Hydrous silicate melts and the deep mantle H2O cycle

https://doi.org/10.1016/j.epsl.2022.117408Get rights and content
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Highlights

  • Hydrous melts of the mantle above and below the transition zone have 40-50 mol% H2O.

  • Structure, density, and viscosity of hydrous melts are determined using ab initio MD.

  • Hydrous melts with Mg#s > 25 will be less dense than PREM mantle and are inviscid.

  • Hydrous melts are highly mobile and move upward above and below the transition zone.

  • Modelling shows the upper mantle and transition zone have similar H2O contents.

Abstract

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.

Keywords

hydrous melts
density
viscosity
transition zone
molecular dynamics
mantle H2O cycle

Data availability

The ab initio trajectories and other simulation parameters are available at Drewitt, J.W.E., 2021. Ab initio Molecular Dynamics Trajectories for Hydrous Silicate Melts. NERC EDS National Geoscience Data Centre. Dataset: https://doi.org/10.5285/c61af45c-8386-4c1a-83ee-61a769843dc2.

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