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The Garnets Have Eyes · PlanetGeo: The Geology Podcast

• 0:01
  SPEAKER_01: 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: There's no uh the one thing I don't like about this is there's no like choo-too going on when you hit record.
• 0:19
  SPEAKER_00: Right.
• 0:19
  SPEAKER_01: There should be something it's a b it's a big bummer when that happens.
• 0:23
  SPEAKER_00: I agree.
• 0:24
  SPEAKER_00: Hold on.
• 0:25
  SPEAKER_00: I know you love it when I do this, but I gotta put my microphone right up tight.
• 0:28
  SPEAKER_00: Yeah, yeah, yeah.
• 0:29
  SPEAKER_01: Let's get right up into it here.
• 0:30
  SPEAKER_01: Uh Chris, I've been uh I've been thinking about you recently.
• 0:34
  SPEAKER_01: Yeah.
• 0:34
  SPEAKER_01: Why?
• 0:37
  SPEAKER_00: You could not have said that with less enthusiasm.
• 0:40
  SPEAKER_00: I mean well, I mean, okay, let's set the record straight.
• 0:43
  SPEAKER_00: What Chris do we have today?
• 0:45
  SPEAKER_00: Well, we have Chris that is mildly under the weather today.
• 0:50
  SPEAKER_01: We have a bedraggled Chris.
• 0:53
  SPEAKER_00: We we do.
• 0:55
  SPEAKER_00: Like this is oh my gosh, it started a few days ago and it just crescendoed to last night.
• 1:01
  SPEAKER_00: It was at its full force.
• 1:03
  SPEAKER_00: So about so about we are dragging this guy out.
• 1:08
  SPEAKER_01: We have we have weekend at Bernie's Chris.
• 1:10
  SPEAKER_01: We have rolled him out, propped him up, and he's here for recording.
• 1:15
  SPEAKER_00: I almost cried.
• 1:16
  SPEAKER_00: I'm like, oh my gosh, I am in so much pain.
• 1:19
  SPEAKER_00: Just where says Jenny, and I'm like, all over the place.
• 1:22
  SPEAKER_00: My skin hurts, my like everything just hurt.
• 1:26
  SPEAKER_00: It was not a good day.
• 1:28
  SPEAKER_00: Yeah, so I apologize to everybody right now for my radio voice is not up to par today.
• 1:34
  SPEAKER_01: We've got a nasally Chris, but you're going, you're gonna step it up and bring the energy because we're talking about cool stuff.
• 1:39
  SPEAKER_01: But before we get to that, I've been thinking about you because, well, we got a new house over here in Pennsylvania, and uh I have been doing some power washing because we have uh a little concrete you know walkway.
• 1:51
  SPEAKER_01: So anyway, we've been power washing the house is old and it needed some some cleaning up.
• 1:55
  SPEAKER_01: And I was just thinking, I love this stuff, and I'll bet we could put together a little Chris's digging company combined with Jesse's power washing.
• 2:04
  SPEAKER_01: This would be very fun.
• 2:05
  SPEAKER_00: I'll I could be a good assistant with power washing.
• 2:08
  SPEAKER_00: I love power.
• 2:09
  SPEAKER_00: It's so satisfying.
• 2:11
  SPEAKER_00: So satisfying when you when you power wash your rocks, it's almost like you know, it got scrubbed clean by a glacier and then just dropped off.
• 2:20
  SPEAKER_00: And that's what the power washer essentially does.
• 2:22
  SPEAKER_00: We're resetting the radiometric clock when we power wash.
• 2:25
  SPEAKER_00: It's true.
• 2:26
  SPEAKER_01: It's amazing, and that's what I've been mostly power washing is we have these like stone walls around the house.
• 2:30
  SPEAKER_01: It looks like a a moss black rock thing, and you power wash it, and the grout stands out, the reds and the yellows, and the textures in the sand, they're all kind of locally derived sandstones.
• 2:42
  SPEAKER_01: It's so fun, so fun.
• 2:43
  SPEAKER_00: So, what kind of power washer do you have?
• 2:45
  SPEAKER_00: Is you is it a little electric thing?
• 2:47
  SPEAKER_00: Is it a gas-powered thing?
• 2:48
  SPEAKER_01: No, I went and got Jesse.
• 2:50
  SPEAKER_01: The little electric one, because you know, it's enough.
• 2:54
  SPEAKER_01: They make them so well these days, and you don't have to deal with I mean it it's like I forget what it is, 2300 PSI or something like that.
• 3:00
  SPEAKER_01: I mean, it's uh it's enough to be it does enough for what I need to do for the most part.
• 3:05
  SPEAKER_01: It'll peel the paint off a car.
• 3:06
  SPEAKER_01: Yeah, yeah, that's right.
• 3:07
  SPEAKER_01: And you know, I found that out the hard way once.
• 3:11
  SPEAKER_01: I don't want to be doing that.
• 3:13
  SPEAKER_01: So, you know, it's good I don't have a 4,000 PSI thing.
• 3:17
  SPEAKER_01: That'd be I'd just I'd break stuff with that.
• 3:20
  SPEAKER_01: So anyway, yeah.
• 3:21
  SPEAKER_01: Uh okay, you can be my assistant.
• 3:23
  SPEAKER_01: I'll assist with tri Chris's tractor company.
• 3:26
  SPEAKER_00: And um yeah, I'll give you the shovel, you do all the hard work, and then I just come along with the tractor and get all the glory.
• 3:31
  SPEAKER_01: There you go.
• 3:32
  SPEAKER_01: Perfect.
• 3:32
  SPEAKER_01: Love it.
• 3:33
  SPEAKER_01: And I can do the same with the bar washer.
• 3:34
  SPEAKER_01: You can uh your power washing sidekick.
• 3:37
  SPEAKER_01: You can kind of you know scrub it a little bit and then I'll come by power washer, take all the glory.
• 3:41
  SPEAKER_01: Sounds great.
• 3:42
  SPEAKER_01: Okay, cool.
• 3:43
  SPEAKER_01: Love it.
• 3:43
  SPEAKER_00: Scrub it a little bit, okay.
• 3:45
  SPEAKER_01: All right, I don't know about that.
• 3:46
  SPEAKER_01: Give you a little wire brush, take the stuff off.
• 3:50
  SPEAKER_01: That'd be good.
• 3:51
  SPEAKER_01: So if you need some rocks cleaned, Chris and Jesse, we'll come, we'll bring the bar washer, we'll uh we'll clean up your rocks for you.
• 3:56
  SPEAKER_00: If you have rocks at your home or rocks at your place and you don't have a power washer, that's a problem that needs to get fixed.
• 4:02
  SPEAKER_00: Yeah.
• 4:03
  SPEAKER_01: You gotta bring the life back out.
• 4:04
  SPEAKER_01: Especially, Chris, if you have rocks with garnet eyes in them, because that's our topic today.
• 4:11
  SPEAKER_01: What's your experience with these things?
• 4:12
  SPEAKER_01: I mean, this was kind of this was your idea.
• 4:14
  SPEAKER_01: Garnet eyes.
• 4:15
  SPEAKER_01: Like, hey, this is a cool thing, cool phenomenon.
• 4:17
  SPEAKER_01: Let's talk about it.
• 4:18
  SPEAKER_00: Yeah, I think these are amazing.
• 4:21
  SPEAKER_00: Uh you know, when you go for a hike in Death Canyon up in the Tetons, and you just come across these outcrops and you can't miss it.
• 4:30
  SPEAKER_00: I don't care how old you are, what your level of geoscience expertise is.
• 4:35
  SPEAKER_00: You look at this and you're like, oh my gosh, these it looks like thousands upon thousands of eyeballs just staring right at you in this beautiful dark colored rock.
• 4:45
  SPEAKER_00: It's amazing.
• 4:46
  SPEAKER_00: And and then you and I have experiences in upstate New York with the Gore Mountain Garnets and the Gore Mountain Garnets.
• 4:53
  SPEAKER_01: Oh that was, I think this is the the origin of the idea.
• 4:57
  SPEAKER_01: It was I I forget what we were talking about, something, and I mentioned Gore Mountain, and you're like, oh, we got to do an episode on the Garnet eyes in Gore Mountain.
• 5:05
  SPEAKER_01: I mean, they are spectacular.
• 5:07
  SPEAKER_01: So, what we're describing here, what we're talking about, is we're gonna talk about a large class of features that are, I don't know, they're called reaction rims, they're called depletion halos.
• 5:20
  SPEAKER_01: There's they're called all sorts of which are two different things, by the way.
• 5:22
  SPEAKER_00: Yes.
• 5:23
  SPEAKER_01: Couple different things.
• 5:24
  SPEAKER_01: But the point is we see a bunch of these types of things.
• 5:27
  SPEAKER_01: They're basically a certain mineral, they're usually found in metamorphic rocks, one mineral that is rimmed by another one.
• 5:33
  SPEAKER_01: And the one you're referring to is a garnet eye, what are called garnet eyes, because there's a garnet and then it's rimmed by plagiarclase, right?
• 5:40
  SPEAKER_01: It's like a really white plagioclase, is that right?
• 5:42
  SPEAKER_00: A really white plagioclase that's in an overall very dark colored schistose rock.
• 5:47
  SPEAKER_01: I think, Chris, maybe we're gonna talk about the broad category of stuff, but kind of center it around this sort of garnet eyes phenomenon.
• 5:53
  SPEAKER_01: But I just want to get a couple definitions here.
• 5:55
  SPEAKER_01: Yeah, sorry, go ahead.
• 5:56
  SPEAKER_00: Well, hold on.
• 5:57
  SPEAKER_00: Before we do that, Jesse, like let's just we always try to center things around well, why is this important?
• 6:02
  SPEAKER_00: Why do people need to know about garnet eyes?
• 6:05
  SPEAKER_00: And here's our take.
• 6:07
  SPEAKER_00: We were just talking about this, and it comes down to really two things.
• 6:10
  SPEAKER_00: One is they're just really cool.
• 6:13
  SPEAKER_00: I mean, enough said you know, yep.
• 6:16
  SPEAKER_00: They they really are.
• 6:17
  SPEAKER_00: You need to go get yourself some garnet eyes.
• 6:20
  SPEAKER_00: Oh, yeah.
• 6:21
  SPEAKER_00: You need to do that if you're into rock collecting and things like that.
• 6:25
  SPEAKER_00: But the other thing is though, from a scientific standpoint, they're really important because this kind of metamorphism that we're talking about, it tells us a history of the rock.
• 6:36
  SPEAKER_00: It gives us a, you know, this kind of geochemistry/slash history.
• 6:41
  SPEAKER_00: What did this rock get subjected to?
• 6:43
  SPEAKER_01: And there are great depths that we could go to in the weeds about various chemical reactions going on, but it all comes down to the sort of chemical reactions that are occurring in the rock.
• 6:54
  SPEAKER_01: And a couple quick definitions to kind of set the stage here.
• 6:58
  SPEAKER_01: First of all, we're talking about minerals that are metamorphic, and they're usually, Chris, in the example you gave, like the garnets, they're much bigger than the other minerals in the rock, like the background mineral assemblage in the rock.
• 7:11
  SPEAKER_01: And we call those porphroblasts or porphyroblasts, depending on which side of the Atlantic you're on.
• 7:17
  SPEAKER_00: But these are Oh my gosh, are you serious?
• 7:19
  SPEAKER_00: You're really good.
• 7:21
  SPEAKER_00: No, no, I'd get a reaction out of that one.
• 7:24
  SPEAKER_00: Serious?
• 7:25
  SPEAKER_01: What did you just say?
• 7:27
  SPEAKER_01: Porphyroblasts.
• 7:28
  SPEAKER_01: If you're if you're in the UK, if you're in the English system, maybe porphroblasts.
• 7:33
  SPEAKER_00: I'm sorry, it's porphyroblasts.
• 7:35
  SPEAKER_01: We'll stick with porphyroblasts.
• 7:36
  SPEAKER_01: Um these these are metamorphic minerals that are growing as the rock is heated up, heated up and put under pressure.
• 7:42
  SPEAKER_01: These garnets in this instance are growing and they're actively growing or they're actively breaking down, but they are metamorphic mineral, they're not there in the rock initially.
• 7:51
  SPEAKER_00: The inside of my head this weekend became a gigantic porphyroblast.
• 7:54
  SPEAKER_00: I mean, it just depleted everything else around.
• 7:58
  SPEAKER_01: You got metamorphosis this weekend, huh?
• 8:00
  SPEAKER_00: I did.
• 8:01
  SPEAKER_00: Uh tortured.
• 8:02
  SPEAKER_00: We've always talked about metamorphic rocks as being tortured rocks, and my head was enlarged.
• 8:10
  SPEAKER_01: And put under a great deal of humanity.
• 8:12
  SPEAKER_01: Torture.
• 8:12
  SPEAKER_01: Okay.
• 8:12
  SPEAKER_01: Yeah.
• 8:13
  SPEAKER_01: It was.
• 8:15
  SPEAKER_01: So these things, Chris, the ones we saw at the Gore Mountain Mine, I mean, these are unbelievably big.
• 8:22
  SPEAKER_01: These things are softball or volleyball-sized garnets.
• 8:26
  SPEAKER_01: I mean, they are enormous and beautiful, and they often have thick rims of amphibole around them.
• 8:32
  SPEAKER_01: So black amphibole or biotite around like rimming them.
• 8:35
  SPEAKER_01: And that's a different mineral, right?
• 8:38
  SPEAKER_01: Like you just described a black rock that has in the Tetons that has garnet rimmed biplagiclase.
• 8:43
  SPEAKER_01: And I just described one in uh upstate New York that has garnet rimmed by amphibole.
• 8:47
  SPEAKER_01: Two very different reactions that are going on here in two different categories of these reaction rims or depletion halos or garnet eyes.
• 8:56
  SPEAKER_01: There are a bunch of different categories of this.
• 8:58
  SPEAKER_01: So the history is kind of the important part, and there's different ways to form this, but they're all about metamorphism and mineral reactions occurring.
• 9:07
  SPEAKER_00: And sometimes, again, dependent upon your previous exposure to this kind of stuff, you might see or have this referred to as augen gneiss, something like that.
• 9:17
  SPEAKER_01: Yes, that's right.
• 9:18
  SPEAKER_01: So I want to come back to that, Chris, at the end.
• 9:20
  SPEAKER_01: I want to kind of touch on this again because the augen or augen, depending on how you want to pronounce it, that word is the German word for eye.
• 9:28
  SPEAKER_01: So I've seen up in the Northwest Territories, a lot of the gneisses that I walk across are actually, we would call them augen gneisses because they have these little eyes in them and they can be really small eyes, they could be bigger eyes, but they have this kind of eye-shaped feature to them, which means that they've been metamorphosed.
• 9:46
  SPEAKER_01: Chris, do the Tetons ones, the the garnet eyes, are the plagioclase rims perfectly circular or are they a little bit like eye-shaped and squished out on the edges?
• 9:55
  SPEAKER_00: Yeah, they're a little bit they're eyeball-shaped.
• 9:57
  SPEAKER_00: So they're, you know, there's been some shearing involved in this as well.
• 10:01
  SPEAKER_00: Not just heat, not just pressure, but the same kind of forces that cause foliation in metamorphic rocks.
• 10:07
  SPEAKER_00: Was a directional pressure, yeah.
• 10:10
  SPEAKER_01: And this is a a kind of a key point that you should think about if you're listening to this.
• 10:14
  SPEAKER_01: Like, okay, the garnets are often very circular, almost always circular, at least if it's a single garnet grain.
• 10:20
  SPEAKER_01: And then the plagiase is kind of the eye shape, the white of your eyes around it.
• 10:24
  SPEAKER_01: There's a reason they're called it, it's kind of like a pupil, circular pupil with a with an eye shape around it.
• 10:28
  SPEAKER_00: So, Jesse, sorry to interrupt you, but but what you're saying is that in this instance, the white around the garnets is deformed, but the garnet is not.
• 10:37
  SPEAKER_00: And so just think about that a second.
• 10:39
  SPEAKER_00: What does that mean?
• 10:41
  SPEAKER_00: Why would certain minerals be deformed and look like they've been stretched and tortured and then the in the center of it untouched?
• 10:50
  SPEAKER_01: So, Jesse, it's a good question.
• 10:53
  SPEAKER_01: I want to give the listener time to think about that.
• 10:56
  SPEAKER_01: Garnet is one of the few minerals that will not deform.
• 11:00
  SPEAKER_01: It is just such a solid crystal structure that it's really, really nearly impossible for garnet to actually deform under these types of conditions.
• 11:10
  SPEAKER_01: So the plagiclase is being actively deformed and growing and getting squished, and the garnet is not getting squished.
• 11:16
  SPEAKER_01: It remains this kind of softball or this round ball shape.
• 11:20
  SPEAKER_01: So that's a really important reason.
• 11:21
  SPEAKER_01: And part of the reason Chris, I think they look so beautiful.
• 11:24
  SPEAKER_01: Like if the entire grain, if all the grains were squished, I don't know, it wouldn't be as striking to me that eye shape.
• 11:30
  SPEAKER_01: It's really around people with a a sort of a curved and squished-out eye shape to it.
• 11:35
  SPEAKER_00: That's right.
• 11:36
  SPEAKER_00: So the Tetons have the definitely the eyeball shape.
• 11:40
  SPEAKER_00: You can see the shearing that happened in this, this kind of like directional force.
• 11:45
  SPEAKER_00: But the Gore Mountain garnets, Jesse, they did not.
• 11:48
  SPEAKER_00: They did not.
• 11:49
  SPEAKER_00: That's just a you have a black rim around these beautiful garnets the size of a of a baby's head.
• 11:56
  SPEAKER_00: Like they're just amazing.
• 11:58
  SPEAKER_00: Oh my gosh.
• 11:59
  SPEAKER_01: I love the way you phrased the two differences there and highlighted the two differences because this is going to give us a really good way to talk about the different ways to form these reaction rims or reaction halos or these eyes.
• 12:12
  SPEAKER_01: There's two fundamentally different ways.
• 12:14
  SPEAKER_01: Which one should we talk about first?
• 12:16
  SPEAKER_00: Well, I think let me take the one that makes like the most sense to me.
• 12:21
  SPEAKER_00: And I'll leave you with the other one because uh it, you know, they're they're very close, but they're distinct processes that are going on.
• 12:28
  SPEAKER_00: They're very different.
• 12:30
  SPEAKER_00: And so we just need to tease that out a little bit.
• 12:33
  SPEAKER_00: So, like you said, there are two ways or two categories of these metamorphic eyes or these porphyroblast eyes that you get.
• 12:41
  SPEAKER_00: The first one is where you have minerals, new minerals that grow in the rock.
• 12:48
  SPEAKER_00: And basically, what are you doing to the rock?
• 12:50
  SPEAKER_00: You're you're cranking up the temperature, you're cranking up the pressure, and that can destabilize the minerals that are already in the country rock, the minerals that were already there.
• 13:01
  SPEAKER_00: And so if the temperatures and pressures are right, let's say, for instance, to form garnets, then the garnets are just gonna bleed those elements out of the host minerals.
• 13:14
  SPEAKER_00: And they're gonna grow right inside of the rock.
• 13:17
  SPEAKER_00: And these are porphyroblasts that you get here, are just some of my favorite things on the in the world.
• 13:22
  SPEAKER_00: I mean, I have this this garnet schist from Wrangell, Alaska, where these garnets are are they're just enormous, but they grew right literally inside of the rock.
• 13:33
  SPEAKER_00: And you can clearly see that because the minerals are kind of like stretched and elongated around the mineral.
• 13:40
  SPEAKER_00: So basically, what's happening is these minerals are just taking what they want because those minerals are stable under the new conditions, the metamorphic conditions.
• 13:52
  SPEAKER_01: And these reactions get really, really complicated, and there's many different forms of the reaction depending upon the composition of the rock.
• 13:59
  SPEAKER_01: And we've kind of talked about the complexity here when you start thinking about the chemistry of a rock, but the gore mountain garnets that we talked about, in the simplest sense, that's the version of this number one.
• 14:08
  SPEAKER_01: The garnet was growing, and one overly simplistic way to think about it is garnet and plagiuclase are kind of two ends of the reaction.
• 14:15
  SPEAKER_01: So garnet is consuming plagiuclase, depleting the rock around the garnet that's growing from plagiuclase because it's kind of soaking up.
• 14:25
  SPEAKER_01: The plagiuclase is breaking down, it's diffusing towards the garnet, and it's forming garnet.
• 14:28
  SPEAKER_01: That's an oversimplification, but we can think of it that way.
• 14:31
  SPEAKER_01: Like it's a depletion halo around the garnet as it's growing, and it's consuming all the other stuff around there.
• 14:37
  SPEAKER_01: And the other stuff's reacting out because the garnet's growing and taking it all in.
• 14:40
  SPEAKER_01: So I like the way you phrase that.
• 14:42
  SPEAKER_01: It just takes what it wants.
• 14:43
  SPEAKER_01: The garnet's taking what it wants and leaving behind what it doesn't want.
• 14:46
  SPEAKER_00: Yeah, so basically you're getting new minerals on both sides.
• 14:49
  SPEAKER_00: You're getting new minerals that are the porphyroblast, the things that we're focusing on, but also the minerals that form the ring around it, in the case of the Gore Mountain Garnets, these very dark rims around it, those are new minerals too.
• 15:02
  SPEAKER_00: They didn't exist in the rock before the metamorphism happened.
• 15:06
  SPEAKER_01: So you get this reaction rim that's kind of you're seeing what the garnet wants and what the garnet doesn't want, I guess.
• 15:12
  SPEAKER_01: Or you're not seeing what the garnet wants, and you are seeing what the garnet didn't want, if you can put it that way.
• 15:17
  SPEAKER_01: So that's one category, and that's I think I think you're right.
• 15:19
  SPEAKER_01: That's a simpler one to kind of understand.
• 15:22
  SPEAKER_01: These are what we call depletion halos.
• 15:24
  SPEAKER_01: New minerals are growing, depleting the rocks around them.
• 15:26
  SPEAKER_01: The other one, number two, the different way, and it's a it's a nuanced difference, it's really very similar process, but there's a kind of a big difference, is that the other way is more of a one-to-one reaction.
• 15:36
  SPEAKER_01: You have a reactant on one side and a product on the other.
• 15:39
  SPEAKER_01: So you're taking the classic example that I've seen is as garnet breaks down, it breaks down into chlorite and quartz.
• 15:48
  SPEAKER_01: And so what I've seen in these old metamorphosed rocks that had garnet in them is as that rock, that rock was metamorphosed into the garnet stability field sometime a long time ago.
• 15:59
  SPEAKER_01: And then at a later time it was metamorphosed again, but at lower temperature.
• 16:03
  SPEAKER_01: And the garnet then breaks down, and then the product of that is chlorite in quartz.
• 16:09
  SPEAKER_01: And so you get this beautiful chloride and quartz rim around a garnet, and you can see the garnets kind of being eaten up.
• 16:15
  SPEAKER_01: It is the one that's being eaten up.
• 16:16
  SPEAKER_01: So it's garnet going to chlorine quartz, it's just a one-to-one reaction, and these get massively complicated.
• 16:22
  SPEAKER_01: Like there's so many different ways to form these one-to-one reactions.
• 16:25
  SPEAKER_00: So, question for you then.
• 16:27
  SPEAKER_00: I'm gonna like I think of this, I want you to see if this is a proper way of thinking about it.
• 16:32
  SPEAKER_00: I think of the process you're describing is an inside-out kind of reaction where you have the garnets that were formed earlier and then it's subjected to new conditions, new temperatures, new pressures, and now they're gonna bleed out into the surrounding minerals and form chloride and quartz in this case, right?
• 16:54
  SPEAKER_00: So the process I described is an outside-in process, and this is an inside out process.
• 17:01
  SPEAKER_00: Is that adequate?
• 17:02
  SPEAKER_00: Like, does that work?
• 17:03
  SPEAKER_01: Yeah, I think that I think that's right.
• 17:05
  SPEAKER_01: Um, inside out is a little hard for me to picture in a way, but I kind of think of it as like, okay, if you have a basketball, brand new basketball, indoor basketball, it's you know, beautiful leather, and you take it and you use it outside for a while, the outside's gonna get really sort of scuffed up and beaten up.
• 17:22
  SPEAKER_01: And that's kind of what's happening to the garnet.
• 17:24
  SPEAKER_01: The garnet's breaking down, the reaction's kind of progressing inside as the garnet is breaking down, or better, maybe in an ice cube in water is a better example.
• 17:32
  SPEAKER_01: Put the ice cube in water and it melts from the outside in, it's reacting out as it's warming up because it's destabilizing.
• 17:39
  SPEAKER_00: Okay, let me give it another crack then.
• 17:41
  SPEAKER_00: Okay.
• 17:42
  SPEAKER_00: We have in previous episodes, we've spoken a lot about Bowen's reaction series.
• 17:46
  SPEAKER_00: And in Bowen's reaction series, as you take these early formed minerals like olivine and calcium-rich plagioclase, and you allow them to cool down in a melt.
• 17:56
  SPEAKER_00: These crystals are suspended in magma, let's say.
• 17:59
  SPEAKER_00: You lower the temperatures, and the olivine and plagioclase will react with the melt and begin to turn into different minerals.
• 18:07
  SPEAKER_00: Well, you can create the same thing that you're describing, these kind of reaction rims where just the surface of the mineral is reacting with what's around it, not all the way through.
• 18:21
  SPEAKER_01: That's exactly right.
• 18:22
  SPEAKER_01: And then it kind of freezes in place.
• 18:24
  SPEAKER_01: So, in the instance of this garnet to goes to chlorite plus quartz, you need garnet plus water, and then you can react to chlorite plus quartz.
• 18:32
  SPEAKER_01: And so you don't react all the way because you don't have enough water in the rock to do that reaction.
• 18:36
  SPEAKER_01: So that's just like one instance of this.
• 18:38
  SPEAKER_01: So the the reaction's kind of frozen part way through.
• 18:41
  SPEAKER_01: And so, Chris, there's tons of examples of this.
• 18:43
  SPEAKER_01: You can have pyroxene that's rimmed by garnet and plagioclase.
• 18:47
  SPEAKER_01: You can have garnet that's rimmed by plagioclase and orthopyroxine.
• 18:51
  SPEAKER_01: There's a crazy, crazy, really high temperature metamorphic ones where you have like saffirine, which is kind of turquoise, rimmed by orthopyroxine, then salemonite around it.
• 19:00
  SPEAKER_01: So you can have these really massively complicated reactions that you kind of get preserved in different rings, kind of growth rings around this porphyroblast.
• 19:10
  SPEAKER_00: If you're listening to this and you want to go down a rabbit hole, then start looking at the chemical formulas for some of these minerals that you just described, right?
• 19:20
  SPEAKER_00: If you look at these chemical formulas, which are really, really complicated chemical formulas, you can start to see what's happening.
• 19:29
  SPEAKER_00: Is the porphyroblast taking what it wants to form that new mineral and then leaving behind other ones?
• 19:35
  SPEAKER_00: You can start to piece this together, but we can't really do that in a podcast because it's it's just it's we need visuals for that.
• 19:44
  SPEAKER_01: Yeah.
• 19:44
  SPEAKER_01: Chris, that is such a great point.
• 19:46
  SPEAKER_01: This is an amazing rabbit hole to go down to go on the Google train and start looking at these reactions.
• 19:52
  SPEAKER_01: It'll teach you a ton about metamorphic petrology and geochemistry.
• 19:56
  SPEAKER_01: This is, you kind of touched on it at the end there.
• 19:59
  SPEAKER_01: This is the story that these rocks tell.
• 20:01
  SPEAKER_01: We can tell so much about what that rock experienced based on these reaction rims.
• 20:06
  SPEAKER_01: And so the garnet eyes back to the Tetons, the garnet rimmed by plagiuclase.
• 20:10
  SPEAKER_01: This is an example of the number two version.
• 20:13
  SPEAKER_01: The garnet is reacting out to form the plagueclase.
• 20:17
  SPEAKER_01: There's debate about how these form.
• 20:18
  SPEAKER_01: And in different rocks, they formed the clouds.
• 20:19
  SPEAKER_01: Of course, there is.
• 20:21
  SPEAKER_00: Yeah, of course, there's debate in this.
• 20:23
  SPEAKER_01: But different rocks have different histories that end up with the same result.
• 20:26
  SPEAKER_01: This garnet rimmed by plagioclase.
• 20:29
  SPEAKER_01: One of the coolest ones, I think, is when garnet's a higher pressure mineral.
• 20:33
  SPEAKER_01: So garnet's happy at higher pressure.
• 20:35
  SPEAKER_01: It's less happy at low pressure.
• 20:37
  SPEAKER_01: And plagioclase is very happy at low pressure.
• 20:40
  SPEAKER_01: And so, in a very oversimplified way to think about it, you take a rock that has garnet in it, the garnet's sitting there very happy, it's high pressure, it's deep in the crust, and you lift that out of the crust.
• 20:50
  SPEAKER_01: You start to like thin the crust or the mountain range kind of collapses out and sloughs out to the side, and rocks come up, and we have this isostatic rebound.
• 20:59
  SPEAKER_01: The rock gets brought to the surface, that garnet can react to plagioclase and sort of start to react out, and you get these plagioclase rims.
• 21:08
  SPEAKER_01: That's kind of one model.
• 21:10
  SPEAKER_01: I mean, to look at that garnet rimmed by plagiaris and say, Oh, I can tell that that rock went from deep to shallow, and that's the result of it.
• 21:17
  SPEAKER_01: This garnet as a result of that is so cool.
• 21:20
  SPEAKER_01: That is just a really cool story.
• 21:22
  SPEAKER_00: Jesse, I want to double-click on this a minute because you know, we did a whole audio book and we've done podcast episodes on the Grand Canyon.
• 21:30
  SPEAKER_00: So, how do we know the depths that the Vishnu schist was exposed to?
• 21:35
  SPEAKER_00: I mean, Jesse, we're talking about ridiculous depths in excess of 12 miles deep.
• 21:39
  SPEAKER_00: How do we know this?
• 21:40
  SPEAKER_00: The garnet tells the story.
• 21:43
  SPEAKER_00: Like you said, these garnets form in really, really high pressures.
• 21:47
  SPEAKER_00: And so it gives us this nice bracket of uh temperatures and pressures, which equate to depth.
• 21:55
  SPEAKER_00: So, yeah, that's I just wanted to double click on that.
• 21:57
  SPEAKER_00: That this is this is something that we've talked about before.
• 22:00
  SPEAKER_00: Four, just maybe not with this nuance to it.
• 22:03
  SPEAKER_01: No, I agree, Chris.
• 22:04
  SPEAKER_01: And I think there's one metamorphic petrology.
• 22:06
  SPEAKER_01: This is certainly in the firmly in the category of metamorphic petrology.
• 22:10
  SPEAKER_01: It's such a cool field because you tell great stories with metamorphic petrology.
• 22:15
  SPEAKER_01: Like, I mean, rocks moving around in the cross and heat getting heated up and cooled down and put under intense pressure.
• 22:20
  SPEAKER_01: I mean, the stories you can tell with metamorphic rocks, it's it's pretty cool.
• 22:24
  SPEAKER_00: You know, Jesse, I was thinking about this actually today, is that I think that we have a clearer understanding of how metamorphic rocks form than we do igneous rocks.
• 22:35
  SPEAKER_00: Intrusive igneous rocks.
• 22:36
  SPEAKER_01: I I agree, in large part because of these reactions.
• 22:39
  SPEAKER_01: The reactions are so crazy and cool, and there's so many different components.
• 22:42
  SPEAKER_01: Like in igneous rock, there's not that many reactions that happen in it because it's it's a magma.
• 22:47
  SPEAKER_01: Like it's it's not like a sediment.
• 22:48
  SPEAKER_01: The sediments get get wild and crazy.
• 22:50
  SPEAKER_01: I agree.
• 22:51
  SPEAKER_01: They they tell uh a more um vibrant or colorful story of how the earth has worked, I think, in metamorphic rocks.
• 23:00
  SPEAKER_01: So that that's certainly true.
• 23:02
  SPEAKER_01: I have a question for you, Chris, to maybe end on here.
• 23:05
  SPEAKER_01: I think, and I struggle with this myself too, so it's not just students, but my perception is that students, when we talk about metamorphic reactions and we talk about the story that metamorphic rocks tell, we often kind of work from low temperature to high temperature.
• 23:20
  SPEAKER_01: So we go from biotite up to starolite and then to garnet, and we eventually end up at the high temperature stuff, and we end up at eclogite, and okay.
• 23:29
  SPEAKER_01: We often forget that these reactions happen on the way down, too.
• 23:33
  SPEAKER_01: So, like the reaction of garnet going to chlorite plus quartz is it is what we call a retrograde reaction.
• 23:40
  SPEAKER_01: It's a rock going from higher temperature back to lower temperature.
• 23:43
  SPEAKER_01: And I'm wondering what you think about this.
• 23:46
  SPEAKER_01: Is this something that people struggle with to even think of as important?
• 23:51
  SPEAKER_01: And if they do, is it a big deal?
• 23:53
  SPEAKER_00: Um, here's my thought on this, here's my take on it.
• 23:56
  SPEAKER_00: You know, we have talked about some really important metamorphic rocks.
• 24:01
  SPEAKER_00: We've talked about eclogite and blue schist and but what gets focused upon in an intro, any intro-level geology class, those don't those don't get mentioned, but those tell us the most information, right?
• 24:14
  SPEAKER_00: They tell us what happened, they tell us what was the tectonic setting.
• 24:18
  SPEAKER_00: But instead we focus on slate, phylite, schist, nice, things like that, right?
• 24:23
  SPEAKER_00: So I I do think it's important.
• 24:26
  SPEAKER_00: I just don't know why I think they're not given their due.
• 24:30
  SPEAKER_00: Why do you think that is?
• 24:32
  SPEAKER_01: Well, it's tough.
• 24:33
  SPEAKER_01: I wish I I wish everybody would learn this in an intro class because it's just so cool and so important, and just shows I think how how deep the fields of petrology are, like how well we can work out the history in Revolver Rock.
• 24:49
  SPEAKER_01: It's really impressive how well we can do that.
• 24:52
  SPEAKER_01: But I do think it's pretty deep.
• 24:53
  SPEAKER_01: Like you gotta have a pretty solid geochemistry mineralogy background to kind of get to that story.
• 24:59
  SPEAKER_01: So there's like a reason that metamorphic petrology is taught in like third or fourth year of college.
• 25:04
  SPEAKER_01: Because I don't know about that.
• 25:08
  SPEAKER_00: No, I think anything can be simplified, anything can be boiled down to here's what's important about this.
• 25:16
  SPEAKER_00: This is why this should be covered in an intro-level class.
• 25:19
  SPEAKER_00: You can distill it enough that we're well, why why is eclogite an important rock?
• 25:24
  SPEAKER_00: Yeah, that's true.
• 25:25
  SPEAKER_00: That's true.
• 25:25
  SPEAKER_00: You know, and we don't have to get into a lot of the specific.
• 25:28
  SPEAKER_01: We don't have to do that.
• 25:29
  SPEAKER_01: You're right.
• 25:30
  SPEAKER_01: I I'm okay.
• 25:31
  SPEAKER_01: We're doing it this semester.
• 25:32
  SPEAKER_01: I just started teaching, I just did minerals this week.
• 25:36
  SPEAKER_01: We're teaching it.
• 25:36
  SPEAKER_01: We're getting into garnet eyes by the end of the semester.
• 25:39
  SPEAKER_01: There we go.
• 25:39
  SPEAKER_01: There we go.
• 25:40
  SPEAKER_01: Absolutely.
• 25:41
  SPEAKER_00: Yeah, yeah.
• 25:42
  SPEAKER_00: So before we end this, Jesse, I just want to encourage our listeners to Google image some of these things.
• 25:48
  SPEAKER_00: Look at the the garnet eyes and the Tetons.
• 25:51
  SPEAKER_00: Look up the Gore Mountain garnets.
• 25:54
  SPEAKER_00: You know, look up Agin Nice or Augen Nice.
• 25:58
  SPEAKER_00: Look up garnet eyes and just allow yourself to go down a little bit of that path.
• 26:03
  SPEAKER_00: It's just, these are shockingly beautiful rocks.
• 26:06
  SPEAKER_01: So beautiful, so beautiful, and tell a very colorful and vibrant story of the rock itself.
• 26:11
  SPEAKER_01: So, with that, Chris, I think that's a wrap.
• 26:14
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• 26:15
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• 26:24
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• 26:26
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• 26:28
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• 26:29
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• 26:31
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• 26:44
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• 26:48
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• 26:52
  SPEAKER_01: Cheers.