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 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 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. % 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 (500 ppm H2O) overlying the ‘depleted’ MORB source (~200 ppm 2O) 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 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.