An experimental study of OH solubility in rutile at 500-900 °C, 0.5-2 GPa, and a range of oxygen fugacities

The solubility of OH in pure synthetic rutile was experimentally constrained at 0.5-2.0 GPa and 500-900 degrees C, in equilibrium with four oxygen fugacity (f(O2)) buffering mineral assemblages: hematite-magnetite (HM), nickel-nickel oxide (NNO), cobalt-cobalt oxide (CCO), and iron-wustite (IW). The hydroxyl concentration ([OH], in parts per million H2O by weight) of equilibrated rutile crystals was characterized by FTIR spectroscopy. Measurements at 1 GPa at individual f(O2) buffers demonstrate that [OH] in rutile depends strongly on temperature: at HM, [OH] increases from 48 to 267 ppm as temperature rises from 500 to 900 degrees C, whereas at NNO, [OH] increases from 108 to 956 ppm over the same temperature range. The [OH] in rutile also increases strongly with decreasing f(O2) at any pressure and temperature, and exhibits a slight, linear, positive dependence on pressure at a given temperature and f(O2). The observed systematic dependences on pressure, temperature, and f(O2) indicate that hydrogen substitutes into rutile as hydroxyl, (OH), via forward progress of the reaction Ti4+O2 + 1/2 H2O = Ti3+O(OH) + 1/4 O-2. Our measured [OH] values are significantly greater than those determined in previous studies on finer-grained, polycrystalline rutile, which likely suffered diffusive loss of H during quenching. This is supported by our observation of narrow, OH-depleted rims on otherwise high-OH run products, pointing to minor but important diffusive H loss from crystal rims during quenching. Fitting of isothermal variations in composition with f(O2) at 1 GPa and temperature indicates nearly ideal, multi-site mixing of the TiO2-TiOOH solid solution. A fit to the entire data set suggests standard volume, enthalpy, and entropy of the hydration reaction of, respectively, 1.90 +/- 0.48 cm(3)/mol, 219.3 +/- 1.3 kJ/mol, and 19.9 +/- 1.4 J/(mol.K) (1 sigma uncertainty). These constraints form the basis for use of [OH] in rutile as a thermobarometer and oxybarometer in experimental and natural systems. The moderate to high [OH] in nominally anhydrous rutile at all investigated temperatures, pressures, and f(O2) values imply that Ti3+ may be higher than previously suspected in some terrestrial geologic settings.