Fluid-melt Mo isotope fractionation: implications for the δ98/95Mo of the upper crust

Abstract

The isotopic composition (δ98/95Mo) of the modern upper continental crust (UCC) remains uncertain. A UCC estimate modelled from the δ98/95Mo of igneous rocks does not converge with constraints derived from the δ98/95Mo of magmatic-hydrothermal molybdenite (MoS2), a mineral used as a proxy for UCC lithologies. To shed light on this discrepancy, we experimentally determined equilibrium Mo isotope fractionation values between exsolved fluids and melts (Δ98/95Mofluid-melt) in shallow felsic magmatic systems. We show that light Mo isotopes are preferentially incorporated in aqueous supercritical fluids in equilibrium with silicic melts, with Δ98/95Mofluid-melt ranging from −0.43 % to −0.17 %. The δ98/95Mo of exsolved fluids equilibrated in upper crustal silicic reservoirs should therefore be lighter than co-existing silicic melts. Since felsic plutonic rocks make ∼50 % of the UCC, estimates of UCC δ98/95Mo entirely based on igneous rock compositions or based on minerals (MoS2) growing in magmatic-hydrothermal systems alone will lead to divergent values. Our results can therefore explain the discordance between current UCC δ98/95Mo constraints and provide new ones, representing a key step toward the determination of a robust estimate.