Physics
Scientific paper
Sep 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000pepi..121...85k&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 121, Issue 1-2, p. 85-102.
Physics
27
Scientific paper
Phase transitions in Mg3Al2Si3O12 garnet (pyrope) were examined at 23-28 GPa and 1600-2000°C using a 6-8-type multianvil apparatus. It was found that pyrope dissociated at 26.5-27 GPa and 1600-2000°C into MgSiO3-rich perovskite solid solution and Al2O3-rich corundum solid solution with a slightly positive dP/dT slope. Perovskite solid solution containing 10 mol% Al2O3 was stabilized at about 26.5 GPa and 1600°C, coexisting with corundum solid solution of 23 mol% MgSiO3. Powder X-ray diffraction of the single-phase aluminous perovskites in the system MgSiO3-Al2O3 showed that these were orthorhombic and that the increase in the b- and c-axes and a slight decrease in the a-axis accompanied increasing Al2O3 content. Our data and other available X-ray data indicate that the unit cell volume of perovskite in the system MgSiO3-Al2O3 is best expressed as V (Å3)=162.35+6.95x, where x represents the mol fraction of Al2O3 in perovskite (0<=x<=0.25). Phase relations on the Mg4Si4O12- Mg3Al2Si3O12 join were examined at 20-27 GPa at 1600°C, and also at about 900°C. A wide two-phase field of garnet+perovskite and a relatively narrow field of ilmenite+garnet were delineated at 1600°C. However, at about 900°C at 21-26 GPa, the stability fields of single-phase ilmenite and of ilmenite+garnet were greatly expanded. Thus, in the cool interior of descending slabs near 660 km discontinuity, there may be a significant depth interval over which garnet gives way to ilmenite-bearing assemblages, before ultimate transformation to perovskite takes place.
Akaogi Masaki
Kubo Atsuki
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