The effect of H2O on the olivine liquidus of basaltic melts:: experiments and thermodynamic models

We designed and carried out experiments to investigate the effect of H2O on the liquidus temperature of olivine-saturated primitive melts. The effect of H2O was isolated from other influences by experimentally determining the liquidus temperatures of the same melt composition with various amounts of H2O added. Experimental data indicate that the effect of H2O does not depend on pressure or melt composition in the basaltic compositional range. The influence of H2O on melting point lowering can be described as a polynomial function (C-H2O(melt)) in wt% : Delta T (degrees C) = 40.4 (C-H2O(melt)) - 2.97 (C-H2O(melt))(2) + 0.0761 (C-H2O(melt))(3). This expression can be used to account for the effect of H2O on olivine-melt thermometers, and can be incorporated into fractionation models for primitive basalts. The non-linear effect of H2O indicates that incorporation of H2O in silicate melts is non-ideal, and involves interaction between H2O and other melt components. The simple speciation approach that seems to account for the influence of H2O in simple systems (albite-H2O, diopside-H2O) fails to describe the mixing behavior of H2O in multi-component silicate melts. However, a non-ideal solution model that treats the effect of H2O addition as a positive excess free energy can be fitted to describe the effect of melting point lowering.