Melting of a Hydrous Mantle: II. Geochemistry of Crystals and Liquids Formed by Anatexis of Mantle Peridotite at High Pressures and High Temperatures as a Function of Controlled Activities of Water, Hydrogen, and Carbon Dioxide

Abstract

The system peridotite-H2O-CO2 serves as a simplified model for the phase relations of mantle peridotite involving more than one volatile component. Run products obtained in a study of phase relations of four mantle peridotites in the presence of H2O- and (H2O+CO2)-bearing vapors and with controlled hydrogen fugacity (fH2) at high pressures and temperatures have been subjected to a detailed chemical investigation, principally by the electron microprobe. Mg/(Mg+∑Fe) of all phases generally increases with increasing temperature and with increasing Mg/(Mg+∑Fe) of the starting material. This ratio appears to decrease with increasing pressure for olivine, and for amphibole coexisting with garnet. DecreasingfH2 from that of IW buffer to that of MH buffer decreases Mg/(Mg+∑Fe) of the partial melt from approximately 0·85 to approximately 0·50, whereas the Fo content of coexisting olivine increases slightly less than 3 per cent and the Mg/(Mg+∑Fe) of clinopyroxene increases about 4 per cent. However, the variations in Fo content of olivines are within those observed in olivines from natural mantle peridotite. The chemistry of other silicate minerals does not significantly reflect variations of fH2. Consequently, the peridotite mineralogy and/or chemistry is not a good indicator for the fH2 conditions during crystallization. All crystalline phases, except amphibole, and to some extent garnet, show increasing Cr content with increasing temperature and increasing Cr content of the starting material, resulting in a positive correlation with Mg/(Mg+∑Fe). Partial melts are depleted in Cr2O3 relative to the crystalline phases. High Mg/(Mg+∑Fe) and Cr2O3 are thus expected in crystal residues after partial melting. The absolute values depend on degree of melting and the composition of the parent peridotite.