Understanding the effect of fluid exsolution on the Mo stable isotopic composition of silicic magmas, a step toward a better upper continental crust estimate
Final Report Abstract
The Mo stable isotopic system is 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 magmatichydrothermal 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, constraints on Phanerozoic UCC do not converge, and deriving a robust estimate requires a better understanding of magmatic-hydrothermal systems. To this end, we experimentally determined equilibrium Mo isotope fractionation values between fluids and melts (Δ98/95Mofluid-melt) at temperatures, fluid salinities, melt compositions and oxygen fugacities relevant to upper crustal silicic magmatic systems. The results suggest that light Mo isotopes preferentially partition into exsolved fluids in silicic magma chambers. Given that 1) most MoS2 for which δ98/95Mo has been measured and compiled crystallized from fluids exsolved from silicic magma or from magmas highly enriched in those fluids; and 2) the δ98/95Mo of UCC igneous rocks is dominantly controlled by that of silicic plutonic rocks; our results can explain the lighter δ98/95Mo of MoS2 global averages compared to that of UCC igneous rocks. Isotope fractionation during fluid exsolution can therefore solve the discrepancy between current Phanerozoic UCC δ98/95Mo estimates and provide new ones. The results represent an important step toward the determination of a more robust UCC 98/95Mo estimate.
