Rutile solubility and TiO2 activity in silicate melts: An experimental study

Abstract

Sixty-eight new experiments on rutile solubility in synthetic silicate melts of a wide compositional range have been conducted at ambient pressure (P) at temperature (T) from 1230 to 1500 °C. The results support the previous findings, namely: (1) at fixed T addition of silica and alumina results in essential decrease of rutile solubility; (2) at fixed composition increasing T results in essential increase in rutile solubility. Based on the new and literature experimental data (350 data points) new equations describing rutile solubility in silicate melts as a function of T, P, and melt composition, both empirical and consistent with the thermodynamic model of MELTS (Ghiorso and Sack, 1995), are derived. The new equation predicts log XTiO2 (TiO2 mole fraction) in the melts with a standard error of 0.066, which is equivalent to ca 15% error, in a wide range of melts major element composition and the XTiO2 value in the range from 0.0008 to 0.39 (TiO2 from 0.1 to 44.8 wt%) covered by the experiments. The equations are applied to calculate the TiO2 activity values relative to rutile saturation (aTiO2, m/rut) in a variety of well documented natural magmatic rocks. The resulting aTiO2, m/rut values for different tested samples vary from 0.1 to close to 1. Simultaneously solving the new equations and the equation for Ti-in-Zircon geothermometer (Ferry and Watson, 2007) gives more reliable T estimates than those based on the assumption of constant aTiO2, m/rut = 0.6 or 1.