• journal_title：American Mineralogist
• Contributor：Clinton V. Colasanti ; Elizabeth A. Johnson ; Craig E. Manning
• Publisher：Mineralogical Society of America
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2011.3708
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：96
• issue：8-9
• firstpage：1291
• section：Articles

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 (<em>fem>_O2) 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 <em>fem>_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 °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 <em>fem>_O2 at any pressure and temperature, and exhibits a slight, linear, positive dependence on pressure at a given temperature and <em>fem>_O2. The observed systematic dependences on pressure, temperature, and <em>fem>_O2 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 <em>fem>_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 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 <em>fem>_O2 values imply that Ti³⁺ may be higher than previously suspected in some terrestrial geologic settings.