Experimental study of the influence of water on melting and phase assemblages in the upper mantle

Abstract

The role of water in the uppermost mantle has been explored to 6GPa (~200 km) by a novel experimental approach in which the silicate melting solidus, the stability of hydrous phases and the H2O contents in nominally anhydrous minerals (NAMs) were determined. The composition studied is a fertile lherzolite 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 from water-rich vapour phase.The vapor-saturated solidus (waterrich vapor) of fertile lherzolite increases in temperature (T) from a minimum of 9708C at 1·5GPa (~50 km) to 13758C at 6GPa. The Ca-rich amphibole pargasite is stable to the vapour-saturated solidus to 3 GPa (~100 km). Based on normative components, at 2·5 GPa the near-solidus melt (1-2%) in mantle with very low H2O content is transitional between sodic-dolomitic carbonatite and olivine melilitite.With higher melt fraction (~5%) at higher Tor higher H2O content it is olivine-rich basanite. Both immediately below and above the solidus, the H2O content in residual lherzolite is ~200 ppm retained in NAMs at 2·5 and 4GPa.The experimentally determined vapour-saturated solidus corrects recent numerical models of melting of lherzolite+H2O based on inferred high solubilities of H2O in NAMs and accounts for a discrepant experimental determination of the vapour-saturated solidus in which very high water/rock ratios were used. At 2·5±0·1GPa, the water content of experimental charges was varied from 0·05 to 14·5wt %. Below the solidus and with increasing water content from 0·05 to 2·9 wt %, pargasite decreases in K2O and Na2O content and is absent in experiments with 7·25 and 14·5wt % H2O. Also with increasing water content from 0·05 to 14·5wt % H2O, the Na2O content of clinopyroxene decreases from 1·6 wt % to below the limit of detection (0·2 wt %). The destabilization of pargasite and change of clinopyroxene composition at 2·5GPa and 10008C are attributed to the leaching role (Na2O and c particularly) of the water-rich vapour at high water/rock ratios. The hydrous mineral pargasite is the major site of H2O storage in fertile uppermost mantle lherzolite but pargasite is unstable at pressures (P) > 3GPa (~100 km depth), causing a sharp drop in the water storage capacity of the upper mantle from > 2000 to ~200 ppm. For small H2O contents (<2000 ppm approximately), the temperature of the vapourundersaturated solidus of fertile upper mantle lherzolite decreases sharply with increasing P at ~90 km depth.The negative dT/dP for the vapour-undersaturated solidus has important rheological and geodynamic consequences. In oceanic intraplate settings, the geotherm passes from subsolidus pargasite-bearing lherzolite to garnet lherzolite with incipient melting, creating the rheological boundary at ~90 km depth, between lithosphere and asthenosphere.The asthenosphere becomes geochemically zoned with the 'enriched' intraplate basalt source (>500 ppm H2O) overlying the 'depleted' MORB source (~200 ppm H2O) in the deeper asthenosphere.Water also plays a significant role at convergent margins, where hydrous silicate melting in the mantle wedge is initiated at the vapour-saturated solidus. Melting of lherzolite at or near the vapour-saturated solidus does not fully dehydrate residual lherzolite or harzburgite. Residual lithosphere returned to the upper mantle may carry ~100-200 ppm H2O. At 6 GPa the low K/Na model mantle composition (MORBsource mantle) with > 200 ppm H2O has normal rather than supercritical melting behaviour with the solidus at 1375°C, which is ~350°C below the C+H-free solidus.