Dr. Jesse Reimink: Of two, okay, yeah. what really happened, this stemmed from a listener question. Mark sent us a listener question, well, several suggestions, kind of on, Earth's oldest materials, [00:01:00] Earth's oldest rocks, and was kind of asking about this, and, and asked about the Acasta which, rang a bunch of alarm bells in my brain.

and Chris, you probably were like, Oh God, Mark, why did you ask about the Acasta Dices? Cause Jesse is just going to go nuts on this one.

Chris Bolhuis: Uh, yeah, what he actually named the Acasta Gneiss in his question, right? He, he framed that as a part of his question. And yeah, I did roll my

Dr. Jesse Reimink: Yeah. You were like, Oh, great. Here we go.[00:01:30]

Chris Bolhuis: I'm like, Oh my gosh, he really did that. He opened the floodgates. It's

Dr. Jesse Reimink: open your timer, count down to when Jesse calls me if he's seen this email.

Chris Bolhuis: that's right. That's right.

Dr. Jesse Reimink: so I did my PhD on the Acasta Gneiss. So that's why, like, this is

Chris Bolhuis: Well, hold on, Jesse. Hold on. Sorry to interrupt you, but let's back up. What are we doing in this two part thing? What are we talking about

Dr. Jesse Reimink: So if you're listening to this in the podcast, um, you're going to get two parts series here, basically on the Agostinites. And we're kind of going to break them apart into

Chris Bolhuis: [00:02:00] Well, not really the Acasta Gneisss, though. This is about the Earth's oldest rocks, which happens to, Surround or is surrounded by the Acastanites.

Dr. Jesse Reimink: that's a great point. So these are, well, we'll talk about that. This is debatable, whether these are actually Earth's oldest rocks or not. There are other rocks that people, yeah. suggest are super old and we'll get into that in later episodes. So this is kind of a, if you're listening to this in the podcast, this is a two parter on the Acasta Gneiss.

It's kind of two parts because I'm going to go on and on and [00:02:30] on about these, because I did my PhD on this, this part of the world, the Acasta Gneiss. Um, if you're listening to this on the app, this is one chapter in a audio book on Earth's oldest materials, really. So we've talked about Earth's oldest minerals, and we're talking about Uh, Earth's oldest rocks, kind of where they are globally, what they look like in different locations.

So on the audio book, you're going to get images on the podcast. You're not going to get the images. So you've got to go to the Camp Geo app to get access to the images and a little bit of extra content there in our [00:03:00] Earth's oldest rocks book there that we have on the app. So. That's the outline. okay.

I'm going to stop talking, Chris, because I got to save my voice. I got to save my vocal cords here where there's a lot more talking to be done.

Chris Bolhuis: Well, I'm going to be the moderator, so I think we're going to be okay in this, but I think Jesse, like I have some things to interject here during the course of these two episodes that you'd perhaps don't

Dr. Jesse Reimink: I, Chris, you know what? I was thinking about this as I was kind of putting this [00:03:30] together. I don't know. And you know, we've had many a Chris Bolhuis front porch, sit and chat with a beer in our hand about geology and rocks. I don't think we've gone this deep into this topic before.

I don't think we've talked, in this much detail. So this is kind of new territory for you and I a little bit.

Chris Bolhuis: think so too. And they're going to be some things that perhaps I'm going to bring into, this episode that you don't know about me, you know? So I think it's going to be interesting.

Dr. Jesse Reimink: it'll be fun. An [00:04:00] adventure with Chris and Jesse.

Chris Bolhuis: that's right.

Dr. Jesse Reimink: Buckle up your seats, kids. We're getting into the big yellow

Chris Bolhuis: right.

Dr. Jesse Reimink: driving down the road.

Chris Bolhuis: Oh man, speaking of which speak, can I say this? I don't like, I don't know. I,

Dr. Jesse Reimink: yeah. Chris Bolhuis was in the news.

Chris Bolhuis: well, hold on. I don't want to date this episode really. But you know, this is really on the heels of the total solar eclipse that ran through the belly of the United States.

And, this is what, Thursday now. And I took [00:04:30] 102 people, most of which were students, down to Ohio to view the eclipse in totality. And it was amazing. Amazing. It was really a cool experience. Um, no, that doesn't do it justice. This was amazing. It was just, it's one of these things that is like a once in a lifetime opportunity for a lot of the people.

This is not my first totality, but it will be the first and the last for a lot of the people that I took there.

Dr. Jesse Reimink: I mean, so cool. And, uh, you know, you [00:05:00] had a little nice little write up and a little video in the local news too. It's great. I mean, it's, I mean, it was excellent. got a lot of traction. So that was, excellent. You know, like we talked about with, um, with John Douglas, actually, you know, teachers who go above and beyond like Chris Bolhuis does all the time they're valued and they, they should be, you know, and it's, an exceptional thing to do to, this was not an easy thing to organize for you, right? This was a stressful thing. I called you the night before you were stressed about it. you know, he didn't know where to go, what the weather's going to be like. Do we have spots? Are we sure we [00:05:30] have a spot to like, get off and get a hundred people off the buses and, that's no small feat and it takes effort and you didn't need to do it and, uh, but everybody's better for it.

Chris Bolhuis: I'm glad I did it. It turned out perfect. everything worked out perfectly, but yeah, you're right. the day before we left, I'm, I'm, I had a place to go and I'm, I'm thinking, well, maybe we need to change because the weather is better in Indiana, maybe than it is in Ohio. So, um, the logistics was just, it was a bit of a nightmare and I lost a lot of sleep.

So, but I [00:06:00] feel rested now. I, it's been a couple of days, so I'm, I'm good to go. Yeah.

Dr. Jesse Reimink: he's, he's

Chris Bolhuis: Relaxed and rested and enthusiastic though, because

Dr. Jesse Reimink: bit.

Chris Bolhuis: today, Jesse, can I lead this off? Like, we're going to talk about the earth's oldest rocks, but we're going to do this in a couple of sections. And this is just part one of two, and I want to start this whole thing by talking about the discovery phase. Of Earth's oldest rocks, Which [00:06:30] leads to your research area. So there's some debate that surrounds this actually, right?

And this goes all the way back to the 1980s. which basically back then we were, you know, we didn't have the technology that we have now. So basically we just recognized that these rocks. We're older than most of the other rocks.

Dr. Jesse Reimink: So Chris, let me just interrupt you real quick and point to image number one, because we're going to talk about this.

And this [00:07:00] is one of the photos that I've taken during my fieldwork. image number one, it's this really, what we're talking about here is a complicated suite of nices, what are called nices. E I S S. ES, right? This is the word for high grade metamorphic rocks. We've talked about this in the Camp Geo content on the Camp Geo app, really deformed stuff.

Image number one is what we're talking about, but Chris, let me kind of turn it back to you here and ask you a question. How do geologists just generally, if you just have two rocks, how do you tell which one's older? Like what is [00:07:30] the sort of a general way that we do that?

Chris Bolhuis: Well, regarding a gneiss or a schist, which are intensely, like, regionally metamorphosed rocks, those were rocks that existed beforehand. Like, let's say that those were shallow marine sedimentary rocks. They existed And then an event happened, and that event then formed the rocks that we're looking at.

when we look at metamorphic rocks, we know that there were older rocks that existed before [00:08:00] that. Does that answer your,

Dr. Jesse Reimink: Yeah, no, that's, that's exactly right. And that's exactly what, these geologists in the late seventies were really doing. They didn't have the modern suite of analytical techniques that we have today or modern suite of geochronological techniques. So they were just walking around the rocks and they noticed, Hey, these Acasta Gneiss rocks, these have been deformed many, many times.

We can see that like there's five different folding events in here. and that's different from. other rocks nearby which had maybe one folding event in it.

Chris Bolhuis: [00:08:30] Now you are dealing with the rocks in your research and the rocks that we're really talking about in the early eighties that said, well, these are really old and they're older than most of the other rocks that we've ever seen. These are gneisses, which you just alluded to. Jesse, my question is, is this an event?

Or is this a sequence of events because a nice is one of the most intensely metamorphose rocks. So how tortured are they? let me ask it that [00:09:00] way.

Dr. Jesse Reimink: that's a great question. These particular rocks, the Acasta Gneiss, are very tortured. they formed many of them formed, you know, 4 billion years ago, 3. 8 billion years ago, we'll talk about that. But we know now with modern tools, we have identified at least five or six, what we call high grade metamorphic events.

So deformation events that cause the minerals to be overprinted, caused folding and faulting and all of these things. So we know there's multiple five or six different events of sort of forming these kinds of nices. [00:09:30] And the most recent one was 1. 9 billion years ago. So these have a very tortured history.

Chris Bolhuis: Can you give me what kind of timeframe are we talking about in terms of this? series of, tortures that happened to this rock. Was this related to one event that lasted a long time or are these punctuated by,

Dr. Jesse Reimink: yeah, this is such a good question, Chris. and we're kind of jumping ahead here. This is what we know now. This is not what we knew at the discovery time, but what we know now, yeah, [00:10:00] what we know now is 4 billion years ago, was the kind of formation event. Okay. Then 3. 8 billion years ago, more rocks were added to it, but also the 4 billion year old rocks were reset, were overprinted.

Then again at 3. 6 billion years ago, then again at 3. 4 billion years ago, then again at 2. 9 billion years ago, then again at 1. 9 billion years ago. So those numbers. And this is where, Chris, I am curious what you think about this, because I'm describing numbers like three or two or something, [00:10:30] but we're talking about hundreds of millions of years in between those events.

So these are discrete events that are literally hundreds of millions of years apart. So they're kind of punctuated, but we're talking about a time period of, Okay. 2 billion years where they experienced five or six metamorphic events in that 2 billion year window. So the rocks are really tortured in part because they're very old.

Chris Bolhuis: So did you have a question for me in that? He's like, what,

Dr. Jesse Reimink: Well, no, I'm curious what your thoughts are. Like, what are your, I don't know. How does that resonate? Cause when we talk about metamorphism, when we talk about, you know, [00:11:00] metamorphic rocks in the grand Canyon at the base of the grand Canyon, the basement nice is there in our grand Canyon chapter, you know, we talk about they formed and then they were metamorphosed and then they're now in the basement stuff,

Chris Bolhuis: Well, I'm going to answer your question. I don't know if this is exactly what you're asking, but here's, here's my take on what you just said, what are, what are my thoughts, right? My thoughts are you're throwing around numbers like 1. 9 or 7. those are small increments.

But that time span that you went through is like [00:11:30] 250 million years. So if I take that to, all right, let's look at where we are now, and let's go back 250 million years ago, well, this is when Pangea was being ripped apart.

Dr. Jesse Reimink: Yeah. Yeah. The earth

Chris Bolhuis: you know, so

Dr. Jesse Reimink: different. Right? Like

Chris Bolhuis: it's totally different. There's a lot that happens within that time span.

So geologists throw about these numbers all the time. Oh, it was 1. 8, it was 1. 7, as if those are close [00:12:00] numbers, but when you talk about 1. 9 billion years compared to 1. 7 billion years, that is not on any stretch of the imagination, close proximity time wise, like that's a long period of time that happened in between, you know, so those are my

Dr. Jesse Reimink: I, that, that's exactly what I was like, I was wondering if you were wondering, I guess, because, you people who study the early Earth, like myself, we kind of bandy around these things, and we forget, actually, that the [00:12:30] difference between 4.

0 and 3. 6 is basically all of animal life on Earth. That is, like, all of the Fenners, like, effectively. And I'm just kind of saying, oh yeah, there's a couple metamorphic events. So we, can't forget that point that these rocks are very old and very tortured, multiple, 7, 8 pretty high grade metamorphic events that happened to them.

So back to the story, the geologists were mapping this area and they walked across and they said, Oh, these look different. [00:13:00] These are really metamorphosis. These have seen five metamorphic events and the other ones, those ones over there, you know, a hundred kilometers that way. They're tortured, but they've only been tortured once or maybe twice.

And so therefore these Acasta ones are most likely really old. They didn't really know how old, but most likely old.

Chris Bolhuis: a question then for, let's say, beginning students that are looking at their NYSEs in their hand samples, right, that they have in lap. are most of these NYSEc hand samples that young students have in their [00:13:30] hands, do they represent multiple tortured events is that like a blanket statement

Dr. Jesse Reimink: that is such a good question. my guess would be probably not, because most, You know, if you go to go out west and you look in Idaho or all the way up into British Columbia, you have these Tetons, you have these metamorphic core complexes that are exposed and often the roots, those roots are, they've been really intensely deformed maybe [00:14:00] once.

The Tetons are different. They've, experienced probably two metamorphic events, serious ones, maybe three. kind of have to be in the interior of a continent where there are old rocks to kind of see multiple deformations. I mean, most of the

Chris Bolhuis: Right. Because the Tetons was at the time was closer to like an active margin, right? To the West coast, the leading edge of North America at that time. so different than the Acasta Gneiss, which is in the cratonic setting, which is the stable interior part of a continent. And [00:14:30] so that's what makes this maybe different.

Dr. Jesse Reimink: That's exactly right. Exactly right. Very well said. So the debate here is kind of like who discovered this. I mean, these were found during mapping campaigns by the Geological Survey of Canada and a geologist called Sam Bowering, who is a geochronologist, really, who was at various universities but ended his career at MIT, was the one who kind of first published on the geochronology and he was part of those mapping campaigns.

Chris Bolhuis: so Jesse, I want to interject a second because know exactly who you're talking about. [00:15:00] In 1994, I did my field camp at the university of New Mexico. And this was a 12 week long field course. and we did New Mexico, Northern New Mexico, Taos region, Southwest Colorado, we were all over that area.

Right. And, it was amazing. It was one of the best geologic experiences in my life. But

Dr. Jesse Reimink: And Chris, let me just back. this was capstone, before your senior year or after senior year or

Chris Bolhuis: No, I had already graduated. Yes. I had already graduated. And, this was like [00:15:30] the last thing that I had to do. And in the last six weeks of the course, the lead professor at that time, cause they had, they brought in several professors to, take sections of the field course.

Carl Karlstrom, who's still at UNM I think anyway, I brought in a friend of his named Sam Bowring, who. was a professor at MIT. And so I spent several weeks with him. And, made somewhat of an [00:16:00] impression on him because, we were doing a mapping project in Southwest Colorado, very complicated area, and he, in Sam's way, and I think you know him personally, right?

You, or you did know him because he passed away,

Dr. Jesse Reimink: yeah, I knew him. I mean, only a little bit. one of his students and I collaborated quite a lot, but I didn't know him very well.

Chris Bolhuis: for whatever reason, we had a connection and I made somewhat of an impression on him and he, in an informal way, offered [00:16:30] me to go to pursue

Dr. Jesse Reimink: Oh, really?

Chris Bolhuis: degree. Yes.

Dr. Jesse Reimink: cool. Oh, wow. That's amazing. his lab, I mean, in that, what'd you say, 94, is that what you said?

Chris Bolhuis: 94.

Dr. Jesse Reimink: 94. So his lab, I mean, really, he was at MIT. So like, obviously a very high profile, you know, very successful sort of academic environment. his lab at that time, that time, they had, this was actually, he published his first paper on Acasta in 1989.

So he's kind of working on these samples, [00:17:00] presumably in his lab at that time. And then in the, in the mid 2000s, like the early 2000s, he He had a series of, advances that they made in geochronology that really took uranium led geochronology and pushed it forward by a massive step function. Like that lab was pumping out amazing papers, very soon after you would have, potentially gone there.

So,

Chris Bolhuis: So, can I ask you, would that have been legitimate? can you see that happening or not?

Dr. Jesse Reimink: Uh, you doing a PhD?

Chris Bolhuis: well, like I'm, I'm asking like, was his [00:17:30] offer, cause he was pretty persistent, you

Dr. Jesse Reimink: You, you, I can tell you for a fact, you don't offer that without having meaning behind it. that's a no joke thing. I mean, you don't, you don't say, Oh, you should come to grad school with me. Unless at least I don't, and I don't think many people do. You don't offer that flippantly.

Chris Bolhuis: yeah, so this was an interesting thing because it was a, this was a pivotal, moment in my life. I didn't have ready contact with Jenny. Because I was out in the field and, you know, cell phones weren't really around then [00:18:00] at the time, you know, they were just beginning to come on, but no, I didn't have a beeper, but I was out in the field, like I was, I had to wait three weeks to get back to Albuquerque and, I'm like, like, Jenny, this just happened and I don't know what to do because I already had a job offer in Portland,

Dr. Jesse Reimink: And what it will, that must've been a difficult decision. Or like at least a lengthy conversation.

Chris Bolhuis: it was, but here's the thing is Jenny was doing her student teaching.

She had it all arranged to do it in Oregon. We [00:18:30] had so much invested in that direction that was my trajectory. it was a very, um, Tempting moment. I don't think you and I've talked about that before. Have we?

Dr. Jesse Reimink: only like offhand, you mentioned that you knew Sam Bowery and that he, uh, he, said you should come to grad school, I thought he was at Washington University when that had happened, but he was already at MIT at that point. So yeah, yeah. I mean, well, you know, this is so interesting how these little one off things, but I, you don't, so that's interesting. Cause this was been very soon after he'd sort of first published on the [00:19:00] Acastanises, which is around 89. And then he'd kind of been, you know, sort of working on them in the lab. And really there was a, uh, several technique developments that kind of got us to the point of being able to date these rocks, and this is, I think Chris is going to be an interesting Part of the discussion.

So image number two in your stack, if you're on the book here, image number two shows a piece of equipment that for me is one of the more impressive pieces of equipment. This is what's called an ion microprobe. And an ion microprobe can [00:19:30] analyze mineral grains, the width of hair.

So tiny, tiny sand grains. They can put tiny, tiny spots on tiny, tiny sand grains and get really good geochronology, uranium lead isotope ratios out of it. So you can basically image these grains and use an ion microprobe to measure the age of them compositions. And this is an image of one.

This is one of the early ones. And this thing takes up a whole room, Chris. Like this is,

Chris Bolhuis: Where does it start? Jesse? Like where do they send the ions through and [00:20:00] where do they count the ions on the back end? Is it on the right

Dr. Jesse Reimink: Great question. they vary, uh, depending on which, you know, brand or make or model of instrument, but in this particular one, which is called the Shrimp 2, the ion beam starts on the right. So the sample will be loaded into a little chamber on the right there.

And then basically you're, you're taking. an ion beam that's either oxygen or cesium ions, and you're hammering your sample with it, what's called sputtering, it just kind of kicks off atoms from the top surface layer, and those atoms then get funneled [00:20:30] through the magnet is the big orange thing or big yellow thing in the back, and uh, and then there's a detector on the

Chris Bolhuis: Oh, wait. So hold on. The magnet is not the horseshoe thing.

Dr. Jesse Reimink: no,

The magnet is inside of that box that's labeled TRIMP2. The big curved thing is what's called an electrostatic analyzer, which just filters not for mass, but for energy. So, it's just curved electric plates. So, it's a way to get higher sensitivity out of these things. So, that's it.

Chris Bolhuis: All right. Because my thought was that the magnet would be there because [00:21:00] that's going to affect the degree of

Dr. Jesse Reimink: the magnet comes next. Yeah, that's right. So basically what happens is you send these, ions, they're charged particles, they're flying really fast. But when it comes off the sample, there's a bunch of different, bunch of different speeds, bunch of velocities of ions that come off, and you want one velocity.

So you have to, they put it through this, sort of racetrack of electric fields that, filters out some of the higher energy ones, and the low energy ones and filters for a particular energy. So anyway, these techniques, we really kind of developed these techniques called ion [00:21:30] microprobes and, a couple other technique developments in the late eighties, early nineties that allowed Geochronology to do what's called in situ work.

So to be able to take an individual sand grain this case, a zircon and blast little parts of it, tiny, tiny little fragments and get reliable ages out of that. And this, this technique transformed our understanding of the early earth. Cause it allowed us to date parts of mineral grains, that preserved a record of a really old record, basically.

Chris Bolhuis: what do you [00:22:00] mean in situ? What do you mean specifically when you say

Dr. Jesse Reimink: So in situ means, in this case means, and maybe this is a good time to turn to image number three, if you're listening to this in the book, image number three is a, image of a, an Acasta zircon grain, a 4. 02 billion year old zircon grain. And this GIF just shows, you know, which parts are old and which parts are younger, but the center part of this grain is old.

So those little, there's these little, circles, tiny little squares and circles on the grain. So in situ means, [00:22:30] imaged the grain and I can put my spots on the grain without destroying the grain itself. Previous techniques, you had to dissolve the whole grain. So you kind of lost that spatial resolution.

Chris Bolhuis: Oh, okay. That That makes sense. So you're taking the zircon in its original state and you're dating from rim through to the core of that zircon grain, the

Dr. Jesse Reimink: That's exactly right. You can, sort of look at the outer part, the inner part, any [00:23:00] metamorphic rims, and, and we talked about these rocks are super deformed. They've been metamorphic times the mineral grains record that.

Chris Bolhuis: Yeah. So Jesse, why is it that the rim of the zircon, that makes sense that the rim of the zircon crystal here that you have on image number three is younger, but man, that's a big difference in time between the rim 3. 3 billion years old. To the core of it as 4. billion years old.

That's a massive [00:23:30] amount of time. Why?

Dr. Jesse Reimink: So the rim is a metamorphic grown rim, so it'll have grown during a metamorphic event. It's like garnets. Yeah, metamorphism was so intense that the zircon started to be dissolved and then some new zircon was grown on the edges of it and that's metamorphic. So think of a garnet zoning and garnet growth.

It's really a metamorphic mineral. The outer

Chris Bolhuis: So there's the rim of the zircon got melted or partially melted and metamorphosed, but it [00:24:00] didn't reach into the core of the zircon grain. Right. Okay. So, for the listener, then when you melt something, And then it crystallizes that resets the clock on it. or at least partially resets the clock,

Dr. Jesse Reimink: it can absolutely reset the clock or partially reset it. It depends on how intense this alteration is. And this is usually, we think of this as fluid grown zircon, so not actually magma. There's not actually molten magma moving around, but. It's a fluid rich environment.

And [00:24:30] usually in these rocks, you'll have things like amphibolite and amphibolite. When that breaks down and starts to get broken down by metamorphism, it'll release zirconium into the fluid. And then that zirconium will start building more zircon. So you kind of have the other minerals around it are breaking down and releasing the building blocks of zircon into the fluid and it grows new zircon around an existing one in that case.

So, that's what's going on here.

Chris Bolhuis: So do we use this kind of, I'm going to call this zoning, if you will, in image number [00:25:00] three, we use the zoning then to put a number on how many events

Dr. Jesse Reimink: Exactly. That's exactly right. Yep, exactly. So you can look at those cores, you can look at the rims. In one rock, you can get a bunch of different ages. And Chris, this brings up a source of debate that has been a kind of a long standing debate about the Acasta Gneisses, and this will become relevant in either in the second part of the podcast or later on in this chapter, because it begs the question, what does an age of a rock mean?

[00:25:30] so when I say a rock is, Some age, just any old age, schist, some metamorphic rock is an age. What do you think, in your view, what does that age mean? Or how do we interpret that age? What age am I giving? If that makes sense. Like if I said this schist, here's a schist.

it originally was a shale and then it's been metamorphosed into a schist and the schist is 1. 8 billion years old. What event am I referring to?

Chris Bolhuis: You're referring to the [00:26:00] metamorphic event, not the event that formed the rock to begin with. In other words, not the protolith to the metamorphic

Dr. Jesse Reimink: Okay, great word. can you, define the proli? Can you define that? Because that's, I think that's gonna be an important word here,

Chris Bolhuis: So if you take a shale, let's say that's the protolith and let's metamorphose that. crap out of this rock, that shale can turn into a schist. So shale is the protolith. It's the parent rock to the metamorphic [00:26:30] rock. So when we date the age of the metamorphic rock, we are not dating the age of the shale that the rock came from.

That's what I'm

Dr. Jesse Reimink: that's right. So let me ask you another question. What if I gave you a granite that had been metamorphos into a nice and told you the age was 1.8? What would you assume Would it be the same?

Chris Bolhuis: no, the 1. 8 is the age of the event that turned the granite, the protolith, which we're not, you're not giving me the age of the granite. You're giving me the age at [00:27:00] which the granite turned into

Dr. Jesse Reimink: Okay, and this is a really, fundamental debate about the Acasta Gneiss complex. Because when I am talking about the age of the Acasta Gneiss complex, when you say it's 4 billion years old, that age is the initial formation age. It's not the latest metamorphic age. The latest metamorphic event here is 1.

9. So if you were saying the, what is the age of the Nices? They're 1. 9 or maybe 3. 2 billion years old.

Chris Bolhuis: I got you. So you are using a different [00:27:30] tool in your toolbox then to do that.

Dr. Jesse Reimink: that is exactly right.

Chris Bolhuis: So there are certain radiometric clocks that you use in your lab, whether it's uranium to lead, rubidium all kinds of different tools that you can use, and some of them are able to see through the metamorphic event and go back to the protolith.

And that's what you've done with the Acasta Gneiss, I, I'm tracking a

Dr. Jesse Reimink: So, and, but [00:28:00] like, I think this is a really fundamental, and I think it's an interesting question, that I think, you know, should challenge listener to think about, what is the age of a rock? A metamorphic rock, what is the age of it? Because we all, we say the, the Acasta Gneiss, oldest rock in the

Middle East. what does that age actually mean? And what I'm talking about is I'm talking about small portions of specifically zircon grains. The inner parts of zircon grains are the only parts of that rock that record the 4 [00:28:30] billion year age, the primary age of the rock.

Chris Bolhuis: I'm sorry, but I want to interrupt you because want to, for our listeners, let's talk about a metamorphic rock. And what tool we would use for dating that metamorphic rock. And then let's take that metamorphic rock. And what tool would we use to see through that event back to the protolith? Let's say a granite.

Dr. Jesse Reimink: This is very good exercise. Yup.

Chris Bolhuis: So walk us through that without providing the answer, because then we'll get to that at the end. [00:29:00] Does that make sense? Just talk about the differences.

Dr. Jesse Reimink: So, geochronologists play this, this game, you know, we say, okay, we've got a bunch of tools. We got, you said, rubidium, uranium, lead, we can apply those specific elemental combinations of parent daughter, or parent, product isotopes. We can apply them to individual mineral grains.

We can apply them to different minerals in the same rock. We can apply them to different rocks in the same kind of package of rocks. And so, you know, we're dealing with a problem of scale a little bit here, but you look at something like [00:29:30] rubidium strontium, that system, rubidium is really mobile in fluids.

So any fluidy event, any metamorphic event that's moving fluids through the rock will reset that geochronometer, the rubidium strontium system. If you look at Smerimnidimium, it's a bit more reliable. Like it doesn't move in fluids so much, but it moves when things are melted. if you think about uranium lead, certain minerals have what's called a low closure temperature.

So they will be reset at 500 degrees centigrade. Zircons won't be reset [00:30:00] until like a thousand degrees centigrade. different minerals will have different kind of ranges that they're sensitive to, or that they get reset at.

Chris Bolhuis: Or another example would be, let's say potassium to argon. And whereas argon's a gas. when potassium decays into argon, metamorphism can squeeze the gas out of it and reset the clock. And so you're not going to be able to date the protolith using that tool in your toolbox.

You're only going to be able to date the time that it was squeezed and tortured.

Dr. Jesse Reimink: [00:30:30] So, the age for these rocks, if you've looked at the potassium argon system, they're going to be 1. 9 billion years, if not younger than that, because that's the last time that rocks reached this temperature, 400 degrees centigrade or something like that.

So in these particular costa nice samples, most of the minerals have a, an overprint at 1. 9 billion years old. Some of them are Zircon is one of them, Titanite is another one, or Sphene, some people call it. Apatite, some of them have older signatures, and Zircon has the oldest, [00:31:00] but only parts of Zircon have the oldest one.

And I, and many other geochronologists, interpret that as the original magmatic age, because that Zircon, if you go back to image number three, that Zircon, of tree ring growth zoning, tells us that that grew in a magma. So that zircon grew in a magma, zircon is 4. 02 billion years old.

So therefore, this rock originally grew from a magma 4. 2 billion years ago, and has since been reset tons of times. But it's kind of a philosophical [00:31:30] discussion that still exists. Like what is the age of the Acasta Gneiss? Are we talking about the metamorphic age or the original protolithic age?

Chris Bolhuis: Yeah. That's a, it's, I'll tell you, Jesse, this is an interesting, this is a rabbit hole in and of itself.

Dr. Jesse Reimink: Oh my goodness. Yes.

Chris Bolhuis: when, when you start digging into and reading about the geology of an area, Right? This is something that you have to sort out early on, because when you read about the geology of, let's say the Precambrian core, and they start throwing out [00:32:00] numbers.

Well, hold on a minute. What do those numbers represent? They say this rock formed 2. 7 billion years ago. Well, what are we talking about? Are we talking about the rock that we have right now in front of us? Or are we talking about the protolith, right? And this can, this is something, Jesse, that I don't think that we as geologists We don't do a good job of this.

We need to tease that out and be more specifically because it leads to confusion. I'm just going to say

Dr. Jesse Reimink: I agree completely. [00:32:30] Massive confusion. And that confusion is really well represented in the Acastan Ice Complex because this debate still kind of exists, as soon as Sam Browring and others discovered the age, really old age of these grains.

People immediately start to say, wait, hold on. That's not right. that's not the right age. You're talking about, some inherited age. Yeah, exactly. So you're kind of talking across each other, talking about two different things. it really kind of also depends on which tool you trust the most.

And whenever a new technique is developed, not many people [00:33:00] trust it. the technique developers have to prove it. Proof that this is a reliable technique. So this ion microprobe technique, nobody believed it for like five years. and so everybody said, no, that one's not right. My samarium neodymium system that I trust and have been using for 20 years is the one that's the geochronometer that's the best.

Your new ion microprobe zircon thing isn't. And so it took a while for this to become like, trusted by the community in a way. so that's kind of, sort of the debate there.

Chris Bolhuis: all right. So Jesse, we need to transition right now. Cause [00:33:30] we, uh, I love, I love doing this, man. I'll tell you, but, but man, I'll tell you, these discussions are just good for the soul.

Like they are.

Dr. Jesse Reimink: I agree. When you just

Chris Bolhuis: Yes. They're good for

Dr. Jesse Reimink: BSing about something you're passionate about and you really

Chris Bolhuis: That's kind of how I feel, you know, you and I are on my front porch right now, and we're drinking a beer, and now we're drinking a bourbon, and we're talking about this, and it's, it's really good, it's good for the soul. So, let's move into, you talked about the ion microprobe, [00:34:00] and, it's kind of like not accepted for a while.

Let's move into more recent work. So, there have been many, many PhD projects that have been done on the Acasta Gneiss. By the way, you gave me Acasta Gneiss. Did you give me good Acasta

Dr. Jesse Reimink: Oh, I give you the best.

Chris Bolhuis: this. Like, did you really? You gave

Dr. Jesse Reimink: no. That's, that's the, that's the old stuff right there. Yep. For

Chris Bolhuis: do you know where I put it?

Do you know where I put it?

Dr. Jesse Reimink: know that, actually. Where is it?

Chris Bolhuis: Jesse.

Dr. Jesse Reimink: in your classroom?

Chris Bolhuis: [00:34:30] my, no,

Dr. Jesse Reimink: stuff doesn't go in the classroom? The good stuff goes in the house?

Chris Bolhuis: no, no, my release, my really good stuff is at my house. Um, so,

Dr. Jesse Reimink: I don't think I knew that.

That's funny.

Chris Bolhuis: yeah, so no, it's on my fireplace mantle, the, the, the rock wall behind my fireplace on the main floor. I have the Acasta that you gave me. So

Dr. Jesse Reimink: in the rock wall?

Chris Bolhuis: it's in the rock wall. So it's never going.

It's, it's a part of, no, I'm not ever moving from my house

Dr. Jesse Reimink: yeah.

Chris Bolhuis: yeah, anyway, [00:35:00] okay, so, let's talk about some notable

Dr. Jesse Reimink: Yeah, so we're going to kind of fast forward a little bit. We left off in kind of the late eighties, early nineties, people have kind of refined the age of these things, you know, started at 3. 96, then it got to, actually, this is an important one.

you'll see the 3. 96 billion year old number. You'll see that in some museums. That's the original age, like in 1989 age. That has since been updated in 1999. And then in my PhD work, it got updated to 4. 02 billion years old, which Chris, it's, I know it sounds like a [00:35:30] small difference, but it's like 50 million

Chris Bolhuis: no, no, it's not, we've already covered this ground, it's not, Insignificant. Cause as you said, you know what, those million years matter. Okay. Like you, you said this when we touched off our series on geochronology, like, and that's an impressive thing to say. I get that. I relate to that.

But, but Jesse, like, this is annoying to me also talking about the confusion. We are not good at updating

Dr. Jesse Reimink: I know [00:36:00] it's, Yeah, I don't know what to do about it because I've gone to museums and I've said, Oh yeah, look at that rock. That's must be an Acastanites. Look at the thing. Yep. Acastanites, 3. 96 billion years old. It's like, guys, that age has been updated since 1999. you know, there's a better number for that.

And it's a little bit

Chris Bolhuis: but here, me too, because let's say that you decide I'm going to the Grand Canyon I want to learn about the geology of the Grand Canyon before I go there. And you start looking into the literature and they start into the numbers of the ages of the [00:36:30] rocks. And you see right away, massive discrepancies of the numbers that are thrown out.

And it's very, very irritating. I mean, it should not be this way.

Dr. Jesse Reimink: it's. There needs to be, Wikipedia for ages in geochronology where the things are kind of open sourced and, community updated things. It's just, it's really hard. anyway, we've got a whole, that's, that's a whole separate thing, but so let, let, Let me skip forward here.

So that age was kind of updated in the late nineties, early 2000s. Then [00:37:00] there's bunch of, PhD work that kind of kicked off in the sort of mid 2000s. And there's several P I mean, there's been dozens of PhD projects done on the Acasta I'll highlight a couple here. and I think we'll end part one.

At the end of this little section here, we're going to go up to when I started my PhD, like, what did we know about the Acasta Gneisses up until that point? And there's some really great work done by Japanese groups. Iizuka was kind of the lead person who was doing their PhD on this place.

And he did a really impressive job mapping. this area. And so [00:37:30] went across and sort of mapped, we're talking about maybe a 30 square kilometer area in the center of the Acasta Gneiss region that he did some mapping on and really showed that there's quite variable strain. And what I mean by strain is like how deformed are the rocks.

And so image number four here shows what are called flaggy gneisses. And Chris, I love this term. The term is flaggy nices because it looks like kind of an American flag or stripes on a flag.

You know, these are really just these rocks are toothpaste, except they have been [00:38:00] smooshed straight. And this little gif shows that there's this really nice tight fold. That's actually my PhD supervisor in the background there, Tom Choco, looking at these, but

Chris Bolhuis: that's not you.

Dr. Jesse Reimink: not me. No, that's, that's Choco. but this is, uh, Flaggy nices, and I think if you've walked across Deformed stuff, you've seen some.

They're just, each layer is centimeter thick at most. They're just stretched into oblivion. that's the, the sort of flaggy gneisses. So, Tsuyoshi Izuka, did this map and showed that there's some [00:38:30] really deformed stuff on the west side and some less deformed stuff on the east side. And image number five in the book here shows some less deformed rocks.

These are obviously not flaggy gneisses. There's like beautiful, uh, Cross cutting relationships. Chris, this reminds me of the outcrop that you taught me about cross cutting relations on up in the upper peninsula, Michigan, where you can kind of trace out the little white veins as they're cutting across things and work out the timeframes of them.

Whereas in that previous image, the flaggy nice is like all cross cutting relationships are lost. They've just been [00:39:00] stretched into oblivion. So really nice PhD that kind of did this mapping project on, on this.

Chris Bolhuis: Very, very cool. I have a question that I want to ask you. let's say that that image number four, the flaggy let's say that we compare a gneiss to granite. And so we have like kind of nice and granite next to each other, there's a contact between them and we're going to shove this up to four mountains, which one is going to be more [00:39:30] resistant to erosion?

Dr. Jesse Reimink: Oh, that is such a good question. Granite probably, in part, because gneiss is going to have, well, granite is going to be kind of homogenous. It's quartz, feldspar, maybe some biotite. those are not very erodible minerals and it's really well put together. It's,

Chris Bolhuis: It's interlocking. The crystals all grow like a jigsaw puzzle into each other till they can't grow anymore. And metamorphism kind of distorts that whole thing and [00:40:00] separates them all out. Right.

Dr. Jesse Reimink: exactly right.

Chris Bolhuis: and so we often, this is just something that I just wanted to throw in for our listeners, because it's something that when you look at mountains, like the Tetons, let's say many of the high peaks are granite.

Not the nice that, makes up the bulk of the Teton range because the granite is more resistant to erosion. It's just like, I don't know, it's just one of these things that fits in geologic, like,

Dr. Jesse Reimink: and [00:40:30] Chris, that is such a great question. Great point. And I just want to build on that because the opposite here, when we have these nices, if you go to image number four, when we have these nices, these things are really strained into oblivion. There's a bunch of different compositions in there.

There's mafic, little mafic parts. There's little felsic parts. There's a little intermediate parts. Some of those will be easily weatherable. And actually the deformation itself, the strain itself. Just having fabric in the rock makes it easier to break down because fluid can kind of percolate in through cracks and kind of exploit [00:41:00] that weakness.

I mean, I just realized this the other day, I was looking at the local geological map in my house in York, and I live on this kind of big sloping hill that goes down towards kind of the level of the city down below. I live on a fault, a little fault that has a bunch of quartz veins running through it, so if you're just walking around in hilly terrain and there's a gully there, odds are there's some sort of geological feature that controls even little gullies, I mean, so these, they're usually little fault veins or little areas where, where their rock has been deformed because it makes it [00:41:30] much more, erodible basically.

Chris Bolhuis: Can I interject a really bad geology pun here? You just

Dr. Jesse Reimink: you may.

Chris Bolhuis: I know, but you just said you lived on a fault, but Jesse, geologists all have their faults. Thanks. But they're stress related.

Dr. Jesse Reimink: That's a good one. Actually, I was wearing my shirt that has that on it. No, that's a good one. I was wearing my shirt the other day that, like, bright colors. You know, my wife gets me these silly [00:42:00] geology pun ones for my birthday because she knows I hate it. So she tortures me. but this one is, um, what was it saying?

It's not my fault, metamorphically speaking, or something like that. you

Chris Bolhuis: that's great. I love it. Yeah, yeah.

Dr. Jesse Reimink: it's a very bright shirt in my, uh,

Chris Bolhuis: Like a plan, you know, metaphorically, you know. Yeah, yeah, yeah, that's awesome. Alright, so Jesse, let's wrap this up real quick. So, to end this part of our presentation, Part one of two, okay, [00:42:30] is the Acasta Gneiss. if you go back to the protolith, You strongly feel that thAcastata Gneiss is not 3. 9 or 3. 96 billion years old, that it is 4. 02 billion years old.

Dr. Jesse Reimink: yes, certain parts of it. Yep, certain rocks within it, but there's also, let's add a little fuel to the confusion fire here, because Suyoshi Iizuka, this excellent Japanese PhD student [00:43:00] who did a PhD project there, also found a not 4. 02, but 4. 2, Zircon core. One single core of a zircon in a rock that was 3.

years old. So the entire rock, all the zircons in this rock are 3. 9 billion years old. They grew in a magma. There's one little core that's 4. 2. And that is a term we call xenochristic. Foreign. Crystal Xeno Stic much like Xeno Lith, we've talked [00:43:30] about Xeno lith before. Chris, like a foreign rock that's intruded in a younger magma.

This is the same thing. It's a 4.2, not oh two, 4.2 billion year old zircon core. So what it tells us is that there used to be 4.2 billion year old rocks in the area.

Chris Bolhuis: somehow this mineral escaped like resurfacing events because this is at a time when earth was in the heavy bombardment era, we were getting smacked and some of these [00:44:00] were resurfacing. These events were big enough to resurface our entire planet, which means a total reset of the clock of the radiometric clock.

So is that right? Like this was perhaps deep enough to escape

Dr. Jesse Reimink: Yeah, and this is deep enough or it didn't get hit, randomly one spot of the earth didn't get hit, by a big impact. But this 4. 2, what it tells us is that this Acasta Gneiss area used to have 4. 2 billion year old rocks around. We haven't found any yet. And we'll [00:44:30] come back to this in the second part because we'll kind of talk about where we're at now and what's coming, The future work or future questions are, but there is evidence for 4.

2 billion year old rocks in this area. We just haven't found them yet. We haven't found the actual rocks. We have the signatures that they used to exist, but we don't see them anymore. And this sample is, funnily enough, it's kind of from a boulder. It doesn't really matter because it's just a random, you know, it's a, It's an old zircon that's, lost the context because it's a core,

Chris Bolhuis: Cause it's not, it's not in

Dr. Jesse Reimink: it's not in play.

So who [00:45:00] cares that it's from a boulder, but it is sampled from a boulder. It's kind of a funny story

Chris Bolhuis: Yeah. We're funny people though. We care about

Dr. Jesse Reimink: that's exactly right. So, okay, Chris, we're almost an hour into this beast. say let's cut it there at, for part one, let's cut it off there and we'll pick up, part two with, where were we at when my PhD kind of started?

How about that?

Chris Bolhuis: Okay. Let's do

Dr. Jesse Reimink: So if you're listening to this on the podcast, we have this content plus images and some discussion about those images in an audio book format on the Camp Geo mobile app. You can go to the first [00:45:30] link in our show notes, click on that link, sends you to the mobile app, download it, log in, and you can get access to a ton of free content and some content like this.

Earth's Oldest Rock's audio book, where you can get images as well and understand what we're talking about in a little bit more detail. So you can follow us on all the social medias. Send us an email, planetgeocast at gmail. com and head over to our website. Another way to support us is go to our website, planetgeocast.

com. There you can support us. And we always appreciate that.

Chris Bolhuis: cheers.

Dr. Jesse Reimink: Peace. [00:46:00]

Jesse Reimink

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