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In order to constrain the high-pressure behavior of kyanite, multi-anvil experiments have been carried out from 15 to 25 GPa, and 1,350 to 2,500°C. Both forward and reversal approaches to phase equilibria were adopted in these experiments. We find that kyanite breaks down to stishovite + corundum at pressures above ∼15 GPa, and stishovite + corundum should be the stable phase assemblage at the pressure–temperature conditions of the transition zone and the uppermost part of the lower mantle of the Earth, in agreement with previous multi-anvil experimental studies and ab initio calculation results, but in disagreement with some of the diamond-anvil cell experimental studies in the literature. The Al2O3 solubility in nominally dry stishovite has been tightly bracketed by forward and reversal experiments; it is slightly but consistently reduced by pressure increase. Its response to temperature increase, however, is more complicated: increases at low temperatures, maximizes at around 2,000°C, and perhaps decreases at higher temperatures. Consequently, the Al2O3 solubility in dry stishovite at conditions of high temperature–high pressure is very limited. [ABSTRACT FROM AUTHOR]

Abstract: In situ X-ray diffraction measurements of CaFe2O4-type MgAl2O4 have been conducted at pressures up to 42GPa and temperatures to 2400K using Kawai-type multianvil apparatus with sintered diamond anvils. Additional measurements have also been conducted at pressures to 12GPa using diamond anvil cell with helium as a pressure medium at room temperature, and at temperatures to 836K at the ambient pressure using a high-temperature X-ray diffractometer. The analysis of room-temperature data yielded V 0 =240.1(2)Å3, K 0 =205(6)GPa, and . A fit of the present data to high-temperature Birch–Murnaghan equation of state (EOS) yielded (∂K 0/∂T) P =−0.030(2)GPa/K and α 0 = a 0 + b 0 T with values of a 0 =1.96(13)×10−5 K−1 and b 0 =1.64(24)×10−8 K−2. The present data set was also fitted to Mie–Grüneisen–Debye (MGD) EOS and we obtained γ 0 =1.73(7), q =2.03(37), and θ 0 =1546(104)K. Density changes of MORB have been estimated using the newly obtained thermoelastic parameters, assuming that the Al-rich phase in this composition possesses the CaFe2O4-type structure under the lower mantle P, T conditions. The calculated densities along geotherms for the normal mantle and subducting cold slabs are both significantly higher than those of typical seismological models, confirming the conclusion of some recent results on MORB by laser-heated diamond anvil cell experiments. [Copyright &y& Elsevier]