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, Roberto
Rudnik 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
crusts 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. Roberto Rodnik is a
distinguished professor in the Department of Earth Science at
the University of California, Santa Barbara.
Roberto Rudnick, welcome to Geology Bytes.
Thank you very much, Oliver. It's a pleasure to be here.
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, IE 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 actually different from that of basalt?
There's a number of approaches that have been taken to trying
to estimate crust composition through time.
People looking at upper continental crust, which is the
most accessible to us either do a go out and sample everything
approach or people have looked at terrigenous sediments or
sedimentary rocks. Things like shales lurse, which
is a windblown dust. And the idea is that these
sediments derive from the exposed 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, etcetera. So the people looking at
sedimentary rocks have approached the composition of
the upper continental crust and pretty much gotten the same
result for all the different studies that have been done,
which gives us confidence that we're honing in on the right
answer. So you said that the shales and
the Lus, because they sample a lot of different rocks on the
surface. I didn't quite understand the
point about chemical weathering in.
There well, so shales in particular are a product of
chemical weathering. So they're formed when, let's
say, igneous rock is exposed to the surface and it's subjected
to chemical weathering. And some elements just dissolve
into water and get washed into the oceans.
Things like magnesium and sodium, they're 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.
And shales are fundamentally sampling the weathering product,
the clays that are produced during chemical weathering.
Is that what we actually want then?
To measure the residue as it were, or So what we're.
After is measuring the rocks before the weathering.
Nevertheless for elements that are insoluble, the shales are
really good proxy for the exposed continental crust, upper
continental crust. But you couldn't go out and take
a shale and look at its magnesium concentration or its
sodium concentration etcetera to try to get those elements.
Or strontium is a trace element that's very, very soluble.
So shales are great, but only for insoluble elements and Lurs
also suffers a little bit from chemical weathering.
And so it's has the same problems.
The only way then that we can pin down for the upper
continental crust those soluble elements 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. So average continental crust is
let's say 35 kilometers thick. So 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 kilometers, 35 kilometers, it's
quite likely that the bulk composition down there is
different. And 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 sort of the surface and metamorphosing them,
changing their assemblage and thereby changing their seismic
velocity? And 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 requires probably a change
in the bulk composition. So how do you get to the lower
cross? Well, we do have lower crystal
samples. We have rocks that are
metamorphosed to add high pressure 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 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
xenoliths rock, foreign rock. And there's certain types of
volcanoes, certain alkali basalts, and also kimberlites.
Kimberlites are the type of rock that brings up diamonds, for
example. So these rocks, these magmatic
rocks, have a whole bunch of volatiles in them, particularly
CO2. And as they decompress, they
erupt explosively through the solid rock that's the Earth's
outer lithosphere. And they bring up fragments of
that rock. And so we can go out to these
volcanoes and collect those fragments, those zenoliths, and
study them and get an idea of what's down there.
Now, the fundamental problem, though, is we don't know how
representative the zenoliths are of what is in the lower crust.
And we don't know how representative that the
granulites that are sitting at the surface might be of what's
presently in the lower crust. And that's why I and others have
turned to geophysical methods to try to get in grand average of
the seismic velocity of the deep crust and use that to try to
infer the bulk composition of the deep crust.
OK, so let's turn to the other side of the paradox, the
basaltic magnetism. 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? I always like to think of
continent cross formation as being a mass transfer from the
mantle to the crust and that's where the basalt comes in
because mantle we know is made of a rock called peridotite.
And when we melt peridotite, we make basalt, and that's what we
see. For example, to generate 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?
What tectonic settings? It has a very strong chemical
signature of subduction zones, which is where the oceanic plate
dives below overriding plate and produces volcanoes, arc
volcanoes. And that's where we see basalts.
But we also see a rock called anbesite, which is more
intermediate composition, igneous rock.
And it turns out that if we frame the continental crust
composition in terms of an igneous rock, we would call it
an anbesite. So there's that connection.
And there's also that we look at the trace element composition of
the bulk continental crust. We see that it has this very
strong fingerprint, chemical fingerprint of magmas formed in
subduction zone settings. So probably subduction zone
settings are the main location of crust formation.
There could be some contribution from interplay.
Things like Hawaii or Yellowstone could contribute,
but their contributions are probably minor, relatively
speaking. But in these settings, don't we
get differentiation of the basaltic magma that rises from
the mantle into more silica rich or Felzik components and more
magnesium rich or matic components, both via partial
melting and sometimes fractional crystallization?
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 maphic to ultra maphic 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. 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 antisitic when on the face of it, it
should be basalting. That is absolutely correct,
yeah. So let's talk about the theories
then that have been proposed to resolve the paradox.
And maybe if you could just give us the high level summary and
then we'll go into each class if you like.
There are basically three different types of hypothesis
that could explain why the crust started perhaps as assault and
now is anthesite. One is the idea of loss of
mafic, ultra mafic cumulus that are complementary to what's
currently in the crust. That could happen through
density foundry and it could happen through a process called
re lamination. It could happen because perhaps
we know that the earth was hotter in the Archaean, and if
there was subduction happening in the Archaean, then the hotter
Archaean earth could have generated more prevalent melts
of that subducting basaltic crust and created granites that
go up to create the crust and the residue then 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.
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.
So it's popularly called delamination, although that may
not be the best name for it, more like foundering because
it's dense material that sinks. That's one version of this and
the other one I mentioned is relamination.
And the idea there is that you could subduct differentiated
crust like continental crust and have the buoyant material rise
and the dense material sink. So you're actually subducting
continental crust? I thought that was almost
impossible to do. Well, this is very interesting.
20-30 years ago, you read any introductory text, they tell you
can't subduct continental crust, it's too buoyant.
But now we know it happens. And the reason we know it
happens is because there are metamorphic rocks that have been
metamorphosed and mantle conditions, they're called ultra
high pressure metamorphic rocks. We know continental crust is
subducting 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. So how would the buoyant
material actually get back up into the crust according to the
re lamination theory? 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 #1
continental crust subducts. OK, we didn't think it did, but
yes, it does. And #2 if you subduct
continental crust, some of it comes back to the surface.
And that's how we know it's subducts because we find
diamonds, for example, in some ultra high pressure metamorphic
rocks, we find high pressure polymorphs of silica like
coacite. So it's subducts.
But what if not all of it comes back to the surface?
And the stuff that's not going to come back to the surface is
the dense stuff. And so you take a differentiated
cross 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.
I see. I think of it as a version of
recycling the mafic recycling. It's just a different way of
doing it than dropping things off the bottom of the crust.
So the buoyant stuff that comes up gets mixed back in or re
laminated. Yeah, re laminated.
It just conjures up a kind of plainer image of a slab being
stuck to the bottom, but that's not really what's meant.
Well, I think some version of that is one version of re
lamination. There's many versions of re
lamination, but the one that most people are talking about in
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. I'm reminded of the podcast
episode we did with Peter Molnar, which he talked about
the thickness of the Tibetan Plateau and why it's so high.
And one of the ideas he mentioned was just exactly what
you alluded to earlier, the foundering of the bottom of the
crust that maybe gets converted into eclogite or some denser
material that then breaks off and falls like a BLOB into the
mantle. Is that what is still being
visualized as one of the ideas? Exactly.
And 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. And is that because you've
looked at things like seismic tomography?
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 Kraton where we see
evidence of fundamental change in the lithosphere between a
Paleozoic when we had an Archaean, typical Archaean
lithosphere to 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.
But how do we know there's been a fundamental change in the
lithos sphere, the North China Craton?
So in eastern China, there's an Archaean craton, and they have a
particular type of lithospheric route that's very distinctive.
And in the Art of vision in eastern China, kimberlites came
up and they brought diamonds and they brought pieces of the
lithospheric group. And we said, all right, we could
see that at that time it was a typical Archaean craton.
But then in the Cenozoic, the salts come up and bring up a
completely different type of mantle lithosphere, completely
changed composition. And so the idea there is that,
well, the North China Kraton got hit on three sides by subduction
and collision. And so that happened during the
Mesozoic. And probably it was during the
Mesozoic that this transformation occurred.
And subduction was probably at least part of the reason why it
happened, although there's a lot of debate about exactly how it
happened. So for sure the mantolithosphere
was removed because the alkali, the salts are sampling in the
exact same place that the kimberlite sampled in the ore
division. And so we see, oh, back in the
ore division, it's Archaean. And then in the Tertiary, oh,
it's not Archaean. So something happened, something
very fundamental happened. And with my colleague GAO Shan,
we also thought we were seeing some magnetism in the Mesozoic
that was produced by melting of these foundering blobs, which
have a very distinctive chemical composition.
So that would imply that not only the mantle lithosphere, but
also the lower cross might have been removed during this event.
But there are other places in the world where we see similar
types of evidence for this happening.
So just in terms of the tectonic context where these ideas about
foundering a thought to take place, is it then that you could
get it on the thickened continental crust where the base
is heated and compressed As for example we see in the present
day in the Tibetan situation? Or is that also something that
we think is associated with continental subduction like you
mentioned earlier? When there is subduction
underneath continents, we also tend to see a lot of crustal
thickening and lithospheric thickening.
For the example, the Andes have some of the thickest crust on
Earth, and that's because there's compression going along
with the subduction. 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 Archaean can
produce anti cytic crust. So that's motivated by the fact
that when you look at Archaean cratons, you see a type of
granite that is sodium rich. It's called a tonolite and
there's this suite called tonolites, trunge mites and
granodiarites, otherwise known as TTGS, that are very typical
of arcane kratons. So the difference between that
and what we see later is that later we see more potassium rich
granites. And so these tonalites, there
have been a lot of people exploring this 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 once the slab continues to subduct.
So the very presence of these TGS 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 solistic, more evolved TTG.
That's correct. And there's still debate though
about TTGS because you could do it by slab melting or you could
do it by differentiation with the crust and then removal of
the dense complement through delamination or density
foundering and. If it's the latter, why don't we
see them being formed today? Or do we?
Well, we do see some tonalites, so Sierra Nevada for example has
some sodium rich granites. But probably that means that the
source rock for the granite has changed over time.
So going from strictly mafic in the Archaean to something a
little more evolved after the Archaean.
OK, let's talk about the third category you mentioned, which is
the weathering, chemical weathering.
How does that help resolve the paradox?
Well, as I said earlier, some elements are very soluble during
chemical weathering and magnesium is one of those and
what we really need to do is remove magnesium from the salt
in order to create a non basaltic crust.
Also calcium and 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 composition across
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 got plenty of sodium, we haven't really
lost sodium. So you have to have the
mechanism get sodium back into the continents if the crust has
been influenced by weathering. So maybe sodium as that ultra
oceanic crust subducts. Maybe sodium comes back up
through arc magnetism preferentially to magnesium and
calcium. That's more or less the
weathering hypothesis. It's not particularly a well
developed hypothesis, but there's one observational fact
that really supports it and this is that if you look at stable
isotope composition, things like oxygen isotopes of any crustal
rock, 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,
has a higher, what's called delta OH 18.
And that's produced by chemical weathering.
And you see that right through the crust, even into the base of
the crust. And so it really is compelling
that weathering has had an imprint on the crust, throughout
the crust. So we're pretty sure that the
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? That's right.
So I said earlier that there's no consensus, but do you have a
favorite theory or favorite approach to this problem?
I think chemical weathering has had an influence, but I'm not
sure how strong of 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 ultra mafic
cumulus is probably the key process by which the continental
cross has gone from up assault to and quote and aside full
composition. And any particular mechanism of
the return of the cumulus? Then you're thinking more of the
thick, thick and continental crust context.
I am agnostic on that. Delamination, relamination, all
of those actually, even the subduction and subduction zone
melts, they're all basically are doing the same thing.
They're taking mphic, ultra mphic materials and returning it
to the mantle, leaving behind the more evolved compositions.
What observations or theoretical work do you think would help us
distinguish the various theories and reach a consensus on why
this paradox exists? 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. And as I said, I started out
being very skeptical of the idea of the delamination or return of
mafic, ultra mafic materials, but now I'm convinced that it
has to happen. There have been good case
studies demonstrating that 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 Andasite bulk
composition since the 1880s and it hasn't changed since then.
Details may change, but that's a fundamental.
I think just continued case studies is probably how we're
going to make headway and new hypotheses.
When Brad Hacker and his colleagues came up with a re
lamination 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. Do we know enough about how the
surface of other rocky planets in the solar system formed to
know if it's consistent with their parent Magma's or not?
In other words, is there the potential for a similar crustal
composition paradox elsewhere in the solar system?
I mean, yes, we know the composition, a pretty good idea,
the compositions of the crusts of 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 density, that's a little closer to the sun, but it
evolved very differently from Earth.
And so a lot of this 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? So 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 from 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 it continues to motivate scientists well into
the future to explore these ideas.
Roberto Rudnick, thank you very much.
It's been a pleasure. Thank you for the invitation to
talk to you today. To see pictures and
illustrations that support this podcast, go to geologybytes.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.