Experimental study of the system peridotite–basalt–fluid: phase relations at supra- and sepercritical P-T parameters

Abstract

To obtain new data on the phase relationships in the fluid-containing upper mantle at P up to 4 GPa, T up to 1400C, partial melting of H2O-containing peridotite, basalt, as well as peridotite-basalt association with an alkaline-carbonate fluid was experimentally studied as a model of the mantle reservoir with protoliths of the subdued oceanic crust. At partial melting of H2O-containing peridotite at P = 3.73.9 GPa, T = 10001300C, critical ratios were observed in the whole studied interval P and T. At partial melting of H2O-containing basalt critical relationships between the silicate melt and the aqueous fluid were observed at T = 1000C, P = 3.7 GPa. At T = 1100C, Na-alkaline silicate melt coexisted with garnetite, at T = 1150 and 1300C with clinopyroxenite. Signs of critical relationships between the carbonated silicate melt and the fluid were observed in peridotite-basalt-alkaline-water-carbonate fluid system at Р = 4 GPa, T = 1400C. The reaction ratios among the minerals of peritotite restite with the substitutions of Ol Opx Ca-Cpx K-Amf indicated a high chemical activity of the supercritical fluid melt. The results of the experiments suggest that in the fluid-containing upper mantle with supercritical Р-Т there are areas of partial melting (asthenosphere lenses), containing near-solidus supercritical fluid-melts enriched with incompatible elements, with high reactivity. Mantle reservoirs with supercritical fluid-melts, similar in geochemical terms to the enriched mantle, can serve as a source of magma enriched with incompatible elements. The modal and latent metasomatism of the upper mantle under the influence of supercritical fluid-melts leads to the peridotite refertilization due to the enrichment of restite minerals with incompatible elements and its eclogitization.