Partitioning of V and 19 other trace elements between rutile and silicate melt as a function of oxygen fugacity and melt composition: Implications for subduction zones

摘要

Vanadium is a multivalent element that can speciate as V2+, V3+, V4+, and V5+ over a range of geologically relevant oxygen fugacities (f(O2)). The abundance of V in planetary materials can be exploited as a proxy for f(O2) when its partitioning behavior is known. The mineral rutile (TiO2) is an important carrier of the high field strength elements Nb and Ta in the solid Earth, but it can also incorporate substantial quantities of vanadium (up to similar to 2000 ppm; e.g., Zack et al. 2002). However, little work has been done to systematically investigate how the partitioning of V in rutile-bearing systems changes as a function of both f(O2) and composition. We measured the partitioning of V and 19 other trace elements (Sc, Cr, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Lu, Hf, and Ta) between rutile and three silicate melt compositions equilibrated at 1 atm pressure, 1300 degrees C and f(O2) values from two log units below the quartz-fayalite-magnetite oxygen buffer (QFM-2) to air (QFM+6.5). Rutile/melt partition coefficients (D-V(rt/melt)) change dynamically over an eight-log unit range of f(O2) and are greatest at f(O2) = QFM-2 in all compositions. Vanadium solubility in rutile declines continuously as f(O2) increases from QFM-2 and approaches unity in air. Trace-element partitioning between rutile and melt is also correlated with melt composition, with the greatest values of D-rt/melt measured in the most polymerized melt systems containing the least TiO2. We do not find any circumstances where V becomes incompatible in rutile. Our results indicate that rutile is a considerable sink for V at terrestrial f(O2) values and will contribute to the retention of V in refractory slab residues in subduction zones. In agreement with previous work, we find that D-Ta(rt/melt) > D-Nb(rt/melt) under all conditions investigated, suggesting that rutile fractionation does not lead to low Nb/Ta ratios in Earth's continental crust.