[tt] Intermediate-Spin Ferrous Iron in Lowermost Mantle

[Eugen Leitl]

http://www.physorg.com/news140775122.html

Researchers discover unexpected properties of materials in lowermost mantle

Materials deep inside Earth have unexpected atomic properties that might
force earth scientists to revise their models of Earth's internal processes,
a team of researchers has discovered.

The researchers recreated in the lab the materials, crushing pressures and
infernal temperatures they believe exist in the lowermost mantle, nearly
2,900 kilometers (1,800 miles) below Earth's surface. They report in the
journal Nature Geoscience the materials exhibit rare and unexpected atomic
properties that might influence how heat is transferred within Earth's
mantle, how columns of hot rock called superplumes form, and how the magnetic
field and heat generated in Earth's core travel to the planet's surface.

The planetary building blocks magnesium, silicon, oxygen and iron are the
most abundant minerals in the lowermost mantle. A team of scientists led by
Jung-Fu Lin at The University of Texas at Austin's Jackson School of
Geosciences synthesized materials from these building blocks in a diamond
anvil cell, a device containing two interlocking diamond pieces that squeeze
the sample like a vice. They subjected the sample to more than 1.3 million
times standard atmospheric pressure. Shining a laser through the transparent
diamonds, they then heated the sample to almost 3,000 degrees Celsius (5,400
degrees Fahrenheit) for several days.

The scientists used the nation's most powerful source of X-rays, a facility
at Argonne National Laboratory called a synchrotron light source, to reveal
the sample's electronic and atomic structure. They determined the high
pressures had caused some of the electrons in the sample's iron, which
normally repel each other, to "pair up" or become bound to each other.
Earlier experiments by Lin and others had found evidence for areas in the
lower mantle in which electrons were either mostly paired up or were mostly
unpaired. This was the first evidence of a broad region in the subsurface
with what scientists describe as "intermediate-spin state," or partially
paired iron electrons.

"We were surprised to find partially paired electrons," said Lin. "That
doesn't normally occur in other geological materials that we know about."

The degree of electron pairing, also known as electronic spin state, can
affect how well the materials conduct heat and electricity. Lin said modelers
who make computer simulations of mantle dynamics will now have to go back and
try to determine how this intermediate-spin state might affect the way heat
is transferred within Earth, how superplumes form, how convection occurs in
the mantle and how Earth's magnetic field might radiate from the core.

The electronic spin state can also affect the speed of seismic waves
traveling through material in the deep mantle. As a result, seismic images of
the lowermost mantle—collected when earthquake vibrations travel through and
reflect off of material in the subsurface—may have to be reinterpreted.

Nature Geoscience will publish the paper, "Intermediate-Spin Ferrous Iron in
Lowermost Mantle Post-Perovskite and Perovskite," in its October 2008 edition
and online Sept. 14.

Source: University of Texas at Austin