Abstract

The solubility of OH in pure synthetic rutile was experimentally constrained at 0.5–2.0 GPa and 500–900 °C, in equilibrium with four oxygen fugacity (fO2) buffering mineral assemblages: hematite-magnetite (HM), nickel-nickel oxide (NNO), cobalt-cobalt oxide (CCO), and iron-wüstite (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 fO2 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 °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 fO2 at any pressure and temperature, and exhibits a slight, linear, positive dependence on pressure at a given temperature and fO2. The observed systematic dependences on pressure, temperature, and fO2 indicate that hydrogen substitutes into rutile as hydroxyl, (OH), via forward progress of the reaction Ti4+O2 + ½H2O = Ti3+O(OH) + ¼O2. 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 fO2 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 cm3/mol, 219.3 ± 1.3 kJ/mol, and 19.9 ± 1.4 J/(mol·K) (1σ 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 fO2 values imply that Ti3+ may be higher than previously suspected in some terrestrial geologic settings.

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