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 6 GPa (∼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 (water-rich vapor) of fertile lherzolite increases in temperature (T) from a minimum of 970°C at 1·5 GPa (∼50 km) to 1375°C at 6 GPa. 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 T or 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 4 GPa. 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 ± 0·1 GPa, the water content of experimental charges was varied from 0·05 to 14·5 wt %. 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 14·5 wt % H2O. Also with increasing water content from 0·05 to 14·5 wt % 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 2·5 GPa and 1000°C are attributed to the leaching role (Na2O and K2O 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) >3 GPa (∼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 vapour-undersaturated 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 (MORB-source 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.