Keeping the Earth's plates oiled
- From: "George" <george@xxxxxxxxxxxxxxx>
- Date: Mon, 13 Aug 2007 07:36:17 -0400
http://www.eurekalert.org/pub_releases/2007-08/esf-kte081007.php
Earth's surface is a very active place; its plates are forever jiggling
around, rearranging themselves into new configurations. Continents collide
and mountains arise, oceans slide beneath continents and volcanoes spew. As
far as we know Earth's restless surface is unique to the planets in our
solar system. So what is it that keeps Earth's plates oiled and on the
move?
Scientists think that the secret lies beneath the crust, in the slippery
asthenosphere. In order for the mantle to convect and the plates to slide
they require a lubricated layer. On Mars this lubrication has long since
dried up, but on Earth the plates can still glide around with ease.
Beneath continents the asthenosphere appears at around 150km depth, while
under oceans it can be as shallow as 60km. Above the asthenosphere lies the
lithosphere: a more rigid layer that includes the crust. By 220km depth the
asthenosphere comes to an end and the mantle goes back to a less flexible
state.
What makes the asthenosphere so slippery and why does it exist on Earth but
not other planets? These are some of the key questions that have puzzled
Earth scientists ever since plate tectonics was discovered, but only now
are the answers starting to emerge. A combination of new experimental
techniques and powerful computational theory is enabling scientists to work
their way through the asthenosphere atom by atom.
Björn Winker, a mineralogist at the Johann Wolfgang Goethe University in
Frankfurt, Germany, believes that the key to the asthenosphere is water.
"We have to have water in the asthenosphere to get it plastically
deforming," he explains. This water is no longer in its liquid state, but
is bound to oxygen in crystal structures to form hydroxyl (OH-) groups
instead.
The question that really interests Winkler is 'where does the water go'?
Which minerals are clinging on to their hydrogen and enabling the Earth to
perform its plate tectonic dance?
Unfortunately we can't get samples from the asthenosphere - no-one has ever
managed to drill a hole deep enough. But seismic wave patterns and magma
spurting out of volcanoes give us clues as to which minerals make up the
majority of the asthenosphere. Winkler finds samples of these candidate
minerals on the Earth's surface and, using specialist experimental
equipment, subjects them to the pressures and temperatures estimated for
the asthenosphere.
The diamond anvil cell is just one of the tools his group uses. A sample is
placed between two diamonds and compressed, to reach pressures of 10GPa -
one million times the pressure at the Earth's surface. When these
experiments are carried out at a synchrotron, which provides extremely
bright x-ray radiation, he is able to use X-ray diffraction to analyse the
way the sample behaves as the pressure is ratcheted up. "It is only
possible to make these measurements at a synchrotron," says Winkler.
"Laboratory x-ray sources are far too weak for such experiments." In other
experiments infra-red radiation shines through the sample and makes the O-H
bonds vibrate. By measuring how much of the infra-red radiation is absorbed
by the sample Winkler can estimate how much water the sample contains and
whether it manages to hold onto it as the pressure increases. However,
spectroscopic measurements can't reveal everything. "They can only give you
a frequency. It is like trying to figure out a car's problems from
listening to the way it rattles," says Keith Refson, a colleague of Winkler's
who is based at the CCLRC Rutherford Appleton Laboratory near Didcot in the
UK.
Afterwards Winkler and Refson use powerful computer calculations to work
out what the atoms are doing and where the water might be held within the
structure. "With computer models we can calculate where the sample should
rattle and match the theory with experiment," says Refson.
Already Winkler and Refson have analysed a number of minerals in this way
including 'diaspore' and 'clinochlore'. "It was known previously that
diaspore would not survive going into the asthenosphere, but we are able to
use the knowledge we have gained and apply it to other minerals," says
Winkler. Meanwhile, clinochlore was found to be good at holding onto water,
but showed some interesting changes in its structure at around 8GPa. "The
nature of the hydrogen bonds start to change and the layers within the
structure slide," explains Refson.
These kind of results have been invaluable for Hans Keppler, a geologist at
the University of Bayreuth in Germany. He has been trying to work out why
the asthenosphere exists.
Previous theories have suggested that this 'wet' and slippery layer exists
because minerals leave their water behind them when they melt and turn into
magma. "This explains why the asthenosphere appears beneath oceans, but it
doesn't explain why we have an asthenosphere beneath the continents," says
Keppler. Lava continually bubbles up at mid-ocean ridges, but continental
plates don't have an equivalent spring of constant magma. It also fails to
explain why there is a lower boundary to the asthenosphere.
Instead, Keppler has been investigating water solubility in the
asthenosphere. Using a loaded piston cylinder apparatus he was able to heat
and pressurise mixtures of aluminium-saturated enstatite (estimated to make
up around 40 percent of the asthenosphere) and water to asthenosphere
values. Similar experiments were also done with olivine (thought to make up
around 60 percent of the asthenosphere).
What he found was that water solubility in olivine continuously increases
with temperature and pressure, whereas in aluminium-saturated enstatite the
solubility reaches a distinct minimum at asthenosphere temperatures and
pressures. "It means that the mantle minerals cannot contain all the water
and the excess water forms a hydrous silicate melt," says Keppler, who
presenting his findings at the 1st EuroMinScI Conference in La
Colle-sur-Loup, France, in March this year. The presence of even small
quantities of melt in a rock in known to drastically reduce its mechanical
strength.
EuroMinScI is the European Collaborative Research (EUROCORES) Programme on
"European Mineral Science Initiative" developed by the European Science
Foundation (ESF).
The water solubility model explains why the asthenosphere has a lower
boundary and why it exists under continental and oceanic plates. Once the
aluminium-saturated enstatite passes through its minimum solubility it
starts to absorb water again and deeper in the mantle (at higher pressures
and temperatures) the mantle becomes dry once more - creating a lower
boundary.
Meanwhile, temperatures increase more slowly underneath continents, meaning
that the minimum water solubility zone for aluminium-saturated enstatite is
not reached until a greater depth under continents, compared to oceanic
plates. (see Fig 4 from the Science paper.)
For now the jury is still out on Keppler's new model. "It is a very
elegant, but simplified model," says Winkler. "Essentially it is based on
two minerals, which is definitely not the whole story. The question is, if
we refine the theory and include a greater range of minerals will it change
things much?"
Some scientists are quite hostile to Keppler's water solubility model. "It
puts a lot of people out of business," says Keppler. Nonetheless, most
people agree that the theory is consistent with what is known about the
asthenosphere and that it can't be discarded. "Only more experiments and
calculations can reveal the truth," says Winkler.
.
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