Mo Isotope Fractionation Between Fluids and Felsic Melts: Implications for the Mo Isotopic Composition of the Upper Continental Crust

The Mo stable isotopic system is a powerful tool to explore both the evolution of the silicate Earth and the paleo-redox conditions of oceans. However, mass balance models produced in both of these applications strongly rely on the composition (δ98/95Mo) of the modern upper continental crust (UCC), which remains uncertain. Indeed, current constraints obtained from exposed igneous rocks (e.g., UCC δ98/95Mo = +0.14 ± 0.07‰ in [1]) or using molybdenite (MoS2) as a mineral proxy for UCC lithologies [2] (e.g., maximum UCC δ98/95Mo based on global mean MoS2 = -0.04 ± 0.84 ‰ in [3]) do not converge. Igneous UCC rocks are dominantly felsic and MoS2 measured for their δ98/95Mo mostly crystallized from magmatic-hydrothermal fluids in differentiated systems [3,4]. Therefore, the derivation of a more robust UCC estimate will require a better understanding of the behaviour of Mo isotopes in silicic magmatic-hydrothermal systems. To this end, we experimentally investigated the partitioning of Mo isotopes between fluids and silicic melts at conditions relevant for typical shallow magma chambers (2 kbar, 700-900̊C).

Our experiments reveal significant isotopic fractionation between equilibrated melts and fluids with Δ98/95Mo fluid-melt ranging from -0.43‰ to -0.32‰. No resolvable correlation between Δ98/95Mo fluid-melt and fluid salinity (0.5-1.5 mol/L (Na,K)Cl), melt alumina saturation index (ASI; 1-1.2) or T were observed, indicating a negligible impact of these parameters over the ranges investigated. Our results suggest that the δ98/95Mo of fluids exsolved from shallow silicic magmas could be lighter than co-existing silicic melts. This could, in turn, explain the lighter δ98/95Mo of global MoS2 averages compared to average silicic rocks, and therefore the discrepancy between UCC constraints derived from them. Fluid exsolution therefore needs to be considered for the derivation of a more robust UCC δ98/95Mo estimate.

[1] Yang et al (2017) GCA 205, 168-186. [2] Greber et al (2014) Lithos 190, 104-110. [3] Willbold & Elliott (2017) Chem Geol 449, 253-268. [4] Breillat et al (2016) J Geochem Explor 161, 1-15.