The equation of state of Al,H-bearing SiO_2 stishovite to 58 GPa

Abstract

We have determined the P-V equation of state of Al-rich H-bearing SiO_2 stishovite by X-ray powder diffraction at pressures up to 58 GPa using synchrotron radiation. The sample contained 1.8 wt% Al_(2)O_3 and up to 500 ppm H_(2)O, 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 Al^3+ 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↔CaCl_2 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.