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Surface topography and bathymetry around South
America (top) overlays variable topography on Earth's upper mantle
phase transition discontinuities at 410 km (middle) and 660 km
(bottom) depth (topography is contoured in 2 km increments).
Topography on the discontinuities is used to characterize
compositional and thermal heterogeneity within the Earth. In this
region, the large depressions are related to subduction processes,
whereby cold oceanic lithosphere descends into the mantle.
Image � by Nicholas Schmerr,
Edward Garnero, Arizona State University
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The simplest model of the mantle
- the layer of the Earth�s interior just
beneath the crust - is that of a convective
heat engine. Like a pot of boiling water, the mantle has parts that
are hot and welling up, as in the mid-Atlantic rift, and parts that
are cooler and sinking, as in subduction zones. There, crust sinks
into the Earth, mixing and transforming into different material �phases,�
like graphite turning into diamond.
�A great deal of past research on mantle structure has interpreted
anomalous seismic observations as due to thermal variations within the
mantle,� Schmerr said. �We�re trying to get people to think about how
the interior of the Earth can be not just thermally different in
different regions but also chemically different.�
The research, which Schmerr conducted with Edward Garnero, a professor
in ASU�s School of Earth and Space Exploration, was published in the
October 26 (2007) issue of the journal Science.
Their article is titled �Upper Mantle Discontinuity Topography from
Thermal and Chemical Heterogeneity.�
Schmerr�s work shows that Earth�s interior is far from homogeneous, as
represented in traditional views, but possesses an exotic brew of down
and upwelling material that goes beyond simply hot and cold convection
currents. His work demonstrates the need for a chemical component in
the convection process.
At key depths within Earth, rock undergoes a compression to a denser
material where its atoms rearrange due to the ever-increasing pressure.
Earth scientists have long known that the dominant mineral olivine in
Earth�s outer shell, compresses into another mineral named wadsleyite
at 410 km (255 mile) depth, which then changes into ringwoodite around
520 km (325 mile) depth and then again into perovskite +
magnesiow�stite at 660 km (410 mile) depth.
These changes in crystal structure, called phase transitions, are
sensitive to temperature and pressure, and the transition depth moves
up and down in the mantle in response to relatively hot or cold
material.
Beneath South America, Schmerr�s research found the 410 km phase
boundary bending the wrong way. The mantle beneath South America is
predicted to be relatively cold due to cold and dense former oceanic
crust and the underlying tectonic plate sinking into the planet from
the subduction zone along the west coast. In such a region, the 410 km
boundary would normally be upwarped, but using energy from far away
earthquakes that reflect off the deep boundaries in this study area,
Schmerr and Garnero found that the 410 km boundary significantly
deepened.
�Our discovery of the 410 boundary deflecting downwards in this region
is incompatible with previous assumptions of upper mantle phase
boundaries being dominantly modulated by the cold temperature of the
subducting crust and plate,� Garnero said.
Geologists and geochemists have long suspected that subduction
processes are driven by more than temperature alone. A sinking oceanic
plate is compositionally distinct from the mantle, and brings with it
minerals rich in elements that can alter the range of temperatures and
pressures at which a phase change takes place.
�We�re not the first to suggest chemical heterogeneities in the mantle,
however, we are the first to suggest hydrogen or iron as an
explanation for an observation at this level of detail and over a
geographical region spanning several thousands of kilometers,� Schmerr
said.
Hydrogen from ocean water can be bonded to minerals within the crust
and carried down as it is subducted into the mantle, Schmerr explained.
When the plate reaches the 410 km phase boundary, the hydrogen affects
the depth of the olivine to wadsleyite phase transition, reducing the
density of the newly formed wadsleyite, and making it relatively more
buoyant than its surrounding material. This hydrated wadsleyite then
�pools� below the 410 km boundary, and the base of the wet zone
reflects the seismic energy observed by Schmerr.
Alternatively, subduction can bring the iron-poor and
magnesium-enriched residues of materials that melted near the surface
to greater depths. Mantle mineral compositions enriched in magnesium
are stable to greater depths than usual, resulting in a deeper phase
transition.
�Either hypothesis explains our observation of a deep 410-km boundary
beneath South American subduction, and both ideas invoke chemical
heterogeneity,� Schmerr said. �However, if we look deeper, at the
660-km phase transition, we find it at a depth consistent with the
mantle being colder there. This tells us that the mantle beneath South
America is both thermally cold and chemically different.�
To make their observations, Schmerr and Garnero used data from the
USArray, which is part of the National Science Foundation-funded
EarthScope project.
�The USArray essentially is 500 seismometers that are deployed in a
movable grid across the United States,� Schmerr said. �It�s an unheard
of density of seismometers.�
Schmerr and Garnero used seismic waves from earthquakes to measure
where phase transitions occur in the interior of Earth by looking for
where waves reflect off these boundaries. In particular, they used a
set of seismic waves that reflect off the underside of phase
transitions halfway between the earthquake and the seismometer. The
density and other characteristics of the material they travel through
affect how the waves move, and this gives geologists an idea of the
structure of the inner Earth.
�Seismic discontinuities are abrupt changes in density and seismic
wave speeds that usually occur where a mineral undergoes a phase
change - such as when olivine transitions to
wadsleyite, or ringwoodite transforms into perovskite and
magnesiow�stite. The transformed mineral is generally denser, and
typically seismic waves travel faster through it as well.
Discontinuities reflect seismic energy, which allows us to figure out
how deep they are. They are found throughout the world at certain
average depths - in this case, at 410 and 660 km,� Schmerr said.
�Because these phase transitions are not always uniform, these layers
are bumpy with ridges and troughs.�
�Right now the big question that we have is about Earth�s thermal
state and its chemical state, and there are a lot of ways we can go
about getting at that information,� Schmerr said. �This study lets us
look at one particular area in Earth and constrain the temperature and
composition to a certain degree, imaging this structure inside the
Earth and saying, These are not just thermal effects -- there�s also
some sort of chemical aspect to it as well.�
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