High pressure crystal chemistry of hydrous ringwoodite and water in the Earth's interior

The crystal chemistry of Fo₉₀ hydrous ringwoodite (Mg_1.7Fe_0.22H_0.15SiO₄) containing 0.93% H₂O by weight has been studied at pressures up to 11.2 GPa by single-crystal X-ray diffraction in the diamond anvil cell. The unit cell edge and volume have been refined at 20 different pressures in this pressure range. The refined bulk modulus for the unit cell is 169.0±3.4 GPa with a K′ of 7.9±0.9. H is accommodated in the structure principally by octahedral cation vacancy. The oxygen position parameter has been refined from X-ray intensity data at 10 pressures over this range. With a K′ fixed at 4.0, the bulk modulus of the Si tetrahedron is 245±31 GPa and that of the octahedral site is 150±7 GPa. Consistent with a previous study, we observe a systematic decrease of bulk modulus with H-content. From these data, it appears that hydration of ringwoodite has a larger effect on seismic velocity than temperature within the possible ranges of these parameters under upper mantle conditions. This means that in tomographic images of the Transition Zone in regions distant from subduction zones, blue is more likely to mean 'dry' than it is to mean 'cold'. Observed seismic velocities in the Transition Zone are consistent with significant hydration of the ringwoodite (γ-(Mg, Fe)₂SiO₄) structure.