Geoscience Reference
In-Depth Information
6
Chemical Composition of the Earth's
Lower Mantle: Constraints from
Elasticity
MOTOHIKO MURAKAMI
Department of Earth and Planetary Materials Science, Graduate School of Science,
Tohoku University, Sendai, Japan
Summary
relevant to the lower mantle is also rapidly
becoming matured very recently. A tightly
constrained lower mantle mineralogical model
would be obtained in the nearest future, which
will resolve the issue of whether the lower
mantle is chemically distinct from the upper
mantle.
A long-standing challenge in geophysics and
geochemistry has been the inference of the chem-
ical composition of the Earth's lower mantle.
Direct comparison of sound velocities of lower
mantle phases under the relevant conditions with
seismological data is one of the most promising
approaches toward constraining lower mantle
mineralogy/chemistry. Despite considerable ef-
forts for the determination of high-pressure
sound velocities for several decades, available
sound velocity data under the lower mantle
pressure regime have been very limited due
to the experimental challenges. Recent experi-
mental progress with the Brillouin scattering
spectroscopy in a diamond anvil cell enables to
determine the sound velocities of lower mantle
phases including silicate post-perovskite phase
under extreme high-pressure conditions up to
the base of the mantle. Mineralogical modeling
combined with the latest sound velocity data
suggests that a lower mantle with at least
6.1
Introduction
The Earth's lower mantle represents almost half
of the total volume of the Earth. Elucidating
the chemistry of the lower mantle is therefore
a key to understand the structural, thermal and
dynamical evolution of the Earth. The Earth's
uppermost mantle, perhaps down to the top of
the transition zone, is widely believed to have a
peridotitic/pyrolitic bulk composition based on
the geochemical and petrologic considerations
(Ringwood, 1975; Sun, 1982). A number of pos-
sible compositional models of the lower mantle
have been proposed to satisfy the constraints im-
posed by seismological models of the interior,
ranging from an olivine-rich pyrolite (Ringwood,
1979) to more chondritic or cosmic compositions
(Anderson, 1989),. However, this issue remains
controversial due to the lack of conclusive exper-
imental evidences.
A chemically homogeneous pyrolite model of
the mantle would imply that a Mg/Si ratio of
∼
90%
silicate perovskite by volume, implying that the
bulk composition of lower mantle is likely not
the conventional peridotitic (pyrolitic) model but
more chondritic (silica-rich) one. While the effect
of high-temperature on the elasticity has yet to be
experimentally elucidated fully, sound velocity
measurement technique under simultaneously
high-pressure AND high-temperature conditions
∼
1.3 based on a mineralogy dominated by olivine,
