Geoscience Reference
In-Depth Information
The mantle is full of mysteries which at first sight seem to be contradictory. It is solid yet it
can flow. It's made up of silicate rock which is a good insulator, yet somehow about 44 ter-
awatts of heat finds its way to the surface. It's hard to see how that heat flow could happen
through conduction alone, and yet, if there was convection, the mantle would be mixed, so
how could it show a layered structure? And how could ocean volcanoes erupt magma with
a different mix of tracer isotopes than that believed to exist in the bulk of the mantle, unless
there are unmixed regions or layers? Resolving these mysteries has been one of the prime
areas of geophysics in recent years.
A diamond window on the mantle
Some of the best clues have come from understanding the nature of the rocks down there.
To find what the rocks are like deep within the Earth, you have to replicate the fantastic
pressures down there. Amazingly, that's possible with just your finger and thumb. The trick
is to get hold of two good, gem-quality diamonds, cut in what jewellers term 'brilliant' cuts,
with a tiny, perfectly flat face at the apex of each. Mount them face to face with a micro-
scopic rock sample between the two, and turn a little thumbscrew to force the faces tighter
together. The force gets so concentrated between the tiny diamond anvils that it's possible
to create pressures more than 3 million times atmospheric pressure (300 gigapascals), just
by turning the screw. Because the diamonds are conveniently transparent, the sample can
be heated by shining a laser in, and viewed with a microscope and other instruments. This
can literally be a window on what rocks are like deep in the mantle.
Professor Bill Bassett was studying a tiny crystal in a diamond anvil one day in his lab
at Cornell University. Nothing much had happened when he'd increased the pressure, so
he decided to go for lunch. As he was leaving, he heard a sudden 'crack' from the anvil.
Certain that one of his precious diamonds had broken, he rushed back and looked down
the microscope. The gems were OK, but the sample had suddenly transformed into a new,
high-pressure crystal form. It was what is known as a phase change: the composition re-
mains the same but the structure changes, in this case into a more dense crystal lattice.
We know from the composition of xenoliths that at least the upper mantle is made of rocks
such as peridotite, rich in the magnesium and iron silicate mineral olivine. Put a tiny sample
of this between diamond anvils and turn up the pressure and it goes through a whole series
of phase changes. At a pressure of about 14 gigapascals, equivalent to a depth in the mantle
of about 410 kilometres, olivine transforms into a new structure called wadsleyite. At 18
gigapascals, 520 kilometres down, it changes again, adopting the structure of ringwoodite,
a form of the mineral spinel. That then changes at 23 gigapascals, corresponding to 660
kilometres down, into two minerals, perovskite and a magnesium iron oxide mineral called
magnesiowüstite. You'll notice that the phase changes happen at precisely the depths at
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