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W. L. Mao (Los Alamos National Laboratory), H-k.
Mao, Y. Meng, J. Shu, Y. Fei, R. J. Hemley
(Carnegie Institution of Washington), W.
Sturhahn, J. Zhao (Argonne National Laboratory),
and V. Prakapenka (University of Chicago)
With the synthesis of the post-perovskite (ppv)
phase in MgSiO3 (Murakami et al.,
2004), we sought to address the question of
whether Fe can play a major role in this phase
as the core-mantle boundary (CMB) represents the
region where the silicate mantle is in contact
with the liquid, Fe-rich outer core. We
conducted a number of diamond anvil cell
synchrotron x-ray experiments at the very
challenging ultrahigh pressure-temperatures of
the CMB that demonstrate that this phase can
incorporate a considerable amount of the Fe
which leads to large increases in density
relative to MgSiO3 ppv (Mao et al.,
2005; Mao et al., 2004). The vast reservoirs of
liquid Fe and Fe-poor silicates at the CMB
provides favorable chelmical-physical conditions
for the formation of Fe-rich ppv silicate which
may hold the key to understanding the
geophysical and geochemical properties of the
CMB.
Using a nuclear resonant inelastic x-ray scattering
coupled with equation of state measurements we
were able to determine aggregate sound
velocities for a ppv composed of 40% of the Fe
endmember, i.e. (Mg0.6Fe0.4)SiO3.
The determined compressional and shear wave
velocities indicate that ppv with Fe can
reproduce the dramatic depression in seismic
velocities in ultralow velocity zones (ULVZ)—i.e.
thin 5-40 km thick patches of depressed
compressional and shear wave velocities
(decreases of ~10 and ~ 30% relative
respectively to PREM) observed just above the
CMB—providing an alternative explanation for the
origin of ULVZ (Mao et al., 2006). Although
further studies of ppv as a function of
temperature and Fe concentration are still
needed, these results provide an exciting new
direction into the dominating role Fe-rich ppv
may play at the CMB.
References
Mao, W.L., Mao, H.K., Sturhahn, W., Zhao, J.,
Prakapenka, V.B., Shu, J., Fei, Y., and Hemley,
R.J., 2006, Iron-rich post-perovskite and the
origin of ultralow-velocity zones: Science, v.
312, p.564-565.
Mao, W.L., Meng, Y., Shen, G., Prakapenka, V.B.,
Campbell, A.J., Heinz, D.L., Shu, J., Caracas,
R., Cohen, R.E., Fei, Y., Hemley, R.J., and Mao,
H.K., 2005, Iron-rich slicates in the Earth’s D”
layer: Proc. Nat. Aca. Sci., v. 102, p.
9751-9753.
Mao, W.L., Shen, G., Prakapenka, V.B., Meng, Y.,
Campbell, A.L., Heinz, D.L., Shu, J., Hemley,
R.J., and Mao, H.K., 2004, Ferromagnesian
post-perovskite slicates in the D” layer of the
Earth: Proc. Nat. Acad. Sci., v. 101, p.
15867-15869.
Murakami, M., Hirose, K., Kawamura, K., Sata,
N., and Ohishi, Y., 2004, Post-perovskite phase
transition in MgSiO3: Science, v.
304, p. 855-858. |
