To extend our knowledge of partitioning of hydrogen between nominally anhydrous mantle minerals and basaltic melts to higher pressures, we conducted 8 experiments at 3-5GPa and 1350-1440 °C on a modified mid-ocean ridge basalt (MORB)+2wt.% 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 8.9wt.%. Eleven measured pyroxenes have H2O concentrations ranging from 430 to 1430ppm, two garnets have concentrations of 73 and 260ppm, and one olivine has a concentration of 29ppm. 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 mineralmodes and compositions along the solidus, we estimate that DHperid/melt increases from 0.006 at 1GPa 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 5GPa, as pyroxenes become less aluminous and less abundant. As a result, incipient hydrous melts beneath oceanic ridges with bulkmantle H2O contents of 50-200 ppmwill have solidus partial melts with H2O contents between 0.5 and 3.8wt.%, while oceanic island mantle with 300-1000 ppmwill have solidus partial melts with H2O content between 3 and 20wt.%.
Bibliographical noteFunding Information:
We thank Erik Hauri for his suggestion of SIMS analyses by counting 16 O 1 H ions. We are grateful for NSF support through grants OCE0623550, EAR0456405, and EAR0757903. Helpful reviews were provided by Erik Hauri and Paul Asimow. Parts of this work were carried out in the Institute of Technology Characterization Facility, University of Minnesota, which receives partial support from NSF through the NNIN program. The SIMS analyses were obtained at the Arizona State University National SIMS facility, supported by EAR0622775.
- Experimental petrology
- Hydrogen partitioning
- Mantle melting