1 High pressure crystal chemistry of hydrous ringwoodite and water in the Earth’s interior Joseph R. Smyth a , Christopher M. Holl a , Daniel J. Frost b , Steven D. Jacobsen b a Department of Geological Sciences, University of Colorado, Boulder, CO 80309 USA b Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth D95440, Germany Abstract The crystal chemistry of Fo 90 hydrous ringwoodite (Mg 1.7 Fe 0.22 H 0.15 SiO 4 ) containing 0.93 % H 2 O 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 ten 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) 2 SiO 4 ) structure. Keywords: Ringwoodite, High Pressure, Equation of State, Crystal Structure, Water. ____________________________________________________________________________________ 1. Introduction Earth is distinguished from its planetary neighbors by the presence of large amounts of liquid water on its surface. Sea level variation studies indicate that sea level has varied relatively little, but perhaps as much as s few hundred meters, through the Phanerozoic (Duval et al., 1998). The presence of quartz-pebble conglomerates of early Archean age indicates that there has been running water (implying the presence of both oceans and land) nearly as far back as we can see in geologic time. Recent geochemical studies of ancient zircons in Archean sediments by Mojzsis et al. (2001) indicate that there may have been liquid water as far back as 4.3 GY ago. Although the oceans cover 72% of the surface area, they constitute only 0.025% of the planet's mass. Silicate minerals of the crust and mantle can incorporate many times this amount of water, so it is possible that these silicates have played a major role in maintaining Earth's oceans over geologic time. Approximately 65% of the total mass of the planet is composed of silicate rocks of the mantle and crust in which the only significant anionic species is oxygen. Bulk hydrogen content is perhaps the most poorly constrained compositional variable in the planet, and the total water content of the planet is unknown within an order of magnitude (Drake and Righter, 2002). Olivine, generally believed to be the most abundant mineral phase in the upper 400 km, can incorporate up to 0.2 weight percent H 2 O (Kohlstedt et al., 1996) at 13 GPa and 1100 ºC. The maximum water content of pyroxenes is not known, but natural clinopyroxenes have been reported with up to about 0.18 weight percent H 2 O (Rossman and Smyth, 1990; Smyth et al., 1991). In the Transition Zone (410-660 km), wadsleyite (β- Mg 2 SiO 4 ) can incorporate up to about 3.3 wt % H 2 O, (Kohlstedt et al., 1996; Inoue et al., 1995), and ringwoodite (γ-Mg 2 SiO 4 ) has been reported with up to about 2.2 wt% (Kohlstedt et al., 1996; Kudoh et al., 2000). Thus, if saturated, these nominally anhydrous phases can incorporate at least