Experimental Study of the Influence of Water on Melting and Phase Assemblages in the Upper Mantle

The role of water in the uppermost mantle has been explored to 6 GPa (similar to 200 km) by a novel experimental approach in which the silicate melting solidus, the stability of hydrous phases and the 1120 contents in nominally anhydrous minerals (NA11,15) were determined, The composition studied is a fertile Iherzolite modelled as a source for mid-ocean ridge basalts (moRB). The use of crushed olivine as traps for melt or fluid inclusions allows a distinction to be made between quenched hydrous silicate melt and quench material tram water-rich vapour phase. The vapor-saturated solidus (water-rich vapor) of fertile Iherzolite increases in temperature (E) from a minimum of 970 C at 1.56Pa, (similar to 50 km) to 1375 C at 6 GPa. The Ca-rich amphibole pargasite is, stable to the vapour-saturated,solidus to 3 GPa similar to 100 km). Based on normative components, at GPa the near-solidus melt (1-27%) in mantle with very low 1190 content is transitional between sodic-dolomitic carbonatite and olivine melilitite, With lugher melt fraction () at higher Tar higher H2O content it is olivine-rich basanite. Both immediately below and above the solidus, the 1190 content in residual the zolite is 200 ppm retained in Mills at 2.5 and 4 GPa. The experimentally determined vapour-saturated solidus corrects recent numerical models of melting of therzolite + H2O based on inferred high solubilities of H2O in MAL and accounts for a discrepant cxperimental determination of the vapour-saturated solidus in which very high water/rock ratios were used. At 2.5 GPa, the water content of experimental charges was varied from 0.05 to 11.5 wt %. Below the solidus and with increasing water content from 0.05 to 2.9 wt %. pargasite decreases in K2O inch Na2O content and is absent in experiments with 7.25 and 11.5 wt % 1120. Also with increasing water content from 0.05 to 11.5 tot hi H20, the.,,Na2O content of clinopyroxene decreases from 1.6wt % to below the limit of detection (0.2 wt %). The destabilization of pargasite and change of clinopyroxene composition at 2.5 GPa and 100020 are attributed to the leaching role (,NU:,,0 and 1X20 particularly) of the water-rich vapour at high water/rock ratios, The hydrous mineral pargasite is the major site of 1120 storage in fertile uppermost mantle lherzolite but pargasite is unstable at pressures. ( P) > 3GPa (similar to 100 km depth.), causing a sharp drop in the water storage capacity of the upper mantle from >2000 to 200 ppm. Far small H2O contents (<2000 ppm, approximately), the temperature of the vapour-undersaturated solidus of fertile upper mantle thergolite decreases sharply with increasing P at similar to 90 km depth The negative df/dY for the vapour-undersaturated solidus has important Theological and geodynamic consequences. In oceanic intraplate settings, the geotherm passes from, subsolidus pargasite-bearing therzolite to garnet thero lite with. incipient melting, creating the Theological boundary at 90 km depth, between lithosphere and dsthenosphere. The intrablate sphere becomes geochemically zoned with the 'enriched intraplale basalt source (>500 ppm 1120) overlying the `depleted "Holm source (similar to 200 ppm H2O) in the deeper asthenosphere. Water also significance role at convergent margins, where hydrous melting in the mantle wedge is initiated at the vapour-saturated solidus. Melting of therzolite at or near the vapour-saturated solidus does not fully dehydrate residual therzolite or harzburgite. Residual lithosphere returned to the upper mantle may curly similar to 100-200 ppm H2O. At 6 GM the low lij..Ya model mantle composition (MORB-source mantle) with >200 ppm H2O has normal rather than super-critical melting behaviour with the solidus at 1375 degrees C, which is similar to 350 degrees C below the C 11-free solidus