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Mineralogy and phase transition in the lower mantle
Shigeaki Ono
Research Program for Geochemical Evolution, Institute For Research on Earth Evolution (IFREE)
1. Introduction
phase transitions. The size of the heating spot was 20-30 and 50100 µm for the YLF and YAG lasers, respectively. The laser
beam was not scanned to heat the sample, because scanning leads
to huge temperature gradients, and thus promotes inhomogeneity
of the chemical equilibrium in the sample. The sample temperature was measured using the spectroradiometric method [Ono et
al., 2005b]. The pressure was determined from the observed unit
cell volumes of gold using the known equations of state of platinum or gold. The samples were probed using an angle-dispersive
X-ray diffraction technique at the synchrotron beam line at the
BL10XU, SPring-8 [Ono et al., 2005c] and at the BL13A, PF
[Ono et al., 2005d]. A monochromatic incident X-ray beam was
used. The X-ray beams were collimated to a diameter of 10-20
µm, and the angle-dispersive X-ray diffraction patterns were
obtained on an online imaging plate (Rigaku, Japan). The
observed intensities on the imaging plates were integrated as a
function of 2θ using the ESRF Fit2d code to obtain conventional,
one-dimensional diffraction profiles.
An investigation of the phase relations of lower mantle materials is essential for evaluating seismic observations and their geodynamic implications [e.g., Sidorin et al., 1999; Ishii and Trom,
1999; van der Hilst and Karason, 1999; Panning and Romanowicz,
2004; Kellogg et al., 1999]. Phase relations of peridotite (pyrolite)
and normal-type mid oceanic ridge basalt (N-MORB) compositions have been determined by means of multianvil apparatus up
to 37 GPa [e.g., Irifune and Ringwood, 1987; Irifune et al., 1993;
Ono et al., 2001] and also in laser-heated diamond anvil cells
(LHDAC) up to 135 GPa [e.g., O’Neill and Jeanloz, 1990; Kesson
et al., 1994; Kesson et al., 1998]. In previous experiments at pressures corresponding to the lower mantle, the phase relations were
confirmed by electron microscope or X-ray diffraction methods
on T-quenched or P-T-quenched samples. This approach suffers
from the disadvantage that some high-pressure phases are
unquenchable. Here, we report on observations made at simultaneous high pressures and temperatures in order to obtain new
insights into the mineralogy of basalt and peridotite under actual
lower mantle conditions.
In order to achieve that objective, the phase relations for natural KLB-1 peridotite, N-MORB compositions have been determined up to 140 GPa and 3000 K, that is, under conditions corresponding to those in the entire lower mantle. And the comparison
between the estimated densities of KLB-1 peridotite and subducted N-MORB with average mantle density profiles reported in
seismological studies has been reported. The results provide
insight into the phase relations of natural mantle compositions and
the dynamics of deeply subducted slabs during their passage into
the lower mantle.
3. Results and discussion
Experiments for the KLB-1 peridotite composition, which is
similar to pyrolite, were conducted at pressures of 27 to 130 GPa
and temperatures of 1700 to 2600 K. The KLB-1 peridotite composition sample crystallized into an assemblage of orthorhombic
Mg-perovskite + cubic Ca-perovskite + magnesiowüstite at pressures below 125 GPa [Ono et al., 2005b]. This result is in good
agreement with those of previous quench experiments using both
multi-anvil presses and DACs [e.g., Irifune and Ringwood, 1987;
Kesson et al., 1998]. After heating, the temperature was decreased
to 300 K and the diffraction pattern recorded. The phase assemblage remained, but the structure of Ca-perovskite had changed.
The observed change in the 200 peak from single to double peaks
(200 and 002), indicates that the cubic Ca-perovskite had transformed into the tetragonal phase [Ono et al., 2004a]. At this fixed
load, the sample was heated again. Although Mg-perovskite
retained its orthorhombic structure, Ca-perovskite changed back
once more to a cubic structure. Then, the sample was compressed
and heated to 120 GPa and 1700 K. New diffraction peaks
appeared from the KLB-1 peridotite composition at 125 GPa and
high temperature. These new peaks indicated that the Fe- and Albearing Mg-perovskite transformed to the CaIrO3-type structure,
which was in good agreement with pure MgSiO3 study [Oganov
and Ono, 2004]. The same heating method was performed in other
heating cycles. The CaIrO3-type phase assemblage remained stable up to 130 GPa and 2500 K (Fig. 1).
Experiments for the N-MORB composition were conducted at
pressures from 30 to 145 GPa [Ono et al., 2005b]. Within a few
minutes of the temperature reaching the desired value, the starting
material crystallized into an assemblage of Mg-perovskite + Ca-
2. Experimental
Synthetic gel powders of natural compositions were used as the
starting materials. The chemical compositions of these gels were
similar to KLB-1 peridotite [Ono et al., 2004a] and N-MORB
[Ono and Yasuda, 1996], and the compositions were confirmed
using XRF analysis. High-pressure X-ray diffraction experiments
were performed using a double-sided laser heating diamond anvil
cell (DAC) system. A symmetric-type diamond cell (SYNTEC
Co. Ltd., Japan) with 60º conical apertures was used [Ono et al.,
2005a]. The sample powders were mixed with gold powder, used
as an internal pressure calibrant. These powders were mechanically ground in an agate mortar for several hours to ensure homogeneity and a small grain size. The sample was pressed into a disc
to a thickness of less than 15 µm. In some runs, argon and NaCl
were loaded into the sample chamber, acting as a pressure transmitting and thermally insulating medium. The samples were heated using the TEM01-mode of an YLF laser or multi-mode YAG
laser to overcome any potential kinetic effects on the possible
perovskite + stishovite + CaFe2O4-type aluminous phase at pressures below 70 GPa (Fig. 1). No transitions or back transformations of stable high P-T phases were observed after the sample
was temperature quenched. Nor did any high-pressure phases,
excepting Ca-perovskite, undergo a transition upon release of
pressure. These results are in good agreement with those of previous multi-anvil experiments [Irifune and Ringwood, 1993; Ono et
al., 2001]. At pressures higher than 70 GPa, the phase transition
of silica phase was observed. The transition boundary from
stishovite to CaCl2-type silica was likely to be about 70 GPa at
high-temperatures. This observation was in good agreement with
pure SiO2 study [Ono et al., 2002]. In the next run, the sample
was cold-compressed to about 150 GPa, and then heated. The
duration of heating was 50 min at about 3000 K and 140 GPa. A
new assemblage of CaIrO3-type (Mg,Fe)SiO3 + Ca-perovskite +
α-PbO 2 -type SiO 2 + CaTi 2 O 4 -type aluminous phase was
observed. This observation of CaIrO3-type phase in N-MORB
composition was consistent with pure MgSiO3 study [Oganov and
Ono, 2004]. The phase transition of silica form CaCl2-type to αPbO2-type structure was also observed in this run. This transition
could be predicted at 100 GPa and high-temperatures. The phase
transition of pure MgAl2O4 aluminous phase from CaFe2O4-type
to CaTi2O4-type structure was calculated at 39-57 GPa at 0 K
[Catti, 2001]. In contrast, Kesson et al. [1994] reported that the
CaFe2O4-type aluminous phase in the basalt composition was stable up to 100 GPa. Present observation and previous experimental
study indicated that the transition pressure of aluminous phase is
much higher than that calculated by an ab initio method [Catti,
2001]. As the chemical composition of aluminous phase is not
pure MgAl2O4, the phase boundary is likely to move to high-pressure region in the natural composition.
According to experimental data, the equation of state of KLB-1
peridotite, which is composed of Mg-perovskite, Ca-perovskite,
and ferropericlase in the lower mantle, was determined. The
Birch-Murnaghan equation of state of KLB-1 peridotite was calculated at room temperature: volume V0 = 22.80 cm3/mol, bulk
modulus K0 = 269 GPa, and its pressure derivative K0’ = 4.0 [Ono
et al., 2004b]. The density of KLB-1 peridotite at lower mantle
conditions was also calculated at high temperatures (Fig. 2). The
calculated densities were then compared with seismologically
derived average density profiles of the mantle, as shown in Figure
2. For a mantle temperature of 2000 K at the top of the lower
mantle, the density of the KLB-1 peridotite composition was
found to be 0.4% lower than the seismologically determined density of PREM [Dziewonski and Anderson, 1981]. This density
mismatch is very small compared with the uncertainties in the
experimental measurements and analysis. Therefore, this study
suggests that KLB-1 peridotite, which is similar to pyrolite,
appears to represent the bulk composition of the lower mantle
[Ono et al., 2004b]. Previous seismic study has indicated the possibility of chemical heterogeneity in the deep lower mantle [van
der Hilst and Kárason, 1999]. However, this experimental study
indicates that such chemical heterogeneity cannot exist in the
lower part of the lower mantle.
According to recent high pressure experimental results, there is
a possibility that slabs cannot subduct to the base of the lower
mantle and may stagnate at middle depths of the lower mantle
because of overall buoyancy [Kesson et al., 1998; Ono et al.,
2001]. In this study, the density of N-MORB was calculated with
mineral volumes obtained at high pressures and high temperatures
appropriate for the lower mantle (Fig. 2). The estimated net density of N-MORB is larger than that of the surrounded mantle estimated by seismic observations [Dziewonski and Anderson, 1981;
Kennett et al., 1995]. The density difference is less than 0.05
g/cm3 at 1200-1800 km depth [Ono et al., 2005b]. However, this
calculation indicates that the density crossover, which was predicted by previous studies is not likely to occur in the lower mantle, as the density difference between the subducted MORB and
the surrounding mantle is nearly constant as the depth increases.
At about 2500 km depth, the perovskite-bearing assemblage
changed to the denser assemblage. The density increase of this
phase change is likely to be about 1%. Therefore, the negative
buoyancy of the subducted N-MORB is likely to increase at the
base of the lower mantle. According to the present experiments,
subducted MORB may decent into the base of the lower mantle
[Ono et al., 2005b].
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Figure 1. Schematic phase relations of KLB-1 peridotite and N-MORB.
Abbreviations in the diagram of phase relations are as follows: PY, pyroxene; GA,
majoritic garnet; OL, olivine; WA, wadsleyite; RW, ringwoodite; CP, Ca-perovskite; MP,
Mg-perovskite; FP, ferropericlase; CI, CaIrO3-type (Mg,Fe)SiO3; CF, CaFe2O4-type aluminous phase; ST, stishovite; CC, CaCl2-type silica; CT, CaTi2O4-type aluminous phase;
AP, α-PbO2-type silica.
Figure 2. Comparison of calculated densities of KLB-1 peridotite and N-MORB with
average mantle densities based on seismic observations.
Plotted values are difference densities from PREM (Dziewonski and Anderson 1981).