STATIC COMPRESSION OF CHONDRODITE - IMPLICATIONS FOR WATER IN THE UPPER-MANTLE

We have determined the equation of state of chondrodite (2Mg2SiO4.Mg(OH,F)2), a possible water-bearing phase in the upper mantle, to 42.3 GPa at room temperature using x-ray diffraction through the diamond cell. The isothermal bulk modulus of chondrodite, determined using the Birch-Murnaghan equation of state, is 136.2 GPa with a pressure derivative of 3.7. Using our results, we have constructed density models for hydrous upper mantle compositions. These models indicate that up to 2 weight percent water could be present in a pyrolitic mantle, and be undetectable using seismically-determined density constraints. We have also investigated the stability of chondrodite under lower mantle pressure and temperature conditions using the laser-heated diamond cell. From in-situ x-ray diffraction at high pressure, and x-ray diffraction and infrared reflectance and absorption spectra of quenched run products, we conclude that chondrodite breaks down to MgSiO3 perovskite, MgO and a hydrated melt at lower mantle pressures and temperatures.