Experimental hydrogenation of pure and Cr-doped rutile at variable temperature, pressure and oxygen fugacity

Abstract Rutile is a nominally anhydrous mineral that can incorporate significant amounts of hydrogen through different trace element related H+-defects. The hydrogen content in rutile has been proposed to be pressure- and redox dependent. To evaluate the influence of temperature, pressure, oxygen fugacity, and rutile trace element composition on hydrogen solubility in rutile, we performed experiments on rutile with three different compositions (two “pure” and one Cr-doped) at 600 °C and 950 °C, 0.8 GPa and 2 GPa, and two different oxygen fugacity conditions (one set unbuffered close to the nickel-nickel oxide buffer and the other buffered at graphite-COH). Under the experimental conditions, only Ti3+-related H+-defects formed, suggesting a kinetic barrier hindering the formation of trace element related H+-defects coupled with divalent and trivalent cations substituting for Ti4+. H2O contents in rutile from experimental run products are between 60–500 μg/g and increase with increasing pressure and decreasing oxygen fugacity. Contrary to previous studies, a significant temperature dependence was not observed. Impurities in rutile, resulting in high oxygen-vacancy defect densities, enhance hydrogen incorporation in rutile, leading to higher H+ contents in rutile with more di- and trivalent impurities. The amount of oxygen vacancies largely controls the H+ solubility in rutile, making it important to constrain the trace element chemistry of rutile before hydrogenation experiments. Natural rutile from low temperature eclogite facies conditions (2 GPa, 600°C) has similar (or higher) H+ contents (200–900 μg/g H2O) to those observed in representative experimental run products (150–500 μg/g H2O). Thus, H+ retention in natural systems is possible to much higher temperatures (∼600–700 °C) than suggested by previous experimental studies on hydrogen diffusion in rutile. High temperature experiments show higher H+ contents in rutile compared to natural high-T rutile, in agreement with diffusive hydrogen loss above 700°C in natural rocks. This suggests that rutile is a viable, qualitative barometer and potential oxybarometer in medium to low-temperature (<650°C) metamorphic rocks.