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

摘要

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_2s 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_2O conditions and variable fO_2s. 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.