Geoscience Reference
In-Depth Information
in the lowermost-mantle (Murakami
et al
., 2004,
2005) and also the center of the core materials
(Dubrovinsky
et al
., 2007; Tateno
et al
., 2010)
under relevant pressure and temperature condi-
tions. A combination of the Brillouin scattering
spectroscopic technique and the diamond anvil
cell therefore provides a powerful means for de-
termining the sound velocities of materials under
the deep regions of the Earth.
Since Weidner first applied the Brillouin
scattering method to the geophysically rele-
vant materials in the 1970s (Weidner
et al
.,
1975), a number of elasticity data of deep
Earth's materials have been collected from
the single-crystal samples under high-pressure
conditions. However, high-quality Brillouin data
generally require clear transparent, colorless,
unstrained single-crystals with perfectly parallel
and mirror-polished surfaces. Most results so far
have, therefore, been limited to conditions below
∼
the sufficient randomness of the orientation
distribution inside the sample. In addition,
the potential uncertainties derived from the
anisotropic feature of the sample can be assessed
by measuring the acoustic wave velocity in
different direction/angle by rotating the sample.
Although the polycrystalline data do not provide
full descriptions of
Cij
, the aggregate elastic
properties directly obtained from polycrystalline
samples are enough for the purpose to evaluate
the lower mantel models to a first approxima-
tion. These studies demonstrated the potential
of laser annealing techniques, which efficiently
promotes the synthesis of polycrystalline sample
and reduces the deviatoric stresses on the sample,
thus
substantially
improving
the
quality
of
Brillouin
spectra
of
aggregate
samples
under
extreme pressure conditions.
6.2.2 Challenges in high-temperature
measurement
20 GPa due to the pressure-induced bending or
deformation of the single-crystal sample by the
deviatoric stresses in the sample chamber. Recent
advances in high-pressure Brillouin spectroscopy
measurements extended drastically the upper
pressure limit, exceeding 200 GPa, which is outer
core pressure (Murakami & Bass, 2010, 2011;
Murakami
et al
., 2007a,b) have first reported
aggregate shear wave velocities of polycrystalline
MgSiO
3
perovskite and post-perovskite phases
within their thermodynamic stability
P-T
fields.
These samples were synthesized in situ in a DAC
from gel starting materials by heating with an
infrared laser up to the pressure of 172 GPa.
Unlike the Brillouin measurement data on
single-crystals, the Brillouin scattering results
on polycrystalline sample give us the direct
aggregate elastic properties without using any
averaging/bounding schemes under the simple
assumption that the Brillouin peaks represents
the average acoustic wave velocity of randomly
oriented polycrystalline specimen. In general,
the grain size of the polycrystalline sample
synthesized from the gel starting material under
lower mantle
P-T
condition is fairly small
below several hundreds of nanometers in size
(Murakami
Simultaneous measurements under pressure and
temperature condition of the lower mantle are
still a technical challenge. Among others, heating
is an important issue. Application of resistive
heating technique in a DAC, which is the con-
ventionally used technique for Brillouin method,
normally works well below 10 GPa and 1000 K
(Sinogeikin & Bass, 2000; Sinogeikin
et al
., 2006).
Generating higher temperatures by resistive
heating is difficult. The advancement of infrared
laser heating technique in a DAC combined with
synchrotron radiation has recently allowed us
to explore successfully the phase equilibria in
the lowermost-mantle and core materials under
relevant pressure and temperature conditions
(P
>
100 GPa, T
>
3000 K). An infrared laser
heating technique would be a favorable alter-
native to overcome this temperature issue,
which can potentially generate over 3000 K at
higher-pressures (Knittle & Jeanloz, 1989; Zerr &
Boehler, 1993; Shen & Lazor, 1995; Zerr
et al
.,
1997; Shen
et al
., 1998).
High-temperature Brillouin scattering mea-
surements with laser heating have been con-
ducted at ambient pressure on single-crystals of
et
al
.,
2005),
which
can
ensure
