PROBING THE EARTH'S DEEP OXYGEN CYCLE WITH VANADIUM: THE TEMPERATURE DEPENDENCE OF PARTITIONING

Oxygen fugacity (fO2) influences the stability of solid phase assemblages, magmatic differentiation trends, and the composition of multi-valent elements in fluids. Thus, the ability to assess the fO2 recorded by diverse rock assemblages with ease is of great interest to the community. The concentration of the multivalent element vanadium (V) in the eclogitic mineral rutile (TiO₂) relative to co-existing silicate melt (D_V^rutile/melt) changes as a function of fO2 [1-3], making V-in-rutile an excellent fO2 proxy for rutile-bearing assemblages, such as eclogites. We present the results of controlled atmosphere experiments that quantify the dependence of D_V^rutile/melt on temperature. We equilibrated rutile and silicate melt (CMAS) in a gas-mixing furnace at 1100°C, 1200°C, 1300°C, and 1375°C at QFM-1. Our experiments indicate that in addition to f_O2, temperature and melt composition have significant effects on partitioning of V between rutile and melt. D_V^rutile/melt appears to be a strong function of temperature, decreasing by a factor of ~2 over a temperature increase of < 200°C. However, this trend is difficult to isolate from the effect of changing melt composition in our experiments because, as temperature decreases at 1 atmosphere, TiO₂ solubility in the silicate liquid drops [e.g., 4]. Resultant rutile-saturated melts are more polymerized at lower temperatures, which demonstrably increases V’s compatibility in rutile [3].

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3. Holycross and Cottrell (2018) Goldschmidt conference, Boston, MA
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