Geoscience Reference
In-Depth Information
( X pv ) than the bulk iron contents ( X Fe ), and
the elastic modulus is fairly insensitive to the
temperature relative to the mineral proportion.
However, recent computational simulations have
demonstrated that density/bulk modulus, derived
from such volume data, do not place unique con-
straints on mantle mineralogy due to an intrinsic
uncertainty, whereas the shear velocity data play
in placing much stronger constraints on lower
mantle models (Mattern et al ., 2005). Indeed,
recent experimental results on the density
measurements of the lower mantle phases under
high-pressure and high-temperature show that
the density difference between peridotitic and
perovskitic lower mantle models is only less than
2000; Sinogeikin & Bass, 1998, 2000, 2002).
With those techniques, substantial experimental
dataset on sound velocities of Earth's deep mate-
rials has been accumulated at pressure conditions
corresponding to the mantle transition zone.
However, only few sound velocity measurements
exist under lower mantle pressure condition
because of the experimental difficulties, which
have
prevented
us
from
making
the
reliable
mineralogical model of lower mantle.
Recently remarkable advances in Brillouin
scattering spectroscopy have been made using
the diamond anvil cell apparatus to explore
the sound velocities of lower mantle phases
under extremely high-pressure conditions. In this
review, we focus on recent technical progress
in the determination of sound velocities under
high-pressure and/or high-temperature condi-
tions corresponding to the lower mantle. First,
the experimental method of Brillouin scattering
spectroscopy combined with diamond anvil cell
is briefly outlined. Then, representative results
on the shear wave velocity measurements of
lower mantle phases will be reviewed. In the final
section, the mineralogical model of lower mantle
offered by those new data will be discussed.
0.5% on average throughout the lower mantle
condition (Fei et al ., 2007; Komabayashi et al .,
2008, 2010; Lundin et al ., 2008; Ricolleau et al .,
2009), which is almost indistinguishable within
the uncertainties in the one-dimensional global
seismic model such as PREM. This is mainly
due to the relatively smaller difference in density
among the lower mantle phases than that in
shear wave velocity. Reliable shear wave velocity
data for the primary lower mantle constituents
under relevant conditions are thereby critical for
understanding the lower mantle composition and
resolving the issue as to the mantle convection
and missing Si.
Considerable effort has so far been focused on
the development of laboratory measurements to
determine the sound velocities at high-pressure
and high-temperature. Recently developed and
improved techniques for the sound velocity
measurements include ultrasonic interferometry
(Kinoshita et al ., 1979; Yoneda, 1990; Yoneda
& Morioka, 1992; Rigden et al ., 1994; Li et al .,
1996, 1998; Liebermann & Li, 1998; Sinelnikov
et al ., 1998; Chen et al ., 1998; Bassett et al ., 2000;
Higo et al ., 2006; Irifune et al ., 2008), impulsive
stimulated scattering (Brown et al ., 1989; Zaug
et al ., 1993; Abramson et al ., 1997, 1999), inelas-
tic X-ray scattering (Mao, 2001; Fiquet, 2004),
nuclear resonance inelastic X-ray scattering
(Lubbers et al ., 2000; Mao et al ., 2001; Shen
et al ., 2004) and Brillouin scattering spectroscopy
(Duffy & Ahrens, 1995; Zha et al ., 1997, 1998,
6.2
Brillouin Scattering Spectroscopy
Brillouin scattering spectroscopy is one of the
powerful techniques in determining the sound-
wave velocities based on the inelastic scatter-
ing related to the photon-phonon interactions
between the focused optical probe (laser) and a
sample. In the Brillouin spectroscopy, the scat-
tered light consists of an elastically scattered
component with frequency ω (Rayleigh scattered
component) as well as inelastically scattered com-
ponents (Brillouin scattered components) with
a frequency shift ω (Brillouin frequency shift)
caused by the interaction with thermally gen-
erated phonons in the sample (analogous to the
Doppler shift). The acoustic wave velocity, V ,in
a symmetric scattering geometry is simply calcu-
lated from the relation (Whitfield et al ., 1976):
ωλ
2 sin ( θ/ 2)
V
=
(6.1)
Search WWH ::




Custom Search