Equation of State of Aluminous H-bearing Stishovite to 60 GPa

The equation of state of water-bearing aluminous stishovite has become an important issue in relation to the both the properties of subducting slabs and also the transport and retention of water in the deep mantle. There were numerous studies on elasticity of stishovite, however most of them were performed on pure SiO₂ compositions, which is not representative of natural stishovites formed in MORB layer of subducting slabs. We performed elasticity measurements of SiO₂ stishovites with various Al³⁺ and H⁺ contents with Brillouin scattering and x-ray diffraction techniques at ambient and high pressures. 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 60 GPa using synchrotron radiation. The sample contained 1.8 %wt Al₂O₃ and about 450 ppm or more of H⁺. This composition corresponds to stishovite that would coexist with aluminous iron-bearing Mg-silicate perovskite in a subducted slab. Our results indicate that Al³⁺ and H⁺ have a less pronounced effect on the elastic properties of stishovite than it was reported earlier. Our results suggest that Al and H are retained in stishovite under extreme P-T conditions and that stishovite is an agent for transporting water to the deepest lower mantle. Brillouin scattering measurements of acoustic velocities on stishovites with various chemical compositions will also be discussed.