This is geology bytes with Oliver Strumple.
The surface of the Earth is capped by a thin rocky layer
called the lithosphere. The lithosphere is composed of
discrete plates which float on the acinosphere, the ductile
upper part of the mantle. Since the acinosphere is far too
deep down for us to access it by drilling, our knowledge of it
comes from mantle rock that finds its way to the surface,
such as in ophulites or Kimber lights, or via geophysical
phenomena that probe the mantle remotely, such as seismic waves,
gravity and magnetic fields. But many of the properties of
the mantle that are thought to determine the mechanisms of
plate movements, subduction and magnetism are poorly understood,
as they depend on the detailed physical and chemical behaviour
of the mantle under the conditions that prevail at
depth. David Colstead has sought to
replicate these conditions in the lab so as to observe these
critical mechanisms directly. He is professor emeritus at the
School of Earth and Environmental Science at the
University of Minnesota. David Colstead, Welcome to
Geology Bites. Thank you.
I am delighted that you have invited me.
What exactly are the properties of the acinospheric mantle that
you try and learn about in the lab?
Our experiments are designed to study the viscosity of the
acinosphere. Question that motivates us is,
why is the acinosphere so weak? Is it simply that the
temperature has reached a critical point and the
stenosphere begins to flow much like warming molasses in
January? Or is it the presence of melt
and water that makes these stenosphere weak?
To answer these questions, we have concentrated our research
on quantifying the dependency of viscosity on the melt fraction
and on water content. And though those two quantities
affect each other, they're two separate things.
And I understand that in your experiments you study the effect
of each of these separately. Let's start with the melt
fraction. Do you experiment on samples of
mantle rock that we can find on the surface?
No, we don't use rocks provided by Mother Nature.
Those rocks that have come to the surface in Kimber Lights or
ophiolites have been on the surface long enough that they
begin to weather and introduce products that we'd rather not
have in our samples during the experiments.
So instead we take gem quality olivine, which is the dominant
mineral in Earth's upper mantle, and we combine it with basalt.
And we take the gem quality olivine, grind it into a powder,
and then grind some of the basalt into powder and mix a
specific amount of a salt in with the olivine.
And the salt at high temperature will melt, but the amount of
melt that you get from the basalt is independent of
temperature, so that we can really isolate the effect of
melt fraction on the deformation behavior of the material.
We also use lurzolite sometimes for our samples.
And lurzolite the melt is formed by melting the aloving plus
orthopyrixine plus colinopyrixine, which react
together to form the melt. But in that case the amount of
melt is a function of temperature.
So we use two different approaches, two different types
of samples, and then the challenge is to demonstrate that
you get essentially the same results taking either approach.
That's interesting. So you actually control the
amount of melt in your samples through really the composition,
the mixture of materials that you put in to your sample to
begin with. Yes, that's exactly right.
And it's a way in which we can really design the experiments to
probe. For example, what is the
strength of material that has 1% melt versus having 10% melt?
And presumably you keep those as dry as possible so as to take
water out of the equation completely for those
experiments. Yes, it's important for us to
try to design experiments where we can vary 1 parameter at a
time. It makes understanding the
physics of what's going on in the problem much more
straightforward. And there's not complications
arise from having three or four different variables changing.
Every time you change one variable, you change two or
three others at the same time. So that's really kind of an
important aspect of the way we try to design our experiments.
Let's talk a bit about water then.
How do you control for the amount of water in your sample?
We work primarily with the mineral olivine and we fabricate
samples again by powdering gym quality single crystals, hot
pressing them and making them a back into a solid cylinder of
material. In this case we put an allobene
sample into a nickel capsule. Nickel helps keep the allobene
in its stability field so that it doesn't oxidize or reduce.
And to that capsule we add a couple of drops of water and
then we seal the capsule. We Weld it shut and then we vary
the water content in the sample by heating assembled a high
temperature and high pressure and the water concentration or
water fugacity in the capsule is a function of temperature and
pressure. Once said high temperature and
pressure, the water dissociates and produces hydrogen ions which
diffuse fairly rapidly into the Aladdin.
So you introduce a fixed amount of water into these experiments
at the beginning, and then you control for the amount of water
by varying the temperature and pressure.
And somehow you know what amount of water, what the effective
amount of water is inside the sample as a function of
temperature and pressure. We do.
We've done calibration experiments prior to doing the
deformation experiments, in which we simply go through this
routine sample to a certain temperature and certain pressure
and then quench it, and then use infrared spectroscopy to measure
the amount of water in the sample so that when we do a
deformation experiment, we know what the temperature and
pressure conditions are, we know what the water content in the
sample will be. What are the size of the samples
that you use in your experiments?
Typically a sample size is about 1 centimeter in diameter and 2cm
in length. So the conditions in the mantle
are extreme, enormous pressures and high temperatures.
What conditions do you aim to achieve in your samples during
the experiments? We typically would go to a
pressure of 300 mega pascals and a temperature of about 1200
thirteen 100°C. Now that corresponds only to a
relatively shallow depth in the Earth of 10 kilometers, so we
certainly need to then be able to determine the flow laws that
allow us to extrapolate to higher pressures and higher
temperatures. Do you not try and simulate
those greater depths and higher temperatures because it's just
technically too difficult? It is technically difficult,
that's for sure. And our limitation is we're
doing experiments in a steel pressure vessel which has a hole
drilled down the axis of the vessel, which limits our ability
maybe 1 giga. Pascal would kind of be an upper
limit of what would be possible, but there are other researchers
who have been very clever and designed apparatuses that can go
to significantly higher pressures, up to 10 gigapascals,
which will get you down to roughly 300 kilometers depth in
the Earth. And these pressure vessels are
taken to the synchrotron, A synchrotron source for X-rays.
And the high intensity beam of X-rays is showing on the samples
during the deformation experiment.
And that allows you to measure the length of the sample and
therefore monitor what the rate of deformation of the sample is
and from X-ray diffraction to measure the stress that's being
applied at the same time. So if you have stress and strain
rate, you have viscosity. But in those experiments that
get to these much higher pressures and temperatures,
presumably they don't do it in the sample size of a few
centimeters like you're doing. That's a really good point.
Oh, the sample size typically is a millimeter by a millimeter, so
the samples get much smaller as the pressure goes up.
Sample sizes tend to get smaller progressively.
OK, so you have this centimeter scale sample.
How do you actually sustain these high temperatures and also
these incredibly high pressures? So in our lab, we focus on using
gas as a confining medium for the sample and the pressure
vessel that we use is about a foot across and about 3 feet
long. And down the center of that
vessel, there's a hole been drilled that's about 3 inches in
diameter. So it's a relatively small hole
in a relatively large vessel. And then the trick is that you
put end caps on the vessel that seal the the vessel.
So you can pump gas into the vessel and increase the pressure
through a series of pumps that each stage increases the
pressure of the gas. That comes basically out of a
welder's argon tank at maybe 10 mega pascals.
And you can increase the pressure with the series of
intensifiers until you get up to 300 mega pascals.
And inside that bore slides a furnace.
And the furnace design in some ways is relatively simple.
There's a lot of insulation that keeps the heat from the center
of the furnace from going out and overheating the steel
vessel, and the furnace is basically simply a wire wound
aluminum oxide tube and it works much like your toaster works at
home. The whole thing is cooled on the
outside to keep the steel basically at room temperature
while the center of the furnace gets up to high temperature.
And you can run a piston through one of the plugs at the end of
the pressure vessel and it moves by a mechanical motor on the
outside of the vessel. And then all the instrumentation
that allows us to measure the stress and displacement gauges
are all inside the vessel. And that design allows us fairly
high resolution experiments. You put all the instrumentation
outside. You could do similar type of
measurements, but you then have to deal with the fact that
you're also deforming the piston at the same time, at least
elastically if not plastically. And you have other distortions
that affect the stress. There's friction on the piston,
so you have to account for the friction with the
instrumentation inside. You get around those
complications. How do you make a sensitive
instrument survive under those conditions inside your
apparatus? This is a really good question,
and it takes clever engineering to make it happen.
If you just buy a commercial strain gauge off the shelf,
they're typically attached to a sample via an epoxy.
Put epoxy inside of a pressure vessel.
When the gas gets up to a pressure of 300 megapascals, it
permeates the epoxy really quickly, and that's not a
problem at that point. But when you lower the pressure
back down, all that argon gas trapped inside the epoxy wants
to get out and it tries to diffuse out, and when it does,
it forms bubbles. And when you now have a high
pressure bubble and you take it down to one atmosphere, the
bubbles will explode and your strained age is destroyed.
So it's complicated, but people have worked on this problem to
try to design displacement transducers that will work under
very high pressure conditions. So how do you actually go about
determining the viscosity of your sample under these
conditions? Well, our focus has been in two
types of experiments. Until about 2000 or the turn of
the century, we did most of our experiments in uniaxial
compression. So we would put a sample inside
of the pressure vessel, confined it with high pressure gas and
then use a piston coming through the lower seal in the vessel to
push on the sample and compress it and make it grow shorter as a
function of time. The other approach we've taken
is to torsion experiments, where you can twist the sample to
fairly high strain. You do one full revolution of a
sample and torsion. You can get strains on the order
of 10. Just to explain the units,
strain is defined as the ratio of a change in a dimension to
the original dimension, and strain rate is the strain change
per second and. That's kind of fun to do because
the Earth does deform rocks to fairly high strains and you can
begin to reproduce that. And again, what we measure in
these experiments typically is the length of the sample as a
function of time. That is how long the stress has
been on the sample, and you can watch the sample get shorter.
Or if you're twisting, you can watch the deformation of the
sample rotate through. And we also measure the stress
on the sample at the same time. So we typically apply a constant
rate of deformation and measure the stress that it requires to
keep that rate of deformation going.
And for us, we've really focused on the steady state part of the
problem where you twist the sample at a constant rate and
eventually the stress will level off.
And when the rate of deformation is fixed by the stress level,
you reached into a steady state condition and that's kind of
where we've largely focused our effort.
So what are the time scales of your experiments?
I mean, I know on earth these things happen over millions of
years. How long do you take to rotate
your sample, say through 360 or two revolutions, and achieve
that steady state that then you're trying to measure?
For torsion experiments, we typically work at strain rates
of about 10 to the -4, maybe 5 * 10 to the -5.
But one of the points about gas medium deformation apparatuses
is that they really do take some level of monitoring.
So we typically would start the experiment early in the morning
and run it for maybe a 12 hour period and that's what really
dictates the strain rate. We could deform more slowly and
in compression experiments we do, we might deform at 10 to the
-6 per second, but the Earth is still out there at 10 to the
-14. So the difference between 10 to
the -410 to the -6 doesn't get you hugely closer to what's
going on in the Earth. And so that again the element
that's important is to try to design experiments where you
really take advantage of what you can do well.
And then again as we've talked a little bit about have equations
or flow laws that allow you to extrapolate that with some level
of confidence to the Earth. How long does it take roughly
then to get up to that steady state?
Within an hour or two you can get from ambient conditions to a
steady state deformation. And the nice part about that is
that then if you want to determine, for example how the
viscosity of the rock depends upon stress, you can look at
several different stresses during the experiment and then
have the ability to go back, analyze that data and determine
what is the constitutive relation that describes the
viscosity dependence in this case on stress.
Or you could do temperature stepping experiments and maybe
look at three or four different temperatures on the same sample.
So we try to extract more than just getting to study state.
We try to kind of tickle the sample a little bit once we have
it at conditions and get it to tell us exactly how it wants to
behave. Let's talk about your results
then. How does the viscosity at
various temperatures and pressures depend on the melt
fraction and the water content? So the viscosity of these Alvin
rich rocks at a given pressure and temperature decreases
approximately linearly with increasing water content in the
sample. For melt fraction, the viscosity
decreases approximately exponentially with increasing
melt fraction in the sample. So it's very very sensitive to
melt fraction. It is very sensitive to melt
fraction. If you add 1% melt, you get a
decrease in viscosity by about a factor of 2.
If you go to 10% melt, decrease the viscosity very well over a
factor of 10 O. If you can store much melt in
the sample, it will be very effective at reducing its
viscosity. Of course, you have to keep in
mind that Melt has a lower density than the solid art
around it, so it does want to rise buoyantly out of the
mantle. So there is some competition
between how much Melt you can maintain and its effect on the
viscosity of the rock. But then the melt fraction then
also as you just said, if it's more mobile.
And so does that actually escape from your sample?
And do you find it all kind of clustering around the edge of
your capsule? Fortunately, gravity doesn't
play a very big role because our sample sizes are so small, so in
fact surface tension is more than adequate to keep the melt
distributed essentially homogeneously throughout the
sample. OK, so the melt stays within the
sample, but how is it structured?
Does it form in particular regions and does that give us
any insight as what happens to melt when it occurs naturally
100 kilometers down? It's a really interesting
question. The first thing that happens if
you start twisting A partially molten sample is the melt
pockets. The melt, which sits in triple
junctions between grains, begin to get themselves oriented, and
they know about the stress field and about pressure gradients in
the sample. If you start to twist the higher
strains, typically a strain of one or higher, the melt actually
begins to segregate. So at first the melt pockets at
the grain scale Orient and then those melt pockets feel a
pressure gradients develop throughout the sample and they
melt will segregate and they'll form melt rich regions, which
we've called melt rich bands or sheets of melt that are kind of
separated by regions that are basically melt free.
So you produce a band that are space maybe 100 microns apart
and the amount of melt in those bands might reach 25% while the
regions between the bands are actually drops way down.
So you produce these really melt rich regions that can act as
shear zones because they pine melt fraction regions have low
viscosity so you can share on them easily, and they're also
high permeability paths. And those paths may be important
as one way in which you can remove melt from the melt source
region and get it to volcanic structures sitting near the
surface of the Earth. In the real Esteemosphere, we
think there was both melt and water present.
Can we use your results to predict what happens in those
circumstances? We can certainly make some
inferences from our experimental results.
I think ultimately these results go into people who are doing
numerical models of convection and subduction, but we can
understand where these results might be important.
One area that we've talked a little bit about is what happens
beneath the mid ocean Ridge as upwelling mantle begins to melt,
as it depressurizes and that melt moves upward, and as it
does, the water that's in the mantle partitions into that melt
and the melt then as it goes up buoyantly to the surface to form
oceanic crust has to strip the mantle in that region of its
water and therefore you should be strengthening that part of
the mantle. It's one way in which you could
think about forming A lithosauric plate is by having
the melt move through the rock. Suck out all the water and leave
behind dry rocks that are stronger than the wet rocks.
So that's one way in which you can think about evolution of a
lithosphere. And there's been a group in
Japan who've looked at what happens in the oceanic
lithosphere and seismically looking at the transition
between the lithosphere and the athenosphere, and that interface
is often called the lithosphere athenosphere boundary.
And that transition is relatively sharp.
It occurs over a distance of about 10 kilometers.
And it's really hard to see how you can do that with just
changing the temperature, which changes relatively gradually as
you go down to greater depths. And they've argued that if you
had even 1% melt in the acinosphere and you allowed it
to segregate into these melt rich sheets that we talked
about, that the net viscosity of that part of the mantle or the
acinis or part of the mantle would be in fact fairly
significantly lower than in the lithosphere above it.
Again, just not with a large amount of melt, but with a small
amount of melt, but where that melt is kind of localized or
focused into these melt rich bands.
Well, that's really interesting. So it suggests that really the
mechanical contrast between the lithosphere and the acinosphere
is fundamentally down to the melt fraction.
Certainly you can make the case for that.
There were experimental results from the lab would support that
point of view. Again, it requires in that
particular model that melt segregate into melt with sheets,
which provides some areas that are really quite weak sitting
beneath the lithosphere. I think there's still quite a
bit of debate about exactly how this works.
There have been a number of papers written more recently
that kind of take a more detailed look at the rules of
water and the rule of melt. We already alluded to the
enormously different time scales and then of course there are
also the space scales between your experiments as compared to
those that prevail in the tectonic settings that we're
trying to understand. How do we deal with that?
And are you able to develop scaling laws that somehow rather
enable us to get from the 24 hour centimetre scale of your
experiments to the varsity slower millions of years and
kilometer scale of the actual Earth?
Well, fortunately we're not the first people to work on the
strength of materials. There's whole fields of material
science and engineering, for example, that has been very
concerned with the strength of materials and not only with the
strength of materials or the viscosity of materials.
They also have engineering problems where you need to
extrapolate from time scales. Not as extreme as we're
interested in, but they might need to extrapolate from testing
materials over a day or two, much like we do.
And they have applications that might need to be extrapolated
out to maybe 100 years. That might be the case in
buildings or other kind of structures.
Are you're testing jet engines that need to last for 30 or 40
years. One really wants to do testing
where you can have the scaling laws that allow you to go out to
at least out to 30 or 40 years. So that helps a lot.
And there's also a lot known about the details of physics of
ductal deformation materials. We know that deformation occurs
primarily by movement of defects.
Those defects can be point defects like atoms and ions
moving around, or they can be dislocations, which are line
defects moving through the material.
So we at least have the starting skeleton of what we're trying to
work out in terms of constitutive relations or flow
laws for extrapolation. And once we've done the
extrapolation that we take our lab results and extrapolate it
to a depth of 300 kilometers and a temperature of 1500°C, we can
look in the Earth and find places where there are
geophysical measurements that give us some constraints.
People have used analysis, for example, of the geoid and
analysis of the rate of post glacial rebound as ways of
constraining the strain rate and calculating the stresses that
should be applicable under those deep Earth conditions.
And it turns out that the lab results do a pretty good job of
predicting what, in fact, the Earth is telling us it's doing.
Is it possible to say that we know what the viscosity is on
geological time scales? Just say 300 kilometers depth
and 1500°C to within a factor of 10% or a factor of 2 or just the
right order of magnitude. Right.
Order of magnitude might be roughly where we're at right
now. I think these things can be
tightened up with additional experimental work, but there are
a lot of complications that haven't even been put into the
experiments at this point. I've been talking about olivine,
but the Earth is made-up of more than olivine.
What happens if you add a little bit of parenting to that sample?
Or what happens if you change the grain size?
So it's a big parameter space and I would argue a real need
for additional experiments and additional experimentalists to
do those experiments in order to make the flow laws and
extrapolation of those flow laws more robust than it is right
now. I did an earlier session with
Alan McNamara who models the deep mantle, does computer
simulations and no doubt uses what he can from experiments
such as yours as inputs to his model.
And I asked him what the biggest uncertainty was in his models.
And he said that he really thought that we didn't know the
viscosity at depth to within several orders of magnitude
because it's extremely non linear in the way it depends on
these parameters. So getting down to one order of
magnitude is something we should be extremely proud of.
Well, I think it's quite amazing actually that we can do that.
Well, if if you think about all the complications in the Earth
in terms of knowing what the temperature is, the pressure, we
can calculate quite well. But there's just a lot of trying
to constrain what the physical parameters are in terms of rock
composition and water content and melt fraction.
There are a lot of variables in the system and then what's down
there is a long way away. Let's talk a bit about some of
the implications of your experimental results on our
understanding of plate tectonics.
For example, does your work help us understand whether the
acinosphere might have the convection cells that have been
theorized with lithospheric plates being carried along above
them? The viscosity measurements from
the lab do help constrain what's going on in the convection
cells. For example, when I first
started thinking about this problem, people were modeling
convection as a linear viscosity and Newtonian viscosity, where
the rate of deformation was linearly proportional to the
stress. That has really changed due to
the laboratory experiments which demonstrate quite clearly that
the viscosity indeed is a function of the stress, not a
Newtonian or linear viscosity. And I think that has entered
into people's calculations of convection cells and had had
some effect on how people think about modelling convection, kind
of locally near the surface, but even at greater depths in the
Earth. In an earlier episode, David
Berkovici talked about how stressed mineral grains in
oceanic lithosphere can radically weaken oceanic plates,
which in turn could help explain how such plates can sag down
into the mantle and start subducting.
Does your work support this idea?
So we've recently have undertaken experiments which we
combined 2 minerals, still not all the complexity of the
mandrel, but taking two minerals, olivine and paroxene
and mixed the two together and then deformed the very large
strain and watch the evolution of the microstructure and the
strength of the rock as the strain increases.
And much as was predicted, the two phases as they mixed
together often form shear zones that are weak and these are the
shear zones that extrapolated back to the subduction problem,
allow the plate to bend and deform and to soften as it goes
down deep into the mantle. So yes, there's a really direct
connection between David Bergavici's work and what we've
been doing. And our hope is that we're able
to provide him with, for example, constitutive equations
that describe the strength of the rock or the viscosity of the
rock as a function of, for example, how much peroxine is
present, what is the allavine to peroxine ratio, How quickly does
the strength evolve? As you go to higher strains,
these rocks tend to get weaker and weaker and weaker.
Do your results on the melt fraction help us understand how
the mantle melts and causes volcanism?
Once the melting has occurred, our results really focus on
telling you something about how strong the rock is and how
rapidly you can transport melt from the source region to the
surface. Again, if you form these melt
rich bands, they really are high permeability bands and allow
melt transport to occur much more quickly than it would.
If the melt had to move through the Alabama grains like water
moves through sands, it would move fairly slowly.
But if you can concentrate the melt into certain regions, that
permeability and rate of melt transport is really is much
quicker through that was the region than through the water,
through sand kind of analogue. Geo experimental results help us
understand the conditions under which rocks give way suddenly
under applied stress and trigger an earthquake.
If you think about it in terms of depth in the Earth, we've
mapped out the lower part of the deformation behavior and there
have been studies that have focused totally on brittle
behavior that have mapped the upper part near the surface as a
function of depth. And if you take as a crude but
first order approximation that the intersection of those two
real logical laws, the one for brittle deformation up above and
the one for ductal deformation down below, and look where they
intersect, they kind of begins to get you into the right regime
of where you have to have interaction between brittle
inductal processes. If you look at the location
where these two geological laws intersect each other, you're
getting yourself into the earthquake forming region and
then it's a matter of mapping out in detail.
Now I get to have both brittle processes and ductal processes
happening. And how do we interact?
Does one stimulate the other? Does one enhance the other?
Does one suppress the other? You begin to to know where the
earthquakes are occurring, and the complication really is
trying to understand those physical details at the
microstructural level of how, in fact, does a crack forming cause
dislocations to be activated, combining the brittle behaviour
of cracks and the ductile behaviour produced by
dislocations. David, Cole said.
Thank you very much. It has been a pleasure chatting
with you, Oliver. To see pictures and
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