Fluid-melt Mo isotope fractionation: implications for the δ98/95Mo of the upper crust
Rachel Bezard, Hu Guo
Abstract
The isotopic composition (δ 98/95 Mo) of the modern upper continental crust (UCC) remains uncertain.A UCC estimate modelled from the δ 98/95 Mo of igneous rocks does not converge with constraints derived from the δ 98/95 Mo of magmatichydrothermal molybdenite (MoS 2 ), 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 (Δ 98/95 Mo fluid-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 Δ 98/95 Mo fluid-melt ranging from -0.43 ‰ to -0.17 ‰.The δ 98/95 Mo 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 δ 98/95 Mo entirely based on igneous rock compositions or based on minerals (MoS 2 ) growing in magmatic-hydrothermal systems alone will lead to divergent values.Our results can therefore explain the discordance between current UCC δ 98/95 Mo constraints and provide new ones, representing a key step toward the determination of a robust estimate.