Hydrogen partitioning between melt, clinopyroxene, and garnet at 3 GPa in a hydrous MORB with 6 wt.% H₂O

To understand partitioning of hydrogen between hydrous basaltic and andesitic liquids and coexisting clinopyroxene and garnet, experiments using a mid-ocean ridge basalt (MORB) + 6 wt.% H₂O were conducted at 3 GPa and 1,150-1,325°C. These included both isothermal and controlled cooling rate crystallization experiments, as crystals from the former were too small for ion microprobe (SIMS) analyses. Three runs at lower bulk water content are also reported. H₂O was measured in minerals by SIMS and in glasses by SIMS, Fourier Transform infrared spectroscopy (FTIR), and from oxide totals of electron microprobe (EMP) analyses. At 3 GPa, the liquidus for MORB with 6 wt.% H₂O is between 1,300 and 1,325°C. In the temperature interval investigated, the melt proportion varies from 100 to 45% and the modes of garnet and clinopyroxene are nearly equal. Liquid composition varies from basaltic to andesitic. The crystallization experiments starting from above the liquidus failed to nucleate garnets, but those starting from below the liquidus crystallized both garnet and clinopyroxene. SIMS analyses of glasses with >7 wt.% H₂O yield spuriously low concentrations, perhaps owing to hydrogen degassing in the ultra-high vacuum of the ion microprobe sample chamber. FTIR and EMP analyses show that the glasses have 3.4 to 11.9 wt.% water, whilst SIMS analyses indicate that clinopyroxenes have 1,340-2,330 ppm and garnets have 98-209 ppm H₂O. D_H^cpx-gt is 11 ± 3, D_H^cpx-melt is 0.023 ± 0.005 and D_H^gt-melt is 0.0018 ± 0.0006. Most garnet/melt pairs have low values of D_H^gt-melt, but D_H^gt-melt increases with TiO₂ in the garnet. As also found by previous studies, values of D_H^cpx-melt increase with Al₂O₃ of the crystal. For garnet pyroxenite, estimated values of D_H^{pyroxenite-melt} decrease from 0.015 at 2.5 GPa to 0.0089 at 5 GPa. Hydration will increase the depth interval between pyroxenite and peridotite solidi for mantle upwelling beneath ridges or oceanic islands. This is partly because the greater pyroxene/olivine ratio in pyroxenite will tend to enhance the H₂O concentration of pyroxenite, assuming that neighboring pyroxenite and peridotite bodies have similar H₂O in their pyroxenes.