Oxidation State of Arc Mantle Revealed by Partitioning of V, Sc, and Ti Between Mantle Minerals and Basaltic Melts

Abstract

Abstract Whether arc mantle is more oxidized than oceanic mantle is persistently debated. The behavior of multivalent vanadium (V) is oxygen fugacity (fO 2 ) sensitive, and the ratios of V to a homovalent element (e.g., Sc, Ti, or Yb) in basalts were commonly used as fO 2 proxies. Similar ratios, such as V/Sc, between arc basalts and mid‐ocean ridge basalts were previously taken as evidence for similar fO 2 s in their mantle sources. However, this claim may be problematic because elemental ratios are primarily controlled by partition coefficients ( D values), which are further affected by various factors. Here we determined D values of V and other transition elements between mantle minerals and basaltic melts at typical arc T–P‐H 2 O conditions and variable fO 2 s. Combining experimental results with published data, the effects of fO 2 , T, P, and phase compositions on D V , D Sc , and D Ti for olivine, orthopyroxene (opx), clinopyroxene (cpx), and spinel were evaluated using multiple linear regressions. The results show that D V values for these four minerals all increase with decreasing fO 2 and temperature, leading to higher D V / D Sc and D V / D Ti ratios at low temperatures than those at high temperatures given a certain fO 2 . Thus, similar V/Sc and V/Ti ratios between arc basalts and mid‐ocean ridge basalts reflect a relatively oxidized arc mantle due to its lower melting temperatures. In light of the highly incompatible behavior of Ti during mantle melting, V‐Ti systematics are regarded to be more superior than V‐Sc systematics in the fO 2 estimation. Partial melting modelling results using V‐Ti systematics reveal that arc mantle is, on average, ~0.9 log units higher in fO 2 than oceanic mantle.