Experimental constraints on melting in the MgO-SiO2 system at the lower mantle conditions - Marzena A. Baron

IMPMC - UPMC, 4, Place Jussieu 75005 Paris. Tour 23 - Barre 22-23 - 4e étage, salle 401

Lundi 10 avril 2017 à 10 h 30

Marzena A. Baron - Center for Earth Evolution and Dynamics, University of Oslo, Norway

Abstract

Eutectic melting curves in the system MgO-SiO₂ have been experimentally determined at lower mantle pressures using laser-heated diamond anvil cell (LH-DAC) techniques. We investigated eutectic melting of bridgmanite plus periclase in the MgO-MgSiO₃ binary and bridgmanite plus stishovite in the MgSiO₃-SiO₂ sub-system as the simplest models of natural peridotite and basalt. The melting curve of model basalt occurs at lower temperatures, has a shallower dT/dP slope and slightly less curvature than the model peridotitic melting curve. Overall, melting temperatures detected in this study are in good agreement with previous experiments and ab initio simulations at ~25 GPa (Liebske and Frost, 2012; de Koker et al., 2013). However, at higher pressures the measured eutectic melting curves are systematically lower in temperature than curves extrapolated on the basis of thermodynamic modelling of low-pressure experimental data, and those calculated from atomistic simulations. The curvature of the model peridotite eutectic relative to an MgSiO₃ melt adiabat indicates that crystallization in a global magma ocean would begin at ~100 GPa rather than at the bottom of the mantle, allowing for an early basal melt layer. The model peridotite melting curve lies ~500 K above the mantle geotherm at the core-mantle boundary, indicating that it will not be molten. The model basalt melting curve intersects the geotherm at the base of the mantle, and partial melting of subducted oceanic crust is therefore expected.