This is geology B with Oliver stimple.
We don't know very much about the first six, hundred million
years of Earth history, which we call the hidden Eon nor indeed
do we know that much about the beginning of the archean, Eon
that followed but that is beginning to change as
researchers deploy the modern tools of geochronology and
geochemistry. There are two main questions
being investigated, the first concerns, the crust when and by
What mechanisms did it become something like the crust we have
today with, its thick. Tenants and thin subducting
seafloor. the second question, which is closely related to the
first concerns life when did the Earth first become habitable, In
an earlier episode, Peter cawood talked about the first question.
And in particular, about when modern style plate, tectonics
may have started. And in another episode John
Valley, discussed the early Earth, suggesting that the Earth
might have cooled more rapidly than was previously thought and
thus become habitable very early in its history.
Nacho bonds research is also directed to the study of the
early Earth by analyzing a newly.
Discovered Trove of extremely ancient Zircon crystals.
She has given us a new lens into the hadean and early archaea
Neons. This has enabled us to witness
some of the processes that created and destroyed the hadean
and early, archean crust. We can then use this knowledge
to make inferences about when the earth first became
habitable. Nadia turbine is assistant
professor of Earth, and planetary Sciences at Harvard
University. Not yet Robin.
Welcome to Joel g, b. Thank you so much for having me.
Before we talk about the newly discovered zircons, can you
remind us as to why zircons are so useful as probes of the early
Earth? Yeah, so dragons are an
incredibly powerful tool for, for different reasons.
So firstly zarkon is really abundant and felsic crust.
Secondly, it's very durable. It's very resistant to
alteration weathering. Then, thirdly, of course, we can
date it and I believe your listeners have heard plenty
about Zircon, uranium that geochronology already and then,
lastly, this tiny zarkon's actually capture a lot of
geochemical information, so that includes isotopic records as
well as trace and worth laments that tell us about crustal
conditions experienced during zarkan crystallization.
Okay, good. We'll get into that in a moment,
but just to clarify, its the Zircon crystals themselves.
That are indestructible and we're dealing with detrital
zircons here. So these has a concert got
eroded out of the igneous rocks in which they originally formed,
and then later on got incorporated into sedimentary
rocks in an earlier podcast off liniment talked about using
detrital zircons to trace the Assembly of Europe.
In the Paleozoic here, we're dealing with much older detrital
zircons to probe processes that happened in the hadean, an early
archean. How old are the Oldest Zircon
crystals and how old are the oldest intact rocks?
This is really a problem we face when we study early Earth
history, right? Because as we go further back in
time we lose the rock record so the archean Eon represents 1.5
billion years of Earth's history.
So really significant chunk of resistor e but only 5% of rocks
of that age. So the oldest rock is 4 point 0
3, billion years old, the struggles from the accursed
ignites, complex and Canada. The sleeves.
The first over 500 million years of Earth's history without a
rock record. We find a few locations around
the globe where the hadean zarkon's but the oldest circle
on that we have found is about 4.4 billion years old.
Okay. So that's not much but still
older than the actual oldest rock itself.
Yeah. That's about 400 million years
older. So can you tell us about the
ancient zircons that were known before this new source that I
just mentioned was discovered? Yeah so the Most famous location
with Heidi, and Zircon is that jackets and Western Australia.
So the first hadean zarkon's there were discovered in the 80s
and since then people have dated about 200,000 zarkon's.
So that's really a huge amount of work and of these 200,000
zarkon's about 10,000 where hadean and Age and what is
really so great about this location is that it allowed us
to first start addressing some of these big questions that we
have about the early Earth. So those questions When did the
first crust form the early Earth was initially covered by magma
ocean. So when did we start forming
crust? And when did we start forming
oceans on this crust. What was the composition of that
question? Does it tell us anything about
Geo Dynamics? So one big question that I'm
trying to address and my research is the evolution of
tectonic processes. So Earth is the only planet in
the solar system that has paid tectonics.
We don't really know when plate tectonics started and why it
started And then the last big question of course is when did
life evolve and that question is really two components when did
Earth first become habitable. And then, when do we actually
start finding physical evidence for life?
The problem that the jackals is that it's only a single
location. So that, of course, it's not
very representative holder whole earth.
So, tell us about the newly discovered zircons.
You found in the Barberton greenstone belt in South Africa.
We found a new location that. Hey, Ends are caught in South
Africa and the Barberton greenstone belt.
So the Barberton rinse them Boulders between about 3.6 the
3.2 billion years old. And within this greenstone belt
we have data about 100 different sand Stones throughout the unit
and we found one single Sandstone that contained hadean
zarkon's and the Sandstone is called the green cents on bad
because it's very green. So it contains a lot of chromium
Rich faces, which gives it that green color and What is so
fascinating about this unit? Is that the zircons?
We find in them have ages, that do not correspond to any known.
Felsic igneous rock within the Barberton group.
So it really yields information about a Terrain that has since
been lost and this terrain has an age between 4.15 to about 3.3
1 billion years. So that's 800 million years of
history that we can tap by studying the ages and the
geochemistry of these zircons being able to look at this.
Complimentary record to the Jack hilts allows us to start testing
some of the hypotheses. They were put forward based on
the results from the Jack Hills and what is nice about the green
Sandstone bed is that The Rock experience lower temperatures
compared to the jackals so it's somewhat better preserved.
Unfortunately what is not as good as the jackals that in the
Jack Hills between 3 and 12 percent of the zircons.
I hate Ian and age versus in the green sense in bed.
It's only half a percent of the zircons, okay?
So let's talk about the Way, you analyzed these zircons from
South Africa in the previous episode, with geology, B, I
talked to John Cottle about a technique for rapid and
self-consistent dating and measuring of Trace, element,
abundances and crystals such as zircons.
Did you use his methods? We did not use his methods.
So John Cotton use laser ablation, split stream analysis,
which allows you to measure the ages and the hafnium isotope
composition. Perhaps also Trace elements of
exactly the Amount of material. And we did not do that in part.
Because we needed much more precise results as I mentioned,
only a half, a percent of the Zircon are hadean and age.
So first, we analyze as many zircons as we can at a
relatively rapid speed, to just be able to find the oldest
grains. And then we take the mounts with
the zircons on them and we put them into a shrimp.
Let me interject that this shrimp is not a crustacean, but
an acronym for an Instrument called a sensitive high
resolution iron microprobe. Yeah, shrimp allows much more,
precise dating. So we dated the old grains.
And then we also use shrimp to do Trace, elements analysis, and
this is really important to use shrimp for that.
A because some elements that we are studying have incredibly
low, abundance of, but also because some of the elements
that are particularly important have interferences.
So, you need really high resolution to really analyze the
Correct Peaks. So you performed then, several
geochemical analyses on the zircons, can you tell us about
those? I use three different analytical
approaches. The first one is we measured
hafnium Isotopes to get at crust mantle differentiation, then we
measure trace and Rare Earth elements, which tells us about
the composition at the magma at Zircon crystallization.
And then, lastly, we used oxygen Isotopes, which can tell us
about the It's of a hydrosphere in the hadean.
Let's go through these one at a time.
Let's start with the hafnium analysis.
How does that work? What can the results of that
analysis? Tell us when we study hafnium
Isotopes and Zircon. What we really focus on is the
decay of 176 lutetium, 2176 hafnium.
So lutetium and hafnium have very different behavior lutetium
is relatively compatible. So it's going to preferentially
stay. The mantle versus hafnium is
much more compatible. So it's going to prefer actually
go into the crust. What we ultimately end up
measuring, is the 176 hafnium. So the daughter product of that
decay over the stable, 177 hafnium, which is relatively
constant through Earth history. So because of this different
behavior. When we look at these different
reservoirs, they show a different change in the daughter
products, 176 hafnium over 177 happening.
Ooh time because the mental contains more lutetium.
The increase in the 176 hafnium over. 177 hafnium is going to be
much more than, for example, continental crust, because
continental crust has less lutetium.
That means, it produces less 176 hafnium.
And that means that the increase in the ratio of 176 hafnium
over. 177 hafnium is going to be smaller Zircon is a really
fantastic tool to track. What they have neum isotopic.
Composition was of the magma at the time, the Zircon
crystallized because when Zircon crystallizes it takes up to
several rate percent of hafnium, but almost no lutetium, so it
essentially freezes the signal, the isotopic signal of the
hafnium Isotopes at the time it crystallises.
So depending on the ratio of the 176 hafnium over 177 happening,
we can now say What's that crust extracted from the mantle like
immediately prior to Zircon crystallization?
Essentially, or did the circle actually formed by remelting of
old crust? So, as I mentioned, again,
order, continental crust has a different change in the ratio of
176 hafnium over 177 hafnium. So it really deviates a lot from
the evolution of the mantle. So, if we take this old crust
and remelt it, the Zircon can again capture the signal, and we
can use that too. Confer those are conformed from
this order crust or from more juvenile.
So like newly-formed crust from the metal.
Wow, that's really impressive. We can actually use the hafnium
isotope ratios and zarkon's to tell whether hadean and eliachim
crustal rocks, which themselves have long since disappeared,
were made directly by melting of the mantle or by melting of
pre-existing older crust. So, can you tell us about the
results of your hafnium isotope analysis?
Yes, And the green, send some bad.
When we look at the oldest zarkon's, we can see that there
are extracted from the mantle. About four point two to four
point four billion years ago. And for the next four to six
hundred million years. There's a constant not record
any evidence of new mental additions.
Instead it seems like that that initial cross that was extracted
from the mental stayed. Relatively isolated, there was
some internal remoting but there were no new edition.
Of mantle material and then that signal abruptly changes about
3.8 billion years ago, anything younger than 3.8.
We see a lot of new additions from their Mentor.
So a lot of juvenile, felsic crust production together with
some remelting of older crust but that isotopic signal of
they're really old. Hadean crust is essentially lost
after about 3.8 billion years ago now, that's fascinating.
So points to a change that happened about 3.8 billion
years, you're already getting into the early archean.
Let's talk about the other analyses first before we Circle
back and then talk about what you manage to conclude from
putting all this together. So, the second analysis you
mentioned earlier. It was about looking at the
trace and Rare Earth element abundances that you measured in
the zarkon's. Yeah, that's right.
So trace and Rare Earth elements have been used for decades at
this point, but it's only really been the last 10 years.
Is that researchers have recognized that they really
provide a lot of information in terms of that setting in which
the zircons crystallized. So for this query Grimes
analysts are cons from felsic igneous, rocks in different
tectonic, magnetic settings so that includes depleted.
Mantle, so mid-oceanic ridges under pleaded mental.
So that's something that would be represented by ocean islands.
Like, Hawaii or Iceland. And then, lastly, he looked at
Continental arcs. So, Subduction zones, right?
That's a very different mechanism.
You're pushing crust below a continent and that setting.
And you've got this type of hydro smelting signature.
So he quite Grimes ultimately found that there's some very
specific Trace elements that can really distinguish these
different tectonic magmatic processes and these include
uranium niobium Scandium serum and ytterbium.
So what do our zarkon's show right that's the big question
here and For this long-lived crust that was extracted from
the mantle in the hadean that I mentioned earlier.
The question is really okayed, this Quest has really long lived
but how did it form initially? Did it form in a type of Arc
setting, which would be evidence of plate tectonics?
Or did it form in a more mental dominated, setting.
That is something. We cannot distinguish by using
hafnium Isotopes. But now, by using the trace
elements, we actually find that these earliest zircons have a
really strong mental signature and very little evidence for
Just melting that we would expect for example, in a
subduction setting because you'd expect the hydras in volcanic
rock in a subduction zone because that would normally be
happening under the ocean. Yeah.
Because you have got the oceanic crust that is interacting with
the ocean. And then that crossed that is
hydrated is being pushed underneath another crust and
that water is being used in the melting process and other parts
are very characteristic your chemical signature.
So, by looking at the Green Sandstone bad.
I mentioned earlier, the hafnium Isotopes that we have evidence
that there was an initial cost extraction and that that crust
has really long lived by using the trace elements.
We are able to say that that initial crust was extracted from
the mantle and so it doesn't have that strong Hydro smelting
signature. So this is really remarkable.
Because this Oceanic type crust was stable for several hundreds
of millions of years, which is something that is really not
typical. When you look at the Water and
environment today. So when you think for example,
about oceanic crust, oceanic crust typically doesn't get
older than 200 million years verse 8, he didn't cross that
formed the source to the green sense and that was stable for
hundreds of millions of years. So this is already starting to
give us some indication that some of the early Earth.
Geodynamic processes may have been rather different to today.
So you also analyze oxygen Isotopes.
What does that? Tell us, oxygen Isotopes and
Zircon. Tell us something different.
Aunt oxygen Isotopes and Zircon is I really sensitive Tracer of
fluid solid interactions. So tells us if the magma from
which the Zircon crystallised has assimilated across, that was
out at low temperature or potentially something that
incorporated sediments. So that implies there was some
kind of hydrosphere already at the surface of the Earth.
It can also tell us if that interaction with a potential
hydrosphere happened at high temperatures, is it possible to
explain in simple terms, why? With the hydrosphere would
affect the oxygen, isotope ratios.
So, this is something that is based on an isotope equilibrium
process. That really depends on the mass
of the oxygen. And you have different exchange
between the material that you are altering and the fluid and
that's going to vary depending on the temperature.
But this is an entirely in organic process.
Correct. So, we already know from the
jackals that most of those are concept of mental, like,
signature in a few of them to show evidence.
For alterations with low temperature fluid.
So that there's some evidence for hydrosphere, but here the
question of course, always is. Okay, it's only one location.
So was the global Ocean or was it?
Maybe just something more localized in South Africa.
What we find is also a lot of Sir concert at elevated oxygen
isotope signature. So this is providing us with a
better picture of a potentially more.
Widespread hydrosphere already in the hadean.
Okay. So can you put the results of
these three sets of analyses together?
The hafnium Isotopes. The trace and Rare Earth
elements, and the oxygen Isotopes and tell us what you
were able to learn about the crust of the early Earth from
the South African zircons. Yeah.
Sure. So in South Africa, we learned
that there's really two phases in the crust.
Evolution prior to 3.8, billion years ago.
We Crust formation by extraction of material from the mantle and
this material from the mentors extract.
It and was kept relatively isolated.
So we don't see a lot of juvenile editions, it's
relatively stable, but we do find evidence that there was
some present water and then after 3.8, we see this really
dramatic change, very knows that seeing a lot of new juvenile
editions. The crust becomes really
unstable. You're still mixing some older
material, but we do start seeing this really Mork Hydra smelting
signatures so is this a period of history that we did not have
access to before? Then when we were just looking
at the Jack Hills, it is really providing a complimentary
record. So tying the have new Isotopes
and the trace elements. It's starting to add a lot of
different puzzle pieces to this picture of the hadean that we
are trying to put together. What was the earliest earth like
was a very similar to today. I was different and there's this
really wonderful study by any Our.
And she looked at hafnium Isotopes from a number of
different locations around the globe and she found that this
Evolution and the hafnium Isotopes showing really
long-lived crust during the early Earth until about 3.8 3.6
billion years ago. And then suddenly we see this
destabilization and more juvenile Quest, editions, but
again having him Isotopes, can't really tell us if that
long-lived crust was formed by some kind of plate, tectonic
process, or something different. So now Tying the trace elements.
We can say that at least for the green Sandstone bad.
We see this transition of something that is more mental,
dominated until about 3.8. And then the signatures start
becoming a little more similar, does that suggest that plate
tectonics might have started at that time?
Or are there other explanations for what we've seen in the
record? That's a really good question
because that's something we have to be really careful about.
So starting at three point eight and the trace elements, we start
seeing a hydro smarting signature and the thing is that
Different processes that could explain the type of signature
one way is the standard way that we see today, right?
Plate tectonics. That's the tectonic model that
we are used to, where we talked about this earlier already where
we're putting hydrated, oceanic crust into the mantle and I turn
it of way. How this might have happened
during the early Earth? Is that when you form a crust at
the surface and why it's at the surface, it's interacting with
the hydrosphere potentially, you are generating sub.
Is that are being deposited, if that costs us buried.
So, imagine just a lot of really heavy mantle material coming up
to the surface. So the crust thickens and it
starts sinking down. So we call that sect action or
partial convective. Overturn, you might be able to
generate a similar signature and that Trace elements.
And I have new, my sister topes sack duction as opposed to
subduction sag so just like a sag a block just falls off.
Yeah it just pods are so psyched.
Action was She possible and the achaean because the heat flux on
The Mantel was higher. So the crossbows generally
hotter and because of that, the crust started behaving in
different ways potentially allowing for that sagging versus
today, the heat flux on the mental is much lower, which
makes the crust, much more rigid, and does not allow for
that sagging Behavior. Sagged action has been proposed
for ancient rocks, but we don't see you today.
What exactly does the signature look like?
I hope that one day. Some Modelers will go and model
that and can tell us if there's ways to distinguish the two
different. Signatures I said in my
introduction that you can use the knowledge, your honor thing
about the early crust to make inferences as to when the earth
first became habitable. What does all this have to do
with habitability? I think that there are three
important takeaways and regards to habitability.
So the first one relates to the presence of water, we all need
water. So The question is, when did we
start forming a hydrosphere? When did we start farming?
And ocean and based on previous results and the jackals and our
new results now and South Africa, we can say that at least
by about 4.3 4.2 billion years ago.
That was probably a hydrosphere present.
The second one is about the creation of typography.
So today, the most effective mechanism of creating Topography
is through plate, tectonics and convergent settings.
So, this is becomes really important when you think about
what the conditions at the surface were today, with
creating topography reinitiate silicate, weathering, and that
draws CO2 out of the atmosphere, it fixes it into carbonate, that
makes the climate much more moderate.
So if we don't have plate tectonics, be a volcano, still
producing CO2 going into the atmosphere, making it really
hard but you don't have a good way to draw that.
CO2 down. So one question is okay.
When did we start creating all of that topography?
And it looks like in terms of mechanisms, it seems like
starting a 3.8 that might have been much more efficient
mechanisms of creating that typography.
So, just really stabilizing the climate.
If the climate was really changing and fluctuating up and
down, you can still have life, but the conditions are going to
be more favorable for more substantial biomass.
Once you start stabilizing the climate and then the last
evidence is more direct path and Bal in the Jack Hills, studied
the mineral, inclusions of zircons and she found at least
one zarkon that has to graphite inclusions.
So graphite as carbon and carbon could potentially be evidence
for life and she measured the carbon isotope composition.
So carbon has heavier and lighter isotope.
So carbon 13 and carbon 12 and live, always prefers, the light
isotope if the Carbon Isotopes are relatively - this could
potentially be evidence for life.
And this graphite inclusions that she measured, she found
carbon isotope value of minus 24, which is very much
consistent with life already being present during the early
Earth and I'm saying consistent because it just one inclusion.
So we need to be really careful. There are some abiotic mechanism
that could produce carbon Isotopes like that but this is
something that I'm hoping to do in future researchers.
Search for more graphite inclusions and measure the
carbon The topes to see if we continuously get these light
carbon isotopes. That could be reflective of
early life or if we find more graphite, inclusions do receive
range and carbon Isotopes which would speak to more abiogenic
mechanisms. You mentioned how useful it
would be to look for other locations.
Are you actually hunting for new sources of ancient zircons?
I think that absolutely are more locations with hadean zircons
and on a lot of my research projects, we just analyze a On
spotted zarkon's. We are always hunting for more
locations with Heidi and zarkon's.
Do you have any promising leads at this point?
We haven't found any new locations, but we did some more
analysis on one location in India and one in Wyoming.
And here we know that people have found old Zircon.
So we went in and we are just trying to analyze thousands of
circles just to see how old can we get, how old is the oldest
Zircon going to be? And I'm happy to report.
We found 14. You hate Ian.
Zircons in India and at least one and Wyoming.
Now that your mom thank you very much.
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