Where does that come from?
• 17:04
  SPEAKER_00: Looked at uranium and oh wait, there's a tiny little mass of uranium-235 next to a big 238 signal.
• 17:11
  SPEAKER_00: And then in the late 30s, Nier improved this and makes effectively the template for the modern mass spectrometer and determines that the Earth contains lead that is not produced by the decay of uranium, basically.
• 17:23
  SPEAKER_00: And then we get into this World War II era and and all of the nuclear physics that goes on associated with that after the discovery of fission, fission of uranium atoms and the focus on uranium-235 in the Manhattan Project leads to this ability to separate isotopes from one another.
• 17:40
  SPEAKER_00: So all of the focus on building nuclear weapons was focused on separating uranium-235 from uranium-238 and plutonium from both of those.
• 17:49
  SPEAKER_00: But 235, that that technology allowed for basically an explosion of geochronology use cases.
• 17:55
  SPEAKER_00: Uh, and in large part because we could separate individual isotopes from one another, we could then use those separated, those pure separated isotopes as a spike.
• 18:06
  SPEAKER_00: And we could put that into our sample and say, oh, we're we're doping our sample with this pure isotope so that we can trace that throughout our entire chemical and analytical process, and we can use it to calibrate the ages.
• 18:19
  SPEAKER_00: Before that, we were kind of dealing with uncalibrated measurements.
• 18:22
  SPEAKER_00: Um, and so this is just a way to do what's called isotope dilution.
• 18:27
  SPEAKER_01: Can I pause you again to ask a couple more nerd questions?
• 18:30
  SPEAKER_00: Because this might hold on.
• 18:31
  SPEAKER_00: This interlude might become a fairly large interlude if this keeps going on.
• 18:35
  SPEAKER_00: But yes, please.
• 18:36
  SPEAKER_01: No, no, no.
• 18:36
  SPEAKER_01: Well, I mean, you know, I'm the audience here.
• 18:38
  SPEAKER_01: I don't know anything about this stuff.
• 18:40
  SPEAKER_00: Okay, perfect.
• 18:41
  SPEAKER_01: So, like, you know, you're talking about was it near, you said, um, doing these measurements and stuff.
• 18:47
  SPEAKER_00: The mass spectrometer, developing mass spectrometer, yeah, yeah.
• 18:50
  SPEAKER_01: So developing mass spectrometers, but were that they weren't separating for for during World War II, they were using they weren't using mass spectrometers to separate isotopes.
• 19:00
  SPEAKER_00: Well, it's kind of yes, in a way.
• 19:02
  SPEAKER_00: I mean, one of the mechanisms for separating 235 from 238 was uh the calutron, which is the Berkeley cyclotron, the cal cyclotron uh name.
• 19:14
  SPEAKER_00: Um, and and this is effectively a mass spectrometer.
• 19:17
  SPEAKER_00: It's an extremely large mass spectrometer that can separate the uranium isotope.
• 19:20
  SPEAKER_00: So that was one of the ways.
• 19:22
  SPEAKER_00: I forget which plant might have been Oak Ridge that did that used the calutron method.
• 19:26
  SPEAKER_00: There are other gaseous diffusion methods that um you know relied on uranium-235 diffusing slightly faster than uranium-238.
• 19:34
  SPEAKER_00: Uh there's anyway, that that's a little bit outside, but there are some of them were using mass spectrum, effectively mass spectrum.
• 19:40
  SPEAKER_00: Ionize a thing, spin it through a big magnet, and you get basically the mass separation, then collect it.
• 19:46
  SPEAKER_00: Just put a big bucket at one side and collect all of it, and you'd have to do that a couple cycles through to get pure stuff.
• 19:51
  SPEAKER_00: I mean, it's amazing, right?
• 19:52
  SPEAKER_00: I mean, but that is so cool.
• 19:54
  SPEAKER_00: So I mean, like, this is it's amazing.
• 19:55
  SPEAKER_00: So I've heard stories that when you turn these isotope separator machines on, when you are trying to separate a pure isotope of one element, you're usually after the smaller isotope, like not the big isotope.
• 20:08
  SPEAKER_00: And when you turn it on, the intensity of the ion beam is so big that you can like see it glowing sometimes in space.
• 20:17
  SPEAKER_00: Like you can see the like they they are very bright ion beams.
• 20:22
  SPEAKER_00: Like there's this huge amount of material going through these things, which makes it it's amazing technology.
• 20:28
  SPEAKER_00: I would love to see some of these things.
• 20:30
  SPEAKER_00: Uh, go to a national lab and see these things, you know, producing some of these uh isotopes.
• 20:35
  SPEAKER_00: It's just awesome.
• 20:36
  SPEAKER_00: I mean, it's such cool technology.
• 20:38
  SPEAKER_01: You know, I guess it just gives me so much respect for the number of extremely smart people whose time went into this stuff because you know, we're sitting here saying, Oh, this helps resercons, but like this is like buoyed entire economies.
• 20:54
  SPEAKER_00: They were doing this before computers, they were doing this with like extremely compared to modern stuff, rudimentary calculation technologies.
• 21:03
  SPEAKER_00: This is like they they were using they were like dumping you know magnetic sand onto the template and and seeing how it aligned to map out magnetic fields and stuff.
• 21:13
  SPEAKER_00: I mean, this is incredible development.
• 21:15
  SPEAKER_00: Like to do this type of work to develop these things is nuts.
• 21:18
  SPEAKER_00: I have so much respect for these people.
• 21:20
  SPEAKER_01: Well, let's just ionize a bunch of extremely rare stuff and then just shoot it through a magnet and separate it by mass.
• 21:26
  SPEAKER_01: Like that is where it goes.
• 21:28
  SPEAKER_00: I mean, but the fundamentals are so they worked out these fundamental equations and then applied them really strategically.
• 21:34
  SPEAKER_00: It's it's a it's an exceptional thing.
• 21:36
  SPEAKER_00: So that the post-war era led to isotope dilution.
• 21:40
  SPEAKER_00: We had really enriched isotopes of a single uh a single isotope of uranium 235.
• 21:45
  SPEAKER_00: We could have a pure uranium 235 thing, and we could put little pieces of that into our sample, and therefore we could know much more precisely how much uranium was in our sample with far smaller uncertainties.
• 21:58
  SPEAKER_00: Um, same thing with lead, and lead is.
• 22:00
  SPEAKER_00: Kind of the big one for the for uranium blood geochronology.
• 22:03
  SPEAKER_00: In the the post-war era, Tilton and Patterson, who were both at the University of Chicago as as PhD students, they were working kind of in tandem on mass spectrometer development and in clean lab technique development to do to work on meteorites basically.
• 22:17
  SPEAKER_00: But they they developed tech clean lab chemistry techniques that allowed for much purer sampling and means you needed less sample to go in.
• 22:27
  SPEAKER_00: And here's kind of the the grain sizes basically.
• 22:38
  SPEAKER_00: So you had to have a sample that had a ton, you had to have a huge sample with a lot of lead in it to make a measurement, just a single measurement.
• 22:46
  SPEAKER_00: Wow.
• 22:47
  SPEAKER_00: Nier in 1938 needed 10 milligrams.
• 22:51
  SPEAKER_00: So, you know, several orders of magnitude less.
• 22:54
  SPEAKER_00: That's about two grains of rice.
• 22:55
  SPEAKER_00: That's still a ton of lead by modern standards.
• 22:58
  SPEAKER_00: Like two grains of rice of pure lead.
• 23:00
  SPEAKER_00: That's not the full sample, that's just lead.
• 23:02
  SPEAKER_00: So you couldn't measure you if you wanted to measure feldspar, you had to measure, you had to dissolve tons of feldspar to get that much lead out.
• 23:10
  SPEAKER_00: Not tons, literally, but you had to measure a lot of feldspar.
• 23:13
  SPEAKER_00: Lots.
• 23:14
  SPEAKER_00: Lots.
• 23:14
  SPEAKER_00: Yeah, lots.
• 23:15
  SPEAKER_00: Tilton and Patterson, they needed 10 micrograms, which is about the size of one small salt grain.
• 23:21
  SPEAKER_00: So so you know, increasingly they needed a less and less and less material to do this with.
• 23:28
  SPEAKER_00: In modern techniques are nanograms.
• 23:31
  SPEAKER_00: We're talking about the the weight of a human cell, is the amount of lead we need to make a measurement.
• 23:36
  SPEAKER_00: And so we can dissolve fragments of individual mineral grains and dissolve those and get precise measurements now.
• 23:43
  SPEAKER_00: I mean, it's crazy stuff.
• 23:44
  SPEAKER_00: So in 1955, uranium-235 was declassified from top secret, and so all this public, all these publications came out in in 1955-56 kind of era on this type of stuff.
• 23:55
  SPEAKER_00: Then in the 70s and 80s, we moved into like the nanogram range.
• 24:00
  SPEAKER_00: So now we're analyzing lead, the uranium and lead.
• 24:03
  SPEAKER_00: Lead is really the big one.
• 24:05
  SPEAKER_00: And the reason for this is there's a lot of lead in the environment.
• 24:08
  SPEAKER_00: There's lead in the air, there's lead in the water, there's lead kind of everywhere.
• 24:11
  SPEAKER_00: It's hard to get a lab clean enough that doesn't have any lead in it.
• 24:14
  SPEAKER_00: So a lot of the techniques were getting rid of the contaminant lead so you could analyze a lot smaller samples and get a useful precision out of it.
• 24:23
  SPEAKER_00: So get rid of the lead in the environment, get rid of the lead in your lab, get rid of the lead in your the whole sample workflow, which you know basically meant clean acids.
• 24:33
  SPEAKER_00: The development of Teflon, interesting, by DuPont was a big step forward here.
• 24:38
  SPEAKER_00: The just the invention of Teflon meant that you could do clean lab chemistry, you could dissolve individual zircon grains in a Teflon beaker very cleanly compared to before.
• 24:48
  SPEAKER_00: So there's a bunch of kind of technique developments in that era that led to the ability now to analyze in the early 2000s individual single grains of zircon, or usually like an aggregate, you'd collect like three or four zircon grains, or maybe five or six, and that would be one sample.
• 25:06
  SPEAKER_00: You would dissolve all that, dissolve it, separate out the uranium, separate out the lead, and then measure uranium and lead.
• 25:14
  SPEAKER_00: And that would be your sample, would be several zircon grains.
• 25:17
  SPEAKER_00: And in some cases, one zircon grain.
• 25:19
  SPEAKER_00: So just to calibrate, Mike, like not when we started our PhDs, but maybe like a decade before that in the early 90s, people used to collect a full five-gallon bucket of rock for one sample to separate zircons in order to analyze enough zircon to make a measurement, or to make like, you know, five or ten measurements from one sample.
• 25:42
  SPEAKER_00: Nowadays, we, you and I, we do this all the time in my state is we analyze tiny, tiny, tiny fractions.
• 25:50
  SPEAKER_00: We we analyze, take a sand grain and we poke holes in it that are extremely tiny and get ages out of it.
• 25:56
  SPEAKER_00: So the technique development has just been exceptional in the last 40 years.
• 26:02
  SPEAKER_00: Uh, in this 2004 paper we're going to talk about, you know, was 20 years ago already, and there's been technique development since then.
• 26:07
  SPEAKER_00: So um anyway, that that's kind of a short story of how we got to to 2004.
• 26:12
  SPEAKER_01: The amount of material in there, the the the fact that you were able to make that so brief is uh pretty impressive.
• 26:19
  SPEAKER_01: Gossed over about 100,000 PhDs right there.
• 26:23
  SPEAKER_00: That that's shorthand for that was really confusing.
• 26:25
  SPEAKER_00: You went way too fast, and nobody followed me.
• 26:27
  SPEAKER_01: No, no, no, not at all.
• 26:28
  SPEAKER_01: My brain is spinning, but in a good way.
• 26:30
  SPEAKER_01: Like, I mean, it's it's it's it's so cool.
• 26:34
  SPEAKER_01: I have a a question for you as somebody who's done some of this and does some of this really high precision wet chemistry stuff for with respect to lead.
• 26:46
  SPEAKER_00: Yes.
• 26:47
  SPEAKER_01: What makes you nervous about lead?
• 26:49
  SPEAKER_01: And I'm asking this as somebody who like I I do experiment, I'm an experimental petrologist.
• 26:54
  SPEAKER_01: We in the laboratory we we we actually use a lot of lead, like lead foil.
• 27:00
  SPEAKER_00: Oh, good, okay, perfect.
• 27:02
  SPEAKER_01: So, you know, like lead foil, you know, you wrap your experiments in lead foil or whatever.
• 27:06
  SPEAKER_01: And when I've been around Tim's people and talking about using lead foil, I see them recoil with smear over my common lead.
• 27:18
  SPEAKER_00: Okay.
• 27:18
  SPEAKER_01: So yes, does that make you nervous?
• 27:20
  SPEAKER_01: It seems like there's a lot of like um superstitions around doing this work, like not getting common lead in your lab.
• 27:26
  SPEAKER_00: Yeah, yeah, yeah.
• 27:27
  SPEAKER_00: Okay, so this is gonna get a little bit weedy.
• 27:30
  SPEAKER_00: My my real answer will get a little bit weedy.
• 27:33
  SPEAKER_01: Okay.
• 27:34
  SPEAKER_01: This is now finally we're gonna get weedy.
• 27:36
  SPEAKER_01: Yeah, yeah, now finally.
• 27:37
  SPEAKER_00: We haven't been there yet.
• 27:38
  SPEAKER_00: Um, no, I'll give, but I'll give you the the I'll give you a description of what clean lab people do and don't do in the lab to keep it clean.
• 27:46
  SPEAKER_00: So, first of all, people don't wear wedding rings into the lab because there's potential for metal contamination.
• 27:55
  SPEAKER_00: And actually, people often do not ever wear metal wedding rings because you know you can't clean that off your hand basically once you get in there.
• 28:02
  SPEAKER_00: Um, these clean labs are over pressured, so there'll be like an inner chamber that's at really high pressure and super clean, and that blows the all the air in there comes in through a bunch of filters, and that air blows out into an outer chamber, an outer lab that's like a slightly less clean room, and then that air blows into the the anterchamber that you go in to change into, and all that air is blowing out into the hallway, so hallway air never gets into the clean lab and never, never gets into the very, very clean lab in the center.
• 28:32
  SPEAKER_00: So that's kind of the we're worried about airflow for sure and clean air coming into the lab.
• 28:37
  SPEAKER_00: But when people are dissolving zircons, individual zircons, in their little tiny, tiny, tiny Teflon beakers, the extent that they go to, they'll buy new Teflon, brand new Teflon from the manufacturer.
• 28:48
  SPEAKER_00: They'll get it.
• 28:49
  SPEAKER_00: They will clean it for a year.
• 28:52
  SPEAKER_00: Literally clean it for a year, put very clean acid into it, put it in for two, two days or a week, put it in, boil it, take it out, put a new batch acid in, put it back in for two days or a week, take it out, do that over and over and over, like a hundred times over a year before they ever let a sample go into that Teflon.
• 29:10
  SPEAKER_00: Then when the Teflon has sample into it and the sample it the cap is open, they will never move something over the top of that thing.
• 29:19
  SPEAKER_00: In your little, you know, we're not only in the clean, clean, clean chamber, we're in the clean, clean, clean chamber in a hood that has clean, clean, clean, clean, clean, clean air blowing over the sample and out of the chamber.
• 29:30
  SPEAKER_00: So we're in like this super clean, super, super duper clean environment.
• 29:34
  SPEAKER_00: And they won't like pass the there's no metal anywhere, no metal tweezers or anything, but they won't like pass their hand over the beaker because the potential for lead to get in there is high.
• 29:45
  SPEAKER_00: And so the question is, why do you care so much about lead?
• 29:48
  SPEAKER_00: So when you're talking about walking around with lead foils, people see you as like you are like the Charlie Brown character who's just like giving off dust of lead, and like I bet they won't even shake your hand anymore, basically, because you're just like bleeding common, you're just bleeding lead dust around.
• 30:04
  SPEAKER_01: Yeah, you're not gonna see Galena on a on a desk of one of your colleagues.
• 30:08
  SPEAKER_00: Do not bring that anywhere nearby.
• 30:10
  SPEAKER_00: Um just this is not a lab I run.
• 30:13
  SPEAKER_00: My lab is not like this, uh is not this style of lab, just to be clear.
• 30:17
  SPEAKER_00: The the reason that it matters is because if you're making a sample measurement, we're very interested in the 207 lead to 206 lead ratio.
• 30:26
  SPEAKER_00: That's where the uranium-led your chronology power is um in some cases, or really the amount of 206 lead.
• 30:33
  SPEAKER_00: But in the zircon, in the ideal world, the zircon does not take any common lead, lead from the background.
• 30:41
  SPEAKER_00: We're using the term common lead.
• 30:42
  SPEAKER_00: That means lead from the environment.
• 30:44
  SPEAKER_00: It doesn't take any in when it grows, but there's always a little bit of common lead.
• 30:49
  SPEAKER_00: And we have to correct for that.
• 30:51
  SPEAKER_00: We have to like remove the 206, the amount of 206 that's from the background.
• 30:57
  SPEAKER_00: When we remove that, we don't exactly know what the composition of that lead is.
• 31:03
  SPEAKER_00: We don't know if it's like lead from the lab.
• 31:04
  SPEAKER_00: We don't know if it's lead from the the rock, we don't know if it's lead from the magma from which the zircon grew.
• 31:10
  SPEAKER_00: Like it could be from any one of those sources.
• 31:13
  SPEAKER_00: So when we do that correction, there's a lot of uncertainty in doing that correction, unless you know exactly where the lead came from.
• 31:20
  SPEAKER_00: And we never really know exactly what so you kind of anytime there's that common lead, there's like there's environmental lead in your analysis.
• 31:29
  SPEAKER_00: You you can correct for it, but it adds a big batch of uncertainty to the ultimate result of your analysis.
• 31:37
  SPEAKER_00: So that's why people are talking about picagrams of lead in their you know analysis as being like good background lead.
• 31:45
  SPEAKER_00: Like you want that a tiny, tiny, tiny, tiny amount of background lead as small as possible.
• 31:50
  SPEAKER_00: Um, so that's why people get scared of you and recoil in horror when you talk about it's not because of your scientific ideas, Mike, or your you know, opinions about other things.
• 32:00
  SPEAKER_01: Six of one, half of those are the other people.
• 32:02
  SPEAKER_01: Yeah, yeah, it could be one.
• 32:03
  SPEAKER_00: Anyway, we are very far from the the room problem right now.
• 32:07
  SPEAKER_00: So maybe let's let's like get back to the room problem here.
• 32:11
  SPEAKER_00: The room problem.
• 32:12
  SPEAKER_00: All right, so we're talking about how do we get time out of a magma chamber, right?
• 32:17
  SPEAKER_00: This is the whole rant, my whole rant about geochronology was to say that it took a lot of technique development and a lot of work to figure out techniques to get time measurements out of a magma, magmatic system.
• 32:29
  SPEAKER_00: I'm gonna stop saying magma chamber, I promise.
• 32:31
  SPEAKER_00: Out of a magmatic system.
• 32:33
  SPEAKER_01: I say it too.
• 32:34
  SPEAKER_01: Don't don't worry, but it's just it's a default when you're talking to like non like not your colleagues.
• 32:40
  SPEAKER_00: All right, we're getting time out of a magmatic system now.
• 32:42
  SPEAKER_00: 2004, uh, what happens, Mike?
• 32:45
  SPEAKER_00: What is the data that that helps us think about the room problem differently?
• 32:49
  SPEAKER_01: Yeah, so I mean that there's inklings, you know, when people are doing something, for example, like a thermal model.
• 32:55
  SPEAKER_01: If you know the amount of heat that's stored in a magma and you can inject it into the upper crust, and the upper crust is going to be relatively cold relative to the magma, you can do thermal modeling and determine like how likely it is that a magma can uh can stay in the upper crust for a long time.
• 33:14
  SPEAKER_00: And how long is that?
• 33:15
  SPEAKER_01: For like on the order of like less than a million years, they cool off really fast.
• 33:20
  SPEAKER_01: They cool off really fast because it's magma that's hot being intruded into something that's very cold.
• 33:25
  SPEAKER_01: So we're talking like something the size of Tuyumi significantly less than a million years.
• 33:30
  SPEAKER_01: So if if you injected the Tuyumi intrusive suite up into the crust at once, it would cool very, very fast.
• 33:38
  SPEAKER_00: Gotcha.
• 33:39
  SPEAKER_01: So there's this idea then, this kind of background idea that if the Tuolume intrusive suite is a magma chamber, then it should have if you you know, theoretically, if you measure zircons from the outside to the inside, you would think that they're older on the outside, younger on the inside.
• 33:57
  SPEAKER_01: Because that's kind of this conceptual framework, but that there would be very little time difference between them.
• 34:02
  SPEAKER_01: Basically, basically no time difference.
• 34:05
  SPEAKER_01: So the stuff on the interior uh crystallized maybe just a fraction, a couple hundred thousand years or up to a million years younger than the older stuff on the outside.
• 34:16
  SPEAKER_01: So that was our initial idea, like set set the groundwork.
• 34:19
  SPEAKER_01: I don't even know if if in Coleman, if the Coleman 2004, if they talk about like where where the baseline was, like maybe somewhere less than a million years for like the entire Tuang intrusive suite.
• 34:29
  SPEAKER_01: Yeah, this was like the idea for a quote unquote magma chamber, if the Tuang Mi was.
• 34:36
  SPEAKER_00: This is useful because the Tuangnee um intrusive suite, the the age of the broad age of emplacement is roughly around 90 to 80 million years that we know that now.
• 34:46
  SPEAKER_00: Um, but but that was kind of the age range.
• 34:48
  SPEAKER_00: And so just to bring this back to like when we could make these types of measurements, um to make a million-year uncertainty measurement, say that's broadly, you know, just over one percent, in order to resolve a one million year age difference between 90 and 91 million years, that's about I don't know, broadly a little over one percent.
• 35:08
  SPEAKER_00: That technology, we only could really start to do that in like around in the 1980s, we could do one percent probably, but but kind of just, and then that really started shrinking in you know, in the 1980s, into the 1990s, when you could start to answer questions at that one percent precision range.
• 35:27
  SPEAKER_00: That's why I went on this long tangent of like, how do we get to these ages to resolve the difference between 90 and 91?
• 35:34
  SPEAKER_00: You you had to be at like post-1980, 1990 to be able to make those measurements even.
• 35:40
  SPEAKER_00: So that's why it kind of took a while to start to think about this problem to apply geochronology to this type of problem.
• 35:46
  SPEAKER_01: Exactly.
• 35:46
  SPEAKER_00: And and actually, and my lab still, still my lab could never make those measurements.
• 35:51
  SPEAKER_00: Like, you know, laser ablation, not because my lab is bad, because we use a different technique, these in situ techniques, laser ablation, um, secondary mass spectrometry, they're they're nowhere near the 1% level now.
• 36:02
  SPEAKER_00: Whereas this isotope dilution thermal ionization mass spectrometry technique that is used in the the Coleman 2004 paper, and when you dissolve an entire grain or a fraction of a grain, you can make now sub 0.1% precision measurements.
• 36:16
  SPEAKER_00: So um we can get this is easily accessible now, and in 2004 was recently accessible.
• 36:23
  SPEAKER_01: Yeah.
• 36:23
  SPEAKER_01: And it took, you know, someone like Drew Coleman to be trained in a laboratory, do postdocs, get a lab, build a lab, and answer the questions that they wanted to answer, him and Alan Glaisner and John Bartley at UNC in um and in Utah to address this problem.
• 36:40
  SPEAKER_01: Right.
• 36:40
  SPEAKER_01: So, you know, so the technique was around, but it still took, you know, this is just a side note on the necessity for training people in science to produce new data.
• 36:50
  SPEAKER_01: You know, like you're saying, the technique's been around since the 90s.
• 36:53
  SPEAKER_01: Well, it took someone trained in that novel field to build their own lab to answer the questions they wanted to answer.
• 36:58
  SPEAKER_00: That's right.
• 36:59
  SPEAKER_00: That's right.
• 37:00
  SPEAKER_00: That's a great point.
• 37:01
  SPEAKER_00: Yeah, for sure.
• 37:01
  SPEAKER_00: Because there's plenty of other questions to answer when you have precision technique when you can make a geochronology measurement at 1% level.
• 37:08
  SPEAKER_00: There's a bunch of other stuff to do with that.
• 37:10
  SPEAKER_00: Um, so yeah, anyway, good point.
• 37:11
  SPEAKER_00: Okay.
• 37:11
  SPEAKER_00: All right, so 1%, the thermal modeling, which is pretty reliable, says a million years at most uh to cool down this a batch of magma this big broadly.
• 37:21
  SPEAKER_00: Right.
• 37:21
  SPEAKER_00: So that's the the default expectation.
• 37:23
  SPEAKER_00: If there's a room problem, that's the expectation.
• 37:25
  SPEAKER_01: So, you know, I think to a certain extent they were thinking when they went out to do this study that that ended up being this paper in 2004 that they were going to see relatively small timescales.
• 37:35
  SPEAKER_01: And you know, we have to have Drew on to answer this question, but um, but uh you know, I think there was at least a part of them that was thinking, okay, well, we have the resolution now for do really good analysis, we'll be able to see this very narrow time window between the outer units and the inner units.
• 37:52
  SPEAKER_00: Yeah.
• 37:52
  SPEAKER_01: And then, you know, what ended up happening is they saw, you know, seven to eight to depending on how you view some of those outer units, up to 10 million years of crystallization difference between those rocks.
• 38:06
  SPEAKER_00: So that's so let's get into the data a little bit here.
• 38:08
  SPEAKER_00: Uh, they take the the five different units, take a bunch of different samples from the five different units, extract zircons out of them.
• 38:14
  SPEAKER_00: This is in an era I went to their supplements.
• 38:16
  SPEAKER_00: This is in an era where they were doing some in some of the samples, they're doing single grain, one grain gets dissolved and that becomes one measurement.
• 38:24
  SPEAKER_00: In some of the samples, uh, the I'm presuming the zircons were smaller, so they needed multiple grains in each analysis, in each dissolution to get enough uranium and lead in the sample.
• 38:35
  SPEAKER_00: And so to make the measurement, to make an accurate measurement, so they did this for several different samples, you know, generated a bunch of data and saw a big age spectrum in between the different units, not only in between the different units, but within individual units, there are age ranges, substantially more than one million year age ranges, right?
• 38:56
  SPEAKER_00: So that at the face value, does to you, Mike, does that just effectively rule out a simple model of a big batch of magma intruding because of the thermal.
• 39:08
  SPEAKER_00: I mean, does it does it rule it out?
• 39:10
  SPEAKER_00: Does it basically change your opinion immediately?
• 39:13
  SPEAKER_01: Well, I mean, you know, I I that's a really hard question for me to answer because I am, as anybody who might not agree with me who happens to listen to this, will say I'm incredibly biased.
• 39:25
  SPEAKER_01: Yeah.
• 39:26
  SPEAKER_01: So like, you know, this paper came out in 2004.
• 39:29
  SPEAKER_01: You know, I started grad school in 2008.
• 39:31
  SPEAKER_01: So like with those people.
• 39:33
  SPEAKER_00: Yeah, yeah, yeah, yeah, yeah.
• 39:34
  SPEAKER_00: Measuring Titanites from the 12 meters.
• 39:36
  SPEAKER_00: Yeah, exactly.
• 39:37
  SPEAKER_01: So I'm I'm heavily, heavily biased by by that model.
• 39:41
  SPEAKER_01: But I will say when I went to UNC, I was talking to a colleague, an older colleague who had retired from petrologist from a different institution.
• 39:50
  SPEAKER_01: And uh he was like, Oh, you're working with those people, huh?
• 39:54
  SPEAKER_01: Why I I think that they definitely got something wrong there because their model makes no sense.
• 40:00
  SPEAKER_01: And so this is a grizzled petrologist within the field who studies uh you know silicic magmas like granite composition magmas, who basically was like, Oh, these people are full of bologna sausage.
• 40:13
  SPEAKER_01: Interesting, and I'm sure there are still plenty of people out there who can come up with ways to to think that this does not make sense.
• 40:21
  SPEAKER_00: Okay, this is interesting because I I I just plugged this question into Claude uh before we were recording.
• 40:26
  SPEAKER_00: Okay, it's which I love it.
• 40:28
  SPEAKER_00: Kind of which I don't know, it gives a fun.
• 40:30
  SPEAKER_00: I mean, I'm not sure if this is uh this is a bit of a sycophanti response, but I said, what is the impact of the Coleman et al.
• 40:36
  SPEAKER_00: 2004 geology paper on the geology, geochronology of the Twilight Munich suite?
• 40:39
  SPEAKER_01: All right, certainly it's gonna be a sycophant, yeah.
• 40:42
  SPEAKER_01: Claude Claude knows what you want to know.
• 40:43
  SPEAKER_01: Oh, let me tell you how amazing super paper.
• 40:47
  SPEAKER_00: Spectacular.
• 40:47
  SPEAKER_00: Well, it says it's one of the most consequential granite geochronology papers of the last few decades, right?
• 40:52
  SPEAKER_00: Okay, good.
• 40:54
  SPEAKER_00: It generated some pushback for sure, blah, blah, blah, lots of different stuff on here.
• 40:57
  SPEAKER_00: Uh, the 2004 papers, the citation that converted, quote unquote, Plutons as frozen magma chambers into quote unquote Plutons as incrementally assembled, rarely more than mushy systems.
• 41:08
  SPEAKER_00: I thought that was quite good.
• 41:10
  SPEAKER_00: I thought that was like a quite a good response.
• 41:11
  SPEAKER_00: That's pretty good, Claude.
• 41:13
  SPEAKER_00: I'll read it again.
• 41:14
  SPEAKER_00: The citation that converted Plutons as frozen magma chambers, i.e., the room problem, into Plutons as incrementally assembled, rarely more than mushy systems.
• 41:24
  SPEAKER_00: I thought that would that like framed it on a kind of quite nicely.
• 41:28
  SPEAKER_00: Um, and and then the last thing here, Mike, which you'll find interesting.
• 41:31
  SPEAKER_00: I I called our last episode that we talked about, uh, the the something along the lines of the Granite Wars, you know, like the the debates that framed the field basically.
• 41:39
  SPEAKER_00: And it said, um, for this framing, this is arguably the paper that started the modern phase of that war.
• 41:46
  SPEAKER_00: That was quite that was quite cool, right?
• 41:48
  SPEAKER_00: Started the paper that started the modern phase of the war.
• 41:50
  SPEAKER_00: I like that phrasing.
• 41:51
  SPEAKER_00: Anyway, um, it so this is like uh uh this is a relevant paper, right?
• 41:56
  SPEAKER_00: You you have people disagreeing with it at the time, you have people you know thinking hard about it, and it kind of it kind of turns the understanding or the assumption of magmas as pretty short-lived uh systems or these intrusive complexes as relatively short-lived systems into something that has to consider time, and time being much longer than we would have previously thought.
• 42:19
  SPEAKER_00: So it kind of alleviates this the room problem.
• 42:22
  SPEAKER_00: And why might can you give like paint a visual for why it alleviates the room problem for us?
• 42:29
  SPEAKER_01: Yeah, sure.
• 42:30
  SPEAKER_01: Well, so I hope that the conceptual model is relatively straightforward in that if you just visualize the crust as there's no gaping uh chasms in the crust, right?
• 42:42
  SPEAKER_01: I mean a mammoth cave is not sitting there waiting to be filled with a magma chamber, right?
• 42:46
  SPEAKER_01: There's no there's no there are gigantic vacuums of space in the crust.
• 42:51
  SPEAKER_01: Uh so you have to produce the room for that.
• 42:54
  SPEAKER_01: So if you have a massive tool, amine-sized injection coming in, they have to solve that with something.
• 43:00
  SPEAKER_01: So people came up with a whole bunch of mechanisms by which you could do that inflation of the crust or uh stoping, which we one thing that we're really good at as geologists is utilizing depth to our advantage because if something uh doesn't make sense with your model, you can just bury it deeper than you can observe it, right?
• 43:20
  SPEAKER_00: So like oh yeah, it's just down below.
• 43:22
  SPEAKER_00: Don't worry about that thing.
• 43:23
  SPEAKER_00: It's just down below.
• 43:24
  SPEAKER_00: So that's like a common thing.
• 43:25
  SPEAKER_00: We all do it.
• 43:26
  SPEAKER_00: That's extremely funny.
• 43:27
  SPEAKER_01: Well, we all do it.
• 43:28
  SPEAKER_01: So, you know, you can stop.
• 43:29
  SPEAKER_01: So you stoping means like taking huge chunks of wall rock and just dropping them to the bottom and like letting the magma fill in and circulating these big magma chambers.
• 43:38
  SPEAKER_01: Um, magma chambers, quotation marks.
• 43:41
  SPEAKER_01: So that is like how that model worked, but you still uh are really uh deficient in the room problem.
• 43:47
  SPEAKER_01: So what incremental emplacement does, which is uh a very elegant idea, is that instead of uh injecting all that magma or huge volumes of that magma at once into the crust, you can do it uh in smaller.
• 44:00
  SPEAKER_01: Increments over longer periods of time.
• 44:02
  SPEAKER_01: So on periods that align with plate tectonic models.
• 44:07
  SPEAKER_01: So when you have subduction going on at a place like this here in Nevada in 90 million years ago, you can operate subduction in a way that produces room in the upper crust that is on par with the long duration, small volumes of incremental emplacement.
• 44:28
  SPEAKER_00: Yeah.
• 44:29
  SPEAKER_01: So it kind of makes sense in like a holistic geochemistry, geochronology, physical model space.
• 44:36
  SPEAKER_01: This it all kind of ties together in a way that's much more digestible than trying to really bend backward to make sense of a big magma chamber.
• 44:46
  SPEAKER_00: Yeah, that's right.
• 44:47
  SPEAKER_00: And you kind of put things together.
• 44:49
  SPEAKER_00: Magma chamber in quotes.
• 44:50
  SPEAKER_00: Yeah, make in quotes.
• 44:52
  SPEAKER_00: Uh and the the interesting part about I think this data, and here's a a question for you, Mike, about and maybe I'm not sure if you know the answer to this, but within this, so in the uh intrusive suite, you have five different rock types, let's say, that are the zoned things, right?
• 45:07
  SPEAKER_00: More Mafik on the outside.
• 45:08
  SPEAKER_00: In the interior, you have the Johnson Grande Porphyry, which is um the the most felsic.
• 45:13
  SPEAKER_00: This duchronology not only said that the the different units were different times, but also said that within individual units there were different times recorded as well.
• 45:23
  SPEAKER_00: So for instance, the um I forget which one it it might have been, uh, but one of these units took maybe there's like a four-year age difference.
• 45:31
  SPEAKER_00: If you go from the outer part of the right, the I think it's four million for the half dome equagranular or half dome granar diarrhite.
• 45:39
  SPEAKER_01: Half dome.
• 45:40
  SPEAKER_00: If you go from the outer part of the half dome to the inner part of the half dome, which is one of these units, you get four million year age difference.
• 45:48
  SPEAKER_00: And so that struck me as as very interesting.
• 45:51
  SPEAKER_00: Even not only across a complex that is zoned and has multiple different rock types in it, is there a long time period?
• 45:56
  SPEAKER_00: But even within one of them, where there's one it looks like one kind of really homogeneous completely homogeneous, but one very similar rock unit across the entire thing takes a long time.
• 46:09
  SPEAKER_00: Which it was it more akin to like a sediment to me, that you got a sandstone, big sandstone package that had some depositional rate to it.
• 46:17
  SPEAKER_00: There's a long time between the top of the sandstone package and the bottom of the sandstone package.
• 46:21
  SPEAKER_00: Like there's time involved in that.
• 46:23
  SPEAKER_00: And in the same way here for this, do you do you view that as like the is that 20 years on?
• 46:28
  SPEAKER_00: Is that still the state of the art that these individual units have a big time difference between the outer and the inner parts of them?
• 46:36
  SPEAKER_00: Do you know the answer to that?
• 46:37
  SPEAKER_01: Yes.
• 46:38
  SPEAKER_01: So, well, I mean, you know, the more modern we get, the more people are probably going to disagree with me, but that's that's fine.
• 46:43
  SPEAKER_01: Hit us up in the comments.
• 46:45
  SPEAKER_00: Yeah, yeah, there you go.
• 46:46
  SPEAKER_00: Yeah.
• 46:48
  SPEAKER_00: Actually, comment, you know, constructive comments can go in the the comments, you know.
• 46:52
  SPEAKER_00: Uh, negative ones go to Mike's email.
• 46:54
  SPEAKER_00: You can find it, find it online.
• 46:56
  SPEAKER_00: Good luck.
• 46:57
  SPEAKER_01: Yeah, yeah.
• 46:58
  SPEAKER_01: So yeah, but no, I uh I I think it would be great if people commented on this.
• 47:02
  SPEAKER_01: And so I think you guys definitely definitely miss something here.
• 47:06
  SPEAKER_01: Look at these papers or whatever.
• 47:07
  SPEAKER_00: I think you're sure are are missing and oversimplifying for sure.
• 47:10
  SPEAKER_00: That'd be great.
• 47:11
  SPEAKER_00: Yeah, totally.
• 47:11
  SPEAKER_01: But yeah, so something like you know, you're talking about the half-dome granite diorite.
• 47:15
  SPEAKER_01: I think you know, it's hard to necessarily say how I don't know how the history of the mapping of this works specifically, but when you go, when you go out there, you know, if if I had, you know, next time you come to my office, we can put samples of each of these units on my desk.
• 47:31
  SPEAKER_01: And if you just look at one rock versus the other, they look very different.
• 47:36
  SPEAKER_01: Like the cathedral peak granadaurite candid places have potassium false bar crystals that are 10, 20 centimeters big.
• 47:45
  SPEAKER_01: And the the half-dome granadiorite has has big case bars and areas, and in other areas it doesn't.
• 47:53
  SPEAKER_01: Some rocks have a lot of amphiboles, some rocks have none.
• 47:56
  SPEAKER_01: And so you can see how when you're trying to map this out, you can kind of draw boundaries between Half-Dome, Cathedral Peak, and Half Dome and Glen Allen and Kuna Crest and Johnson Grande Porphyry.
• 48:09
  SPEAKER_01: But I'm telling you, when you walk out there, like when you come out there and you come out in the field with us, you can hike between Half Dome and Cathedral Peak.
• 48:18
  SPEAKER_01: And uh I you would be uh hard pressed to put your finger on a boundary.
• 48:25
  SPEAKER_01: You you really can't.
• 48:26
  SPEAKER_01: I mean, there's a lot of textures that you can see.
• 48:28
  SPEAKER_01: You you can see like some gradational repeating gradational areas in the half dome, but you I I mean, I would be you would be really hard-pressed to be able to put your finger on a a unit contact out there.
• 48:42
  SPEAKER_00: Oh, cool.
• 48:43
  SPEAKER_00: Okay.
• 48:44
  SPEAKER_00: So the fact that in this geochronology, when you know, this geochronology plot the so people can kind of visualize it, they're they sort of plot broadly the the age is on the y-axis here, going from like 95 million to 84 million or 85 million.
• 48:59
  SPEAKER_00: And then basically they plot units on the x-axis.
• 49:02
  SPEAKER_00: So from left to right, you're going from the outer units to the inner units.
• 49:05
  SPEAKER_00: And just the way this plot is constructed, it basically looks linear from 96 million years in the outer part, top left, to 84 million years in the the inner part, bottom right.
• 49:16
  SPEAKER_00: It's basically a straight line, it's not like a step function where you're in one unit, it's all the same age, and then it drops down in the next unit.
• 49:22
  SPEAKER_00: It's not discrete, it's kind of continuous.
• 49:25
  SPEAKER_00: And you're saying that makes sense in the field.
• 49:26
  SPEAKER_00: Like these are relatively continuous units across several of them at least.
• 49:30
  SPEAKER_01: Yeah, exactly.
• 49:31
  SPEAKER_01: So, you know, the question of like is like half dome, half dome to me, that's kind of uh more or less an artifact of previous mapping generations.
• 49:40
  SPEAKER_01: Like the Half Dome Graniteurite, I mean, I mean, again, that's something I'm sure a lot of people disagree with.
• 49:46
  SPEAKER_01: But if you uh if you think about the the size of the increments that are being in place, it's not on the scale, it's the the increment is not the size of the entirety of Half Dome.
• 49:57
  SPEAKER_01: Uh in it geochronologically, I I think it's much smaller packets of magmas that are are probably being overprinted by a bunch of other processes as well.
• 50:08
  SPEAKER_01: But you know, if you go out to Half Dome, you can see some areas where you can say, oh, I could convince myself that you know this 150, 200 meter thick uh package was like an increment of magma that was injected cooled.
• 50:22
  SPEAKER_00: Okay.
• 50:23
  SPEAKER_01: But the difference between maybe 150 to 200 meters thick uh package versus uh a two, three, four kilometer thick package is is a big difference.
• 50:32
  SPEAKER_01: And so that's that's I think where this incremental emplacement uh really made a big impact is just trying thinking about the volumes of magma as significantly smaller than like the map unit.
• 50:43
  SPEAKER_01: So Cathedral Peak, you know, in that model, Cathedral Peak would not be one gigantic magma pulse injected into a quote unquote magma chamber.
• 50:52
  SPEAKER_01: It's uh increments of much smaller volumes of magma being intruded into the core of this system over time.
• 51:00
  SPEAKER_00: And a system that remained broadly the region stayed hot for a very long time.
• 51:04
  SPEAKER_01: Warm, yeah.
• 51:05
  SPEAKER_00: And I might maybe we can um we can leave a bit of a cliffhanger here with another term, which might be magma mush.
• 51:14
  SPEAKER_00: Because I think this, you know, and I know there's people who don't like the magma mush, um, and you're one of them, but the mushiness kind of system, or I think it's a good like visual for for the uninitiator for people who haven't thought about these things.
• 51:28
  SPEAKER_00: Sure, we can argue about the term exactly, but like imagining not imagining a magma as a big vat of m of liquid, but imagining it as like mostly crystals with a significant amount of melt sort of percolating through that.
• 51:44
  SPEAKER_00: I mean, is that a good visual or a bad visual?
• 51:47
  SPEAKER_00: And uh, we can talk about this more in the next episode, but but this this visual, what should what is the thing when it's happening?
• 51:54
  SPEAKER_00: Like what does it look like when it's happening in your mind?
• 51:57
  SPEAKER_01: Well, well, I think I think maybe to not answer your question, but to get at kind of what we're talking about, because you know, we're sitting here and you and I are both granite or granitic and ice people.
• 52:08
  SPEAKER_01: What we have been ignoring this whole time is that over systems that are producing granites are systems that are producing volcanoes.
• 52:17
  SPEAKER_01: So underneath Mount St.
• 52:19
  SPEAKER_01: Helens right now is an active magmatic system.
• 52:23
  SPEAKER_01: And so presumably there are granites being formed underneath Mount St.
• 52:28
  SPEAKER_01: Helens right now.
• 52:29
  SPEAKER_01: So we can't just have like the system is way more complicated than just a granite sitting there being injected as like a hundred meter thick packet of magma and just dying.
• 52:40
  SPEAKER_01: It's much more complicated than that because you have to have somewhere in the system, you have to produce magmas that erupt with maybe 10% crystals, not 50% crystals.
• 52:52
  SPEAKER_01: So, you know, this is this idea of a magma mush, it's a very complicated thing that's probably not for for this episode.
• 52:59
  SPEAKER_00: But yeah, yeah, yeah, yeah, yeah.
• 53:00
  SPEAKER_01: Yeah, well, this would be great to bring a geophysicist on board for this conversation.
• 53:04
  SPEAKER_00: Yeah, no, for sure, and talk about what actually can we say about stuff in the crust.
• 53:08
  SPEAKER_00: But but it's not, you know, it's either increments, little small dikes, things that are intruding as dikes progressively over time, or it's some system that is mostly crystals, um, you know, in and has melt kind of percolating through it over again, over long periods of time.
• 53:23
  SPEAKER_00: We know it has to be long period of time now, i.e., the geochronology we've been talking about.
• 53:27
  SPEAKER_00: Um, so I think this is uh this is a really good episode on the room problem or on the space problem.
• 53:33
  SPEAKER_00: Like I I um have we have we tied it off?
• 53:35
  SPEAKER_00: Is there anything else we need to add here?
• 53:37
  SPEAKER_01: No, I think I think we probably we beat this one to death here.
• 53:41
  SPEAKER_00: I think we might have overdone it.
• 53:43
  SPEAKER_00: Uh it's possible.
• 53:45
  SPEAKER_00: If you think we've overdone it, let us know in the comments.
• 53:49
  SPEAKER_00: Uh, but if you know, if you if we haven't overdone it and you're interested in more, uh we got more coming at you.
• 53:54
  SPEAKER_00: But please let us know if you are want to hear more on a specific subject.
• 53:58
  SPEAKER_00: We like uh we like getting comments, we like getting emails.
• 54:00
  SPEAKER_00: You can follow us on all the social medias or at Plant GeoCast.
• 54:03
  SPEAKER_00: Shoot us an email.
• 54:03
  SPEAKER_00: There's a contact us link on our website, plantgeocast.com.
• 54:06
  SPEAKER_00: And again, go to our Camp GeoMobile app.
• 54:08
  SPEAKER_00: First link in your show notes.
• 54:09
  SPEAKER_00: We've got a ton of stuff there uh that gives you kind of the backstory to what we've been talking about in the deep dive we've been doing on the granite.
• 54:15
  SPEAKER_00: Mike, appreciate the time.
• 54:16
  SPEAKER_00: Always fun talking signs with you, man.
• 54:18
  SPEAKER_01: Yeah, of course, of course, man.
• 54:19
  SPEAKER_01: Yeah.
• 54:20
  SPEAKER_01: Till till the next one.
• 54:21
  SPEAKER_00: Till the next one.
• 54:22
  SPEAKER_00: There we go.
• 54:22
  SPEAKER_00: Love it.
• 54:23
  SPEAKER_00: All right, cheers.