Phases and electrical conductivity of a hydrous silicate assemblage at lower-mantle conditions

Abstract

THE presence of a small amount of water in the lower mantle might affect in a significant way the geophysical and geochemical properties of its host mineral assemblage^1–5. Here we present experimental observations of the phase behaviour and the electrical conductivity of a hydrous silicate assemblage synthesized from a mixture of (Mg_0.88Fe_0.12)SiO₃ pyroxene and water under the pressure and temperature conditions of the lower mantle. Previous studies have shown that anhydrous (Mg, Fe)SiO₃ pyroxene transforms to a perovskite structure under these conditions^6–9. We find that, although the hydrous assemblage is also dominated by the (Mg, Fe)SiO₃ perovskite phase, it coexists with the so-called hydrous phase D, of estimated composition (Mg, Fe)SiH₂O₄. Our measurements show that the inclusion of small amounts of water in the silicates can enhance the electrical conductivity of the lower-mantle assemblage by more than three orders of magnitude at these temperatures and pressures.