Hydrogen partition coefficients between nominally anhydrous minerals and basaltic melts

We have measured hydrogen partition coefficients between nominally anhydrous minerals (olivine, pyroxenes) and basaltic melts in 13 hydrous melting experiments performed at upper mantle P‐T conditions (1–2 GPa and 1230–1380°C). Resulting liquids have 3.1–6.4 wt.% H2O and average mineral/melt partition coefficients as follows: DHol/melt = 0.0017 ± 0.0005 (n = 9), DHopx/melt = 0.019 ± 0.004 (n = 8), and DHcpx/melt = 0.023 ± 0.005 (n = 2). Mineral/mineral partition coefficients are DHol/opx = 0.11 ± 0.01 (n = 4), DHol/cpx = 0.08 ± 0.01 (n = 2) and DHcpx/opx = 1.4 ± 0.3 (n = 1). These measurements confirm that water behaves similarly to Ce during mantle melting (DHperidotite/melt is ∼0.009). For mantle water concentrations of 50–200 ppm, the onset of melting is 5–20 km deeper than the dry solidus, less than previous estimates.