Donald H Green

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 (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.

Large-scale tectonic processes introduce a range of crustal lithologies into the Earth's mantle. These lithologies have been implicated as sources of compositional heterogeneity in mantle-derived magmas. The model being explored here assumes the presence of widely dispersed fragments of residual eclogite (derived from recycled oceanic crust), stretched and stirred by convection in the mantle. Here we show with an experimental study that these residual eclogites continuously melt during upwelling of such heterogeneous mantle and we characterize the melting reactions and compositional changes in the residue minerals. The chemical exchange between these partial melts and more refractory peridotite leads to a variably metasomatised mantle. Re-melting of these metasomatised peridotite lithologies at given pressures and temperatures results in diverse melt compositions, which may contribute to the observed heterogeneity of oceanic basalt suites. We also show that heterogeneous upwelling mantle is subject to diverse local freezing, hybridization and carbonate-carbon-silicate redox reactions along a mantle adiabat.

The incorporation of water in olivine and pyroxenes interlayered within fertile lherzolite compositions was explored experimentally near the wet solidus of lherzolite at 2·5 and 4 GPa. The concentrations and activities of water were varied to establish the partitioning of water between nominally anhydrous minerals (NAMs) and the hydrous minerals pargasite and phlogopite. The water content in NAMs was determined by Fourier-transform infrared (FTIR) spectroscopy. The main absorption bands in NAMs from these experiments are very similar to those generally found in natural upper mantle peridotites. Olivine, orthopyroxene and clinopyroxene contain 32–190, 290–320 and 910–980 ppm of water under the studied conditions. The partition coefficients between coexisting clinopyroxene and orthopyroxene (D_cpx/opx) are 2·7 ± 1·1 and 3·5 ± 1·5 at 2·5 and 4 GPa respectively, whereas values for coexisting orthopyroxene and olivine (D_opx/ol) are 6·7 ± 2 and 4·7 ± 1·1, at 2·5 and 4 GPa respectively. The storage capacity in NAMs in a model mantle composition close to the vapour-saturated solidus (water-rich vapour) is ∼190 ppm at both 2·5 and 4 GPa. Pargasite is the most important phase accommodating significant amounts of water in the uppermost mantle. Its breakdown with increasing pressure at 3 GPa at the vapour-saturated solidus (which is at ∼1025°C at 2·5 GPa) results in a sharp drop in the water storage capacity of peridotite from ∼1000 ppm to ∼190 ppm H₂O. At pressures >3 GPa, melting in fertile lherzolite begins at the vapour-saturated solidus if the bulk H₂O concentration exceeds ∼190 ppm.