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 (~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 enables 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 Carich 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 olivinerich 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 an erroneous 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 pargasite 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 destabilisation of pargasite and change of clinopyroxene composition at 2.5 GPa, 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 the fertile uppermost mantle lherzolite but pargasite is unstable at pressure (P) >3 GPa (~100 km depth) causing a sharp drop in 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 approximately 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.