Phase transition and density of subducted MORB crust in the lower ...

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K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251
pressure studies on end-member compositions suggest
that these minerals may undergo structural phase
transitions at greater depths. Phase relation and crystal
chemistry in MORB bulk composition, however,
are not yet well understood under the deep lower
mantle conditions [1,7]. Both experimental and theoretical
studies showed that pure MgSiO3 perovskite
transforms to a CaIrO3-type post-perovskite phase
near the base of the mantle (e.g., [8–10]). MgSiO 3rich
perovskite includes large amounts of FeO,
Fe 2O 3, and Al 2O 3 in MORB bulk composition,
which could significantly affect the stability of postperovskite
phase. Pure SiO2 phase transforms from
stishovite to CaCl2-type structure [11] and further to
a-PbO2-type structure [12] in the lower mantle. Phase
transition from tetragonal or orthorhombic to cubic
structure in CaSiO 3 perovskite has been reported in
both pure [13] and Al-bearing compositions [14].
Funamori et al. [15] demonstrated that CaFe 2O 4type
MgAl2O4 transforms to CaTi2O4-type structure
above 40 GPa, suggesting that phase transition of
CaFe2O4-type Al-phase in MORB composition possibly
occurs in the lower mantle. Here we determined
the phase transition boundaries in MORB composition
up to the condition of core–mantle boundary
region, based on X-ray diffraction measurements insitu
at high-pressure and -temperature. Phase transitions
in subducted MORB crust may be the causes of
seismic anomalies observed locally but in a wide
range of depths in the lower mantle (e.g., [16–20]).
Subduction of basaltic crust gives rise to a strong
chemical heterogeneity in the mantle. The amount of
basaltic component that has ever subducted into the
Earth’s interior over the geological time possibly sums
up to several tens percent of the whole mantle [21].
The fate of basaltic crust may be controlled by the
density relationship with the surrounding mantle. Previous
studies suggested that basaltic crust is buoyant
in the transition zone at 660- to 720-km depth
[2,5,6,22] and possibly in the lower mantle below
1500–2000-km depth [4,23]. In this paper, we determined
the chemical composition of each constituent
mineral by using analytical TEM. The density of
MORB crust in the lower mantle was calculated
from measured volume and chemical composition
data. Results indicate that the subducted MORB
crust is denser than the surrounding below 720-km
depth to the bottom of the mantle.
2. Experimental and analytical procedures
Starting material was a glass (runs #1 to #4) or gel
(runs #5 to #7) (Table 1). Both are chemically identical
with a composition of normal MORB (Table 2).
The same glass starting material was used in our
previous studies [2,5]. High P–T conditions were
generated using laser-heated diamond anvil cell
(LHDAC) techniques. The sample was mixed with a
fine powder of gold (~10 wt.%) that served as an
internal pressure standard and a laser absorber. The
sample mixture (~20-Am thick) was loaded into a
rhenium gasket with an initial thickness of ~50 Am,
being sandwiched by pure basalt layers that was not
mixed with gold (~15-Am thick for both sides). About
50-Am area was heated with a focused multimode
continuous wave Nd:YAG laser using double-side
heating technique, which minimizes both radial and
axial temperature gradient [24]. The laser beam was
not scanned on the sample.
Angle-dispersive X-ray diffraction measurements
were conducted in-situ at high-pressure and -temperature
at BL10XU of SPring-8. A monochromatic
incident X-ray beam with a wavelength of 0.41328
or 0.41688 A˚ was collimated to 20-Am in diameter.
X-ray diffraction spectra were obtained on an imaging
plate with exposure time of 1 to 10 min. Twodimensional
X-ray diffraction image was integrated
as a function of two-theta in order to give a conventional
one-dimensional diffraction profile. The
diffraction patterns of the sample were repeatedly
collected during heating and after quenching to
room temperature.
The uncertainty in temperature within 20-Am area
from which X-ray diffraction was collected could be
F10% [14]. Pressure was determined by applying the
equation of state (EOS) of gold, using two to four
diffraction lines. Pressure determination strongly
depends on the choice of EOS (e.g., [7]). We primarily
used EOS recently proposed by Tsuchiya [25]. Tsuchiya’s
gold scale is practically useful because it is
consistent with platinum pressure scale at 300 K [26].
Pressures calculated by EOS of gold proposed by
Jamieson et al. [27] were also presented in Table 1,
which predicts larger thermal pressure than Tsuchiya’s
EOS. The pressure uncertainties shown in Table 1 are
derived mainly from large errors in temperature to
apply P–V–T EOS.