The distribution of H2O between silicate melt and nominally anhydrous peridotite and the onset of hydrous melting in the deep upper mantle

摘要

The partitioning of H2O between a mantle peridotite assemblage and low degree hydrous melt has been investigated at 6 GPa (corresponding to similar to 180 km depth) at a temperature of 1400 degrees C. Peridotite mineral phases were analysed from 6 melting experiments performed in a natural chemical system. The experiments contained similar to 80 wt% of a low degree hydrous melt that was obtained through a series of experiments where the melt composition was iteratively adjusted until saturation with the appropriate peridotite assemblage was achieved. The melt is fluid-undersaturated at the conditions of the experiment. Ion microprobe measurements of the mineral phases indicate olivine H2O concentrations of 434 +/- 61 ppm wt and average clinopyroxene (cpx) concentrations of 1268 +/- 173 ppm wt H2O. Orthopyroxene (opx) and garnet contain 700 +/- 46 ppm wt and 347 +/- 83 ppm wt H2O, respectively. The H2O content of the hydrous melts was determined by mass balance to be 11 +/- 0.5 wt% H2O. H2O partition coefficients between minerals and melt (D-H2O(min/melt) = X-H2O(min)/X-H2O(melt)) are 0.0040 +/- 0.0006 for olivine, 0.0064 +/- 0.0004 for opx, 0.0115 +/- 0.0016 for cpx and 0.0032 +/- 0.0008 for garnet.