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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. When we compare the average composition of the Earth's continental crust with the composition of magmas generated by melting of the mantle, we find they don't match. Continental crust is estimated to be made up of 60% silica, but the magmas that come from the mantle are basaltic in composition, with only about 52% silica. Continental crust also has much less magnesium than basaltic rocks. Since continental crust is believed to be derived by melting of the mantle, this presents a puzzle.

About 30 years ago, Roberta Rudnick was among the first to recognize what has come to be known as the crustal composition paradox. Since then, she has developed new approaches to improve our estimates of the bulk composition of continental crust, and has used various geochemical and isotopic measurements to help distinguish among the various theories that purport to resolve the paradox. But to this day, there is no consensus among the theories. Roberta Rudnick is a Distinguished Professor in the Department of Earth Science at the University of California, Santa Barbara. Roberta Rudnick, welcome to Geology Bites.

Roberta Rudnick: Thank you very much, Oliver. It's a pleasure to be here.

Oliver Strimpel: Let's start by talking about both sides of the paradox, as it were: the continental crust on the one hand, and the basaltic magma on the other. To start with, the continents are very heterogeneous, both from place to place laterally and from top to bottom, that is, from the upper crust near the surface to the lower crust. So how do we go about determining the overall average composition of continental crust? And can we be confident enough in our measurements to know that the composition of continental crust really is different from that of basalt?

Roberta Rudnick: There are a number of approaches that have been taken to estimate crust composition through time. People looking at the upper continental crust, which is the most accessible to us, either take a go-out-and-sample-everything approach, or they have looked at terrigenous sediments or sedimentary rocks, things like shales and loess, which is a windblown dust. The idea is that these sediments derive from the crust that is exposed to chemical weathering and erosion, and therefore they should provide us with a very robust grand average of what's at the surface. And that works pretty well for elements that are not affected by chemical weathering, that are not removed because they're soluble during chemical weathering: things like the rare earth elements, zirconium and hafnium, et cetera. So the people looking at sedimentary rocks have approached the composition of the upper continental crust and pretty much gotten the same result in all the different studies that have been done, which gives us confidence that we're homing in on the right answer.

Oliver Strimpel: So you said the shales and the loess sample a lot of different rocks on the surface. I didn't quite understand the point about chemical weathering in there.

Roberta Rudnick: Well, shales in particular are a product of chemical weathering. They're formed when, let's say, an igneous rock is exposed at the surface and is subjected to chemical weathering. Some elements just dissolve into water and get washed into the oceans. Things like magnesium and sodium are highly soluble elements. Calcium is highly soluble. But other elements don't dissolve easily into water, and so they remain behind in the weathering product. Shales are fundamentally sampling the weathering product, the clays that are produced during chemical weathering.

Oliver Strimpel: Is that what we actually want, then? To measure the residue, as it were?

Roberta Rudnick: What we're after is measuring the rocks before the weathering. Nevertheless, for elements that are insoluble, the shales are a really good proxy for the exposed upper continental crust. But you couldn't go out and take a shale and look at its magnesium concentration or its sodium concentration, et cetera, to try to get those elements. Or strontium, a trace element that's very, very soluble. So shales are great, but only for insoluble elements, and loess also suffers a little bit from chemical weathering, so it has the same problems. The only way, then, that we can pin down those soluble elements for the upper continental crust is through the brute-force method of going out and analyzing a whole bunch of exposed rocks and taking the grand average of those.

Now, that's the upper continental crust. If we go deep into the crust, it becomes more problematic, because it's less accessible. Average continental crust is, let's say, 35 kilometers thick. If we go down 15 kilometers, what's at the surface may not be representative of what's at 15 kilometers. And certainly if we go down to near the base of the crust, 30 or 35 kilometers, it's quite likely that the bulk composition down there is different. The reason I say that is because we can look at things like seismic velocities, how fast seismic waves travel through rocks, and we can see that seismic velocities increase with depth in the crust everywhere you go worldwide. Now, could that be just due to taking the rocks at the surface and metamorphosing them, changing their mineral assemblage and thereby changing their seismic velocity? Yes, this could and would increase velocity, but the observation is that the increase in velocity is actually too large to be attributed solely to metamorphism of the rocks, and that probably requires a change in the bulk composition.

Oliver Strimpel: So how do you get to the lower crust?

Roberta Rudnick: Well, we do have lower crustal samples. We have rocks that are metamorphosed at high pressures and temperatures. They're called granulites, and we have some of those granulites exposed at the Earth's surface because they've been brought up along fault zones. We also have granulites that we can access that are brought up as rock fragments in volcanoes. That's what I've worked on a lot through my career. They're called xenoliths, which literally means foreign rock: xeno, foreign; lith, rock. There are certain types of volcanoes, certain alkali basalts, and also kimberlites, which are the type of rock that brings up diamonds, for example. These magmatic rocks have a whole bunch of volatiles in them, particularly CO2. As they decompress, they erupt explosively through the solid rock of the Earth's outer lithosphere, and they bring up fragments of that rock. So we can go out to these volcanoes and collect those fragments, those xenoliths, and study them and get an idea of what's down there.

Now, the fundamental problem, though, is that we don't know how representative the xenoliths are of what is in the lower crust. And we don't know how representative the granulites that are sitting at the surface might be of what's presently in the lower crust. That's why I and others have turned to geophysical methods, to try to get a grand average of the seismic velocity of the deep crust, and use that to try to infer the bulk composition of the deep crust.

Oliver Strimpel: OK, so let's turn to the other side of the paradox, the basaltic magmatism. As I mentioned in the introduction, basaltic magma is thought to be the raw material of continental crust formation. What are the various mechanisms that form new continental crust?

Roberta Rudnick: I always like to think of continental crust formation as being a mass transfer from the mantle to the crust, and that's where the basalt comes in, because the mantle, we know, is made of a rock called peridotite. When we melt peridotite, we make basalt, and that's what we see, for example, generating the oceanic crust, which is basaltic. The same processes must somehow be involved in generating the continental crust. And yet, as you mentioned earlier, the continental crust is not basaltic.

So where does continental crust form? In what tectonic settings? It has a very strong chemical signature of subduction zones, which is where the oceanic plate dives below an overriding plate and produces volcanoes, arc volcanoes. That's where we see basalts. But we also see a rock called andesite, which is an igneous rock of more intermediate composition. And it turns out that if we frame the continental crust composition in terms of an igneous rock, we would call it an andesite. So there's that connection. And also, when we look at the trace element composition of the bulk continental crust, we see that it has this very strong chemical fingerprint of magmas formed in subduction zone settings. So subduction zone settings are probably the main location of crust formation. There could be some contribution from intraplate settings. Things like Hawaii or Yellowstone could contribute, but their contributions are probably minor, relatively speaking.

Oliver Strimpel: But in these settings, don't we get differentiation of the basaltic magma that rises from the mantle into more silica-rich, or felsic, components and more magnesium-rich, or mafic, components, both via partial melting and sometimes fractional crystallization?

Roberta Rudnick: That's absolutely correct. The fundamental problem, though, is that if the magma that has risen out of the mantle is basaltic, even if we differentiate it, we still have those mafic to ultramafic cumulates at the base and the more differentiated rock at the top. And as long as those cumulates remain in the crust and we reintegrate the whole crust composition, we end up with a basalt.

Oliver Strimpel: Ah, OK. And just to be clear, the basaltic composition of the magma derived from the mantle is itself already different from the bulk composition of the mantle, which consists of ultramafic peridotite with less than 45% silica. So although I said there is no consensus on how the paradox came about, and we'll talk about the different ideas now, there is consensus that the paradox actually exists, namely that we do know the average composition of the continental crust well enough to know it is andesitic when, on the face of it, it should be basaltic.

Roberta Rudnick: That is absolutely correct, yeah.

Oliver Strimpel: So let's talk about the theories, then, that have been proposed to resolve the paradox. Maybe you could just give us the high-level summary, and then we'll go into each class, if you like.

Roberta Rudnick: There are basically three different types of hypothesis that could explain why the crust started, perhaps, as a basalt and is now an andesite. One is the idea of loss of mafic and ultramafic cumulates that are complementary to what's currently in the crust. That could happen through density foundering, and it could happen through a process called relamination. Another: we know that the Earth was hotter in the Archean, and if there was subduction happening in the Archean, then the hotter Archean Earth could have generated more prevalent melts of that subducting basaltic crust, creating granites that go up to make the crust, while the residue gets subducted away. And even chemical weathering has been proposed as a possible process by which we could change the bulk composition of the continental crust.

Oliver Strimpel: OK, so let's take these one at a time, and start with the theories that invoke the loss of some mafic material, presumably from the base of the crust into the mantle.

Roberta Rudnick: It's popularly called delamination, although that may not be the best name for it. It's more like foundering, because it's dense material that sinks. That's one version of this, and the other one I mentioned is relamination. The idea there is that you could subduct differentiated crust, like continental crust, and have the buoyant material rise and the dense material sink.

Oliver Strimpel: So you're actually subducting continental crust? I thought that was almost impossible to do.

Roberta Rudnick: Well, this is very interesting. Twenty or 30 years ago, any introductory text you read would tell you that you can't subduct continental crust; it's too buoyant. But now we know it happens. And the reason we know it happens is that there are rocks that have been metamorphosed at mantle conditions. They're called ultra-high-pressure metamorphic rocks. We know continental crust is subducted in some cases. And so if you could cause the buoyant material of the subducted crust to rise and the denser material to sink, that's another way of differentiating the crust.

Oliver Strimpel: So how would the buoyant material actually get back up into the crust, according to the relamination theory?

Roberta Rudnick: If you go back to the original paper, they have many different scenarios, but the one that most people latch onto, which was the really novel idea, is that, number one, continental crust subducts. We didn't think it did, but yes, it does. And number two, if you subduct continental crust, some of it comes back to the surface. That's how we know it subducts: we find diamonds, for example, in some ultra-high-pressure metamorphic rocks, and we find high-pressure polymorphs of silica, like coesite. So it subducts. But what if not all of it comes back to the surface? The stuff that's not going to come back to the surface is the dense stuff. So you take a differentiated crust, with a more felsic, buoyant upper portion and a more mafic, dense lower portion. You subduct it, you bring the buoyant stuff back up, and you get rid of the dense stuff.

Oliver Strimpel: I see.

Roberta Rudnick: I think of it as a version of mafic recycling. It's just a different way of doing it than dropping things off the bottom of the crust.

Oliver Strimpel: So the buoyant stuff that comes up gets mixed back in, or relaminated.

Roberta Rudnick: Yeah, relaminated.

Oliver Strimpel: It just conjures up a kind of planar image of a slab being stuck to the bottom, but that's not really what's meant.

Roberta Rudnick: Well, I think some version of that is one version of relamination. There are many versions of relamination, but the one that most people are talking about, the new, interesting idea, I think, was this idea of a diapiric rise of more buoyant material and the sinking of the less buoyant material.

Oliver Strimpel: I'm reminded of the podcast episode we did with Peter Molnar, in which he talked about the thickness of the Tibetan Plateau and why it's so high. One of the ideas he mentioned was exactly what you alluded to earlier: the foundering of the bottom of the crust, which maybe gets converted into eclogite or some denser material that then breaks off and falls like a blob into the mantle. Is that still being visualized as one of the ideas?

Roberta Rudnick: Exactly. This idea came around in the early 1980s, and I have to say, I was extremely skeptical that this could be an important way in which the crust differentiates. And I've really come around to thinking that, yes, for sure it's happening in some places.

Oliver Strimpel: And is that because you've looked at things like seismic tomography?

Roberta Rudnick: You know, it's really hard to catch these blobs on seismic images, because they sink relatively rapidly. So it's more places like the North China Craton, where we see evidence of a fundamental change in the lithosphere between the Paleozoic, when it had a typical Archean lithosphere, and the present day. It's completely different. And I think that's one of the areas where for sure there has been removal of the deep lithosphere.

Oliver Strimpel: But how do we know there's been a fundamental change in the lithosphere of the North China Craton?

Roberta Rudnick: In eastern China, there's an Archean craton, and Archean cratons have a particular type of lithospheric root that's very distinctive. In the Ordovician, in eastern China, kimberlites came up, and they brought diamonds and they brought pieces of the lithospheric root. And we said, all right, we can see that at that time it was a typical Archean craton. But then in the Cenozoic, basalts come up and bring up a completely different type of mantle lithosphere, a completely changed composition.

And so the idea there is that the North China Craton got hit on three sides by subduction and collision. That happened during the Mesozoic, and it was probably during the Mesozoic that this transformation occurred. Subduction was probably at least part of the reason why it happened, although there's a lot of debate about exactly how it happened. But for sure the mantle lithosphere was removed, because the alkali basalts are sampling in the exact same place that the kimberlites sampled in the Ordovician. So we see, oh, back in the Ordovician, it's Archean. And then in the Tertiary, oh, it's not Archean. So something happened, something very fundamental happened. With my colleague Gao Shan, we also thought we were seeing some magmatism in the Mesozoic that was produced by melting of these foundering blobs, which have a very distinctive chemical composition. That would imply that not only the mantle lithosphere but also the lower crust might have been removed during this event. And there are other places in the world where we see similar types of evidence for this happening.

Oliver Strimpel: So just in terms of the tectonic context where this foundering is thought to take place, is it that you could get it in thickened continental crust, where the base is heated and compressed, as we see, for example, in Tibet today? Or is that also something that we think is associated with continental subduction, like you mentioned earlier?

Roberta Rudnick: When there is subduction underneath continents, we also tend to see a lot of crustal thickening and lithospheric thickening. For example, the Andes have some of the thickest crust on Earth, and that's because there's compression going along with the subduction.

Oliver Strimpel: I see. So the two tectonic contexts are actually related. The subduction creates a thick crust, and then the thick crust is where this foundering can happen. So let's turn to the second category that you talked about, the idea that slab melting during the hot Archean can produce andesitic crust.

Roberta Rudnick: That's motivated by the fact that when you look at Archean cratons, you see a type of granite that is sodium-rich. It's called a tonalite, and there's this suite called tonalites, trondhjemites and granodiorites, otherwise known as TTGs, that are very typical of Archean cratons. The difference between that and what we see later is that later we see more potassium-rich granites. A lot of people have explored these tonalites experimentally. How do you make a tonalite? Well, you melt a basalt to create a tonalite. OK, so where was that basalt? Well, if it was in a subducting slab, and the slab was actually melting, because it heated up to the point where it could melt before it was fully dehydrated when it was subducted, then you could generate a tonalite and move that tonalite through the mantle above it to create continental crust. And then the residue is removed as the slab continues to subduct.

Oliver Strimpel: So the very presence of these TTGs is viewed as evidence that they were formed out of basaltic crust, and that the more mafic part is no longer there. And what we're seeing in the crust is this more silicic, more evolved TTG.

Roberta Rudnick: That's correct. There's still debate, though, about TTGs, because you could make them by slab melting, or you could make them by differentiation within the crust and then removal of the dense complement through delamination or density foundering.

Oliver Strimpel: And if it's the latter, why don't we see them being formed today? Or do we?

Roberta Rudnick: Well, we do see some tonalites. The Sierra Nevada, for example, has some sodium-rich granites. But probably that means that the source rock for the granite has changed over time, going from strictly mafic in the Archean to something a little more evolved after the Archean.

Oliver Strimpel: OK, let's talk about the third category you mentioned, which is chemical weathering. How does that help resolve the paradox?

Roberta Rudnick: Well, as I said earlier, some elements are very soluble during chemical weathering, and magnesium is one of those. What we really need to do is remove magnesium from a basalt in order to create a non-basaltic crust. Also calcium; calcium's also soluble. So the weathering hypothesis is that you remove some of these soluble elements and leave behind things like silica and aluminum, and that could drive your bulk crust composition towards a non-mafic composition. But the problem is, well, they're removed, they go into the oceans, and then what happens to them? Well, maybe they get taken up through alteration of oceanic crust. Magnesium, for example, may be enriched in altered oceanic crust; also calcium. So then you subduct it. But then sodium: we've got plenty of sodium; we haven't really lost sodium. So you have to have a mechanism to get sodium back into the continents if the crust has been influenced by weathering. So maybe, as that altered oceanic crust subducts, sodium comes back up through arc magmatism preferentially to magnesium and calcium. That's more or less the weathering hypothesis.

It's not a particularly well-developed hypothesis, but there's one observational fact that really supports it. If you look at stable isotope compositions, things like oxygen isotopes, of crustal rocks: we know that rocks out of the mantle should have a particular composition, around six per mil relative to the standard. And yet almost all of the continental crust, any rock you look at, is heavier than that; it has a higher what's called delta-O-18. And that's produced by chemical weathering. You see that right through the crust, even into the base of the crust. So it really is compelling that weathering has had an imprint throughout the crust.

Oliver Strimpel: So we're pretty sure that weathering has had an impact, but you say there are still some residual problems about what happens to the stuff that's weathered, and how we wind up with the present-day composition in that model?

Roberta Rudnick: That's right.

Oliver Strimpel: So I said earlier that there's no consensus, but do you have a favorite theory or favorite approach to this problem?

Roberta Rudnick: I think chemical weathering has had an influence, but I'm not sure how strong that influence is. I don't rule any of them out. I think all of them have happened, but I think this return of mafic to ultramafic cumulates is probably the key process by which the continental crust has gone from a basalt to, in quotes, an andesitic bulk composition.

Oliver Strimpel: And any particular mechanism for the return of the cumulates? Are you thinking more of the thickened continental crust context?

Roberta Rudnick: I am agnostic on that. Delamination, relamination, all of those, even subduction-zone slab melts: they're all basically doing the same thing. They're taking mafic and ultramafic materials and returning them to the mantle, leaving behind the more evolved compositions.

Oliver Strimpel: What observations or theoretical work do you think would help us distinguish the various theories and reach a consensus on why this paradox exists?

Roberta Rudnick: I don't think there's ever going to be a consensus in my lifetime, which is fine, because debate is what invigorates science and also spurs people to new discoveries. I think case studies are critically important. As I said, I started out being very skeptical of the idea of delamination, or return of mafic and ultramafic materials, but now I'm convinced that it has to happen. There have been good case studies demonstrating that it probably happens in some areas. So that's one approach. There will be continued efforts to refine crust composition, but I can tell you that that's not going to change. We've known it's an andesitic bulk composition since the 1880s, and it hasn't changed since then. Details may change, but that's fundamental. I think continued case studies and new hypotheses are probably how we're going to make headway. When Brad Hacker and his colleagues came up with the relamination idea, it was a new idea, and it was great, and it's probably happening in some places. People have been working hard to identify areas where it's happened.

Oliver Strimpel: Do we know enough about how the surfaces of other rocky planets in the solar system formed to know whether they're consistent with their parent magmas or not? In other words, is there the potential for a similar crustal composition paradox elsewhere in the solar system?

Roberta Rudnick: I mean, yes, we have a pretty good idea of the compositions of the crusts of the terrestrial planets in our solar system. And there is no similar dichotomy anywhere else, which is very interesting. Venus is considered a sister planet in many ways, in terms of mass and density; it's a little closer to the sun, but it evolved very differently from Earth. So you could take one step back and say, OK, why do we have continents, and why do we have continental crust? And I think the answer is because we have plate tectonics and we have subduction, and that's critically important for creating continents. But then why do we have plate tectonics? We don't see evidence for plate tectonics on any of the other terrestrial planets. Is it because we have liquid water? Venus probably never had liquid water. Mars definitely did have liquid water in the past. We can see the evidence for that in the sedimentary deposits. Some people think they see former lakes on the surface of Mars. And yet Mars also did not seem to develop plate tectonics. So these are big questions, and really interesting questions, and I hope they continue to motivate scientists well into the future to explore these ideas.

Oliver Strimpel: Roberta Rudnick, thank you very much.

Roberta Rudnick: It's been a pleasure. Thank you for the invitation to talk to you today.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find transcripts and a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.