Hydrogen partitioning between nominally anhydrous upper mantle minerals and melt between 3 and 5 GPa and applications to hydrous peridotite partial melting

To extend our knowledge of partitioning of hydrogen between nominally anhydrous mantle minerals and basaltic melts to higher pressures, we conducted 8 experiments at 3–5 GPa and 1350–1440 °C on a modified mid-ocean ridge basalt (MORB) + 2 wt.% H2O. H contents in olivine, orthopyroxene, clinopyroxene, and garnet were measured by counting 16O1H ions with low-blank SIMS, and total H2O concentrations in coexisting glasses were measured by FTIR, SIMS, and electron microprobe. The glasses have water contents ranging from 4.1 to 8.9 wt.%. Eleven measured pyroxenes have H2O concentrations ranging from 430 to 1430 ppm, two garnets have concentrations of 73 and 260 ppm, and one olivine has a concentration of 29 ppm. Mineral/melt hydrogen partition coefficients are as follows: DHcpx/melt ranges from 0.014–0.021, DHopx/melt ranges from 0.009–0.019, DHgt/melt ranges from 0.001–0.0033, and DHol/melt has a value of 0.0006. Mineral/mineral hydrogen partitioning data are as follows: DHcpx/opxranges from 1.2–2, DHcpx/gt has values of 5.5 and 14, and a single DHcpx/ol has a value of 27. DHpyx/melt correlates with the concentration of Al2O3 in pyroxene and with the concentration of Al on tetrahedral sites. Combining the mineral/melt hydrogen partitioning data from this study, and estimates of mineral modes and compositions along the solidus, we estimate that DHperid/melt increases from 0.006 at 1 GPa up to 0.01 near the garnet lherzolite/spinel lherzolite transition, owing to the maximum in Al2O3 concentration in pyroxenes. With increasing pressure DHperid/melt for garnet lherzolite diminishes to 0.005 at 5 GPa, as pyroxenes become less aluminous and less abundant. As a result, incipient hydrous melts beneath oceanic ridges with bulk mantle H2O contents of 50–200 ppm will have solidus partial melts with H2O contents between 0.5 and 3.8 wt.%, while oceanic island mantle with 300–1000 ppm will have solidus partial melts with H2O content between 3 and 20 wt.%.