1. Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA
2. Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK
3. Department of Computer Science and Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA

Correspondence to: Lars Stixrude² Correspondence and requests for materials should be addressed to L.S. (Email: stixrude@umich.edu).

The structure and physical properties of hydrous silicate melts and the solubility of water in melts over most of the pressure regime of Earth's mantle (up to 136 GPa) remain unknown. At low pressure (up to a few gigapascals) the solubility of water increases rapidly with increasing pressure¹, and water has a large influence on the solidus temperature, density², viscosity³ and electrical conductivity. Here we report the results of first-principles molecular dynamics simulations of hydrous MgSiO₃ melt. These show that pressure has a profound influence on speciation of the water component, which changes from being dominated by hydroxyls and water molecules at low pressure⁴ to extended structures at high pressure. We link this change in structure to our finding that the water–silicate system becomes increasingly ideal at high pressure: we find complete miscibility of water and silicate melt throughout almost the entire mantle pressure regime. On the basis of our results, we argue that a buoyantly stable melt at the base of the upper mantle would contain approximately 3 wt% water and have an electrical conductivity of 18 S m^-1, and should therefore be detectable by means of electromagnetic sounding.

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