Stable vanadium isotopes as a redox proxy in magmatic systems?

Abstract

Recycling pathways of multivalent elements, that impact our understanding of diverse geological processes from ore formation to the rise of atmospheric oxygen, depend critically on the spatial and temporal variation of oxygen fugacity (fO2) in the Earth's interior. Despite its importance, there is currently no consensus on the relative fO2 of the mantle source of mid-ocean ridge basalts compared to the sub-arc mantle, regions central to the mediation of crust-mantle mass balances. Here we present the first stable vanadium isotope measurements of arc lavas, complemented by non-arc lavas and two co-genetic suites of fractionating magmas, to explore the potential of V isotopes as a redox proxy. Vanadium isotopic compositions of arc and non-arc magmas with similar MgO overlap with one another. However, V isotopes display strikingly large, systematic variations of ∼2 ‰ during magmatic differentiation in both arc and non-arc settings. Calculated bulk V Rayleigh fractionation factors (1000 lnamin-melt of -0.4 to -0.5 ‰) are similar regardless of the oxidation state of the evolving magmatic system, which implies that V isotope fractionation is most influenced by differences in bonding environment between minerals and melt rather than changes in redox conditions. Thus, although subtle fO2 effects may be present, V isotopes are not a direct proxy for oxygen fugacity in magmatic systems.