Hydrous silicate melts and the deep mantle H2O cycle

We report ab initio atomistic simulations of hydrous silicate melts under deep upper mantle to shallow lower mantle conditions and use them to parameterise density and viscosity across the ternary system MgO-SiO₂-H₂O (MSH). On the basis of phase relations in the MSH system, primary hydrous partial melts of the mantle have 40-50 mol% H₂O. Our results show that these melts will be positively buoyant at the upper and lower boundaries of the mantle transition zone except in very iron-rich compositions, where ≳ 75% Mg is substituted by Fe. Hydrous partial melts will also be highly inviscid. Our results indicate that if melting occurs when wadsleyite transforms to olivine at 410 km, melts will be buoyant and ponding of melts is unexpected. Box models of mantle circulation incorporating the upward mobility of partial melts above and below the transition zone suggest that the upper mantle becomes efficiently hydrated at the expense of the transition zone such that large differences in H₂O concentration between the upper mantle, transition zone and lower mantle are difficult to maintain on timescales of mantle recycling. The MORB source mantle with ∼0.02-0.04 wt% H₂O may be indicative of the H₂O content of the transition zone and lower mantle, resulting in a bulk mantle H₂O content of the order 0.5 to 1 ocean mass, which is consistent with geochemical constraints and estimates of subduction ingassing.