Solubility of molecular hydrogen in silicate melts and consequences for volatile evolution of terrestrial planets

We present experiments from 0.7 to 3GPa that quantify solubility of H ₂ in silicate melts under controlled hydrogen fugacities (f _H2). Two experimental series, one on synthetic basalt+COH and other with a synthetic andesite+OH, were conducted using a double capsule technique to impose a range of f _H2, on the samples. Quenched glasses were analyzed by FTIR and SIMS. Both series follow simple solubility laws in which molecular H ₂ concentrations are proportional to f _H2 and with a partial molar volume of molecular H ₂ of 11cm ³/mole. Solubilities in andesitic melt are systematically greater than in basaltic liquid in a relationship consistent with control by the ionic porosity (IP) of the melts. Extrapolation based on IP allows estimation of the solubility of H ₂ in peridotitic melts applicable to magma oceans. The H ₂/(H ₂+H ₂O) ratio in silicate melts (where H ₂O includes molecular H ₂O and OH ^-) increases as conditions become more reduced, with increasing pressure, and with increasing total H. Under some conditions prevailing in the early Earth and terrestrial planets as well as in the deep Earth today, H ₂ can be a significant fraction of the dissolved H and at high pressure it may exceed "water" (H ₂O and OH ^-). Therefore, magmatic H ₂ may influence the initial distribution of volatiles and the redox evolution of terrestrial planets, as well as the ongoing formation and fate of hydrous melts in the deep Earth today. Hydrous species in melts in equilibrium with Fe-rich alloy at high pressure, for example during core formation from a magma ocean, could be chiefly H ₂, rather than H ₂O. Hence, delivery of H ₂ to the core by removal of Fe hydride need not be coupled to oxidation of the residual mantle. Although lunar basalts are much reduced, the fraction of H dissolved as molecular H ₂ is small owing to low total H concentrations. Extrapolation to conditions of potential hydrous partial melting in the deep Earth suggests that the chief magmatic volatile may be H ₂ rather than H ₂O. The very small partial specific density of magmatic H ₂ (0.18g/cm ³ at low pressure) may provide significant positive buoyancy to deep partial melts.