Understanding the effect of fluid exsolution on the Mo stable isotopic composition of silicic magmas, a step toward a better upper continental crust estimate

• Applicant: Rachel Bezard, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2021 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 471125662

The formation of the continental crust (CC) has significantly impacted the chemical composition of the rest of the silicate Earth and the hydrosphere. Exactly how it impacted the stable isotopic composition of these reservoirs, however, remains debated for many systems due to the lack of robust CC estimates. It is particularly the case for the Mo stable isotopic system, a very promising new tool to explore both the chemical evolution of the silicate Earth and the paleo-redox conditions of oceans. Mass balance models associated with both types of applications strongly rely on the composition of the CC, especially its upper layer (UCC). This is because it is highly enriched in Mo and in direct contact with the hydrosphere. While an estimate for the Mo stable isotope composition (δ98/95Mo) of UCC created prior to the so-called Great Oxidation event (GOE; ~2.4-2.2 Ga) exists, constraints on post-GOE UCC δ98/95Mo are scarce and conflicting. The difficulty to constrain the δ98/95Mo of the UCC after the GOE is a consequence of the redox-sensitivity of Mo and its fluid-mobility in oxidizing conditions. One approach to constrain modern UCC has been to use the signatures of Mo-rich minerals, molybdenites (MoS2), mostly derived from magmatic-hydrothermal fluids, as proxies for exposed rocks. It was argued that a global average for MoS2 δ98/95Mo could represent a maximum value for Phanerozoic UCC. This, however, is at odds with a recent Phanerozoic UCC composition derived from igneous rock compositions, since the latter is visibly heavier than the most recent MoS2 δ98/95Mo averages. Clearly, current constraints on Phanerozoic UCC do not converge, and deriving a robust estimate will require a better understanding of magmatic-hydrothermal systems.One geological process having the potential to solve this discrepancy is Mo isotopic fractionation during fluid exsolution in silicic systems. The most dominant igneous rock types in the UCC are plutonic silicic rocks, and Mo investigations of these lithologies suggest that up to 60% of the Mo budget of silicic magmas could be transferred to exsolved fluids. Furthermore, based on known Mo species in fluids and melts, a preferential enrichment of light Mo isotopes in fluids during their exsolution is a strong possibility. Given that most measured and compiled MoS2 δ98/95Mo derive from systems highly enriched in fluids exsolved from silicic magmas, this process could explain the lighter δ98/95Mo of MoS2 averages, compared to silicic rocks. It is therefore the aim of this proposal to establish the first experimental constraints of the fluid/melt equilibrium fractionation factor of Mo stable isotopes at temperatures, fluid salinities, melt compositions and oxygen fugacities relevant to upper crustal silicic magmatic systems. Our results will shed light on the meaning of both MoS2 and silicic rock δ98/95Mo and allow the determination of a more robust UCC δ98/95Mo estimate.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Haihao Guo, Ph.D.