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111.
Dmitry L. Lakshtanov Carine B. Vanpeteghem Jennifer M. Jackson Jay D. Bass Guoyin Shen Vitali B. Prakapenka Konstantin Litasov Eiji Ohtani 《Physics and Chemistry of Minerals》2005,32(7):466-470
We have determined the P-V equation of state of Al-rich H-bearing SiO2 stishovite by X-ray powder diffraction at pressures up to 58 GPa using synchrotron radiation. The sample contained 1.8 wt%
Al2O3 and up to 500 ppm H2O, and had a composition that would coexist with Mg-silicate perovskite in a subducted slab. By fitting a third-order Birch-Murnaghan
equation of state to our compression data, we obtained a bulk modulus K
T0=298(7) GPa with K′=4.3(5). With K′ fixed to a value of 4, the bulk modulus K
T0=304(3) GPa. Our results indicate that Al3+ and H+ have a small effect on the elastic properties of stishovite. Compared with data obtained up to 43.8 GPa, peak intensities
changed and we observed a decreased quality of fit to a tetragonal unit cell at pressures of 49 GPa and higher. These changes
may be an indication that the rutile↔CaCl2 transition occurs between these pressures. After laser annealing of the sample at 58.3(10) GPa and subsequent decompression
to room conditions, the cell volume is the same as before compression, giving strong evidence that the composition of the
recovered sample is also unchanged. This suggests that Al and H are retained in the sample under extreme P-T conditions and
that stishovite can be an agent for transporting water to the deepest lower mantle. 相似文献
112.
The phase relations in hydrous and anhydrous mid-ocean ridge basalt were determined at pressures of 18-28 GPa. Liquidus phase relations in hydrous and anhydrous MORB are different. Garnet is the liquidus phase at pressures below 21 GPa, Ca-Al (CAS) phase and stishovite are the liquidus phases at pressures of 22-27 GPa, and stishovite and Ca-perovskite are the liquidus phases above 27 GPa, whereas Ca-perovskite is a liquidus phase of anhydrous MORB at pressures above 23 GPa. Under subsolidus conditions, we have found that in the hydrous MORB system the stability fields of Al-bearing perovskite and Na-Al (NAL) phase might shift to lower pressure by about 1.5 GPa compared to the dry MORB system. This shift could be explained by oxidation of a garnet-bearing assemblage by hydrous fluid and formation of Fe3+-bearing aluminous perovskite at lower pressures relative to the anhydrous system and/or differences in water solubility of the phases existing in perovskite-bearing assemblages. Our data indicate that hydrous basaltic crust remains denser than peridotite along the geotherm of a subducting slab, i.e. there is no density crossover between peridotite and basalt. Therefore, in slabs going through the 660 km discontinuity, basalt would gravitationally sink into the lower mantle under relatively hydrous conditions. The delamination of former basaltic crust near the 660 km discontinuity might be possible under relatively dry conditions of subduction. There are no stable highly hydrous phases in MORB above 10 GPa even at lower temperatures corresponding to subducting slabs. Therefore, MORB cannot be an important carrier of water to the deep Earth interior. However, it can be constantly supplied by water-bearing fluid from the underlying peridotite part of the descending slab. Thus, it is plausible that water can control subduction of the oceanic crust into the lower mantle. 相似文献
113.
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115.
Takeshi Sakai Eiji Ohtani Hidenori Terasaki Masaaki Miyahara Masahiko Nishijima Naohisa Hirao Yasuo Ohishi Nagayoshi Sata 《Physics and Chemistry of Minerals》2010,37(7):487-496
Fe–Mg partitioning between post-perovskite and ferropericlase has been studied using a laser-heated diamond anvil cell at
pressures up to 154 GPa and 2,010 K which corresponds to the conditions in the lowermost mantle. The composition of the phases
in the recovered samples was determined using analytical transmission electron microscopy. Our results reveal that the Fe–Mg
partition coefficient between post-perovskite and ferropericlase (K
DPPv/Fp) increases with decreasing bulk iron content. The compositional dependence of K
DPPv/Fp on the bulk iron content explains the inconsistency in previous studies, and the effect of the bulk iron content is the most
dominant factor compared to other factors, such as temperature and aluminum content. Iron prefers ferropericlase compared
to post-perovskite over a wide compositional range, whereas the iron content of post-perovskite (X
FePPv, the mole fraction) does not exceed a value of 0.10. The iron-rich ferropericlase phase may have significant influence on
the physical properties, such as the seismic velocity and electrical conductivity at the core–mantle boundary region. 相似文献