Vanadium isotope fractionation during mantle melting: Evidence from Cenozoic alkali basalts in eastern China
It is well known that vanadium (V) is redox-sensitive during magmatism, but whether V isotopes can be used as an oxygen fugacity (fO2) sensor of the mantle remains controversial. It is crucial to understand the behavior and controlling factors of V isotope fractionation during mantle melting before using V isotopes as a redox proxy. This study presents high-precision V isotopic data for alkali basalts with high fO2 from eastern China, which were generated by low degree partial melting of carbonated mantle. Our results show that their δ51V values (-0.85‰ to -0.61‰) are higher than those of mid-ocean ridge basalts (MORBs) and Bulk Silicate Earth (BSE). Chemical alteration, crustal contamination or fractional crystallization negligibly affect the δ51V values of alkali basalts. Although subducted carbonates are involved in the mantle source region beneath eastern China, mass balance calculations show that the incorporation of carbonates did not significantly increase the V isotopic compositions of alkali basalt. In contrast, the observed high δ51V probably reflects the control of partial melting on the V isotopic compositions of mantle-derived melts. The δ51V values of alkali basalts are positively correlated with indicators of partial melting such as the Nb/Y ratios and δ56Fe values, which indicates that V isotopes could be fractionated during mantle melting. Basaltic melts tend to be enriched in V with high valence as it is overall more incompatible than the V with low valence during partial melting, which contributes to the enrichment of 51V in alkali basalts because of the affinity of high valence V and 51V. Furthermore, the fractionation of V isotopes is more significant at a lower degree of melting and/or a more oxidizing condition. Therefore, this study validates discernable V isotope fractionation during mantle partial melting and examines the potential of using V isotopes to trace the oxidation state of magmatic systems.