Fluid Control on the Mo Isotope Budget in High-Silica Magmatic Systems

Mass-dependent variations of molybdenum (Mo) isotopes in magmatic rocks show potential as a powerful tool to study magmatic processes involved in the evolution of the Earth's mantle and crust. There is general consensus that primitive, mantle-derived melts faithfully track the Mo isotope composition of their sources. Yet, little is known about the effect of magmatic processes on δ98Mo in high-silica systems (δ98Mo = [98Mo/95Mo]sample/[98Mo/95Mo]standard - 1). To this end, we studied the δ98Mo composition of a well-characterised dacitic system in Iceland that also carries abundant crustal mafic xenoliths1. The dacites formed by partial melting of mafic crustal rocks in an oceanic within-plate setting and are thus not affected by δ98Mo variations of source rocks due to variable involvement of subduction zone components.

Starting from mantle-like δ98Mo values of ca. - 0.21‰, mafic xenoliths show an inverse trend of decreasing δ98Mo with increasing TiO2, in line with variable content of residual, isotopically light oxide phases dominating their Mo isotope budget2. In contrast, δ98Mo values of dacites vary from sub- to super mantle-like compositions over a range of more than 1 ‰. Their δ98Mo values do not vary with general geochemical indices of magmatic differentiation nor with tracers for specific mineral phases (TiO2, La/Yb, Dy/Dy*). Also, no systematic trends between δ98Mo and indices of fluid addition (Ba/Th, Ce/Pb, Ce/Mo) are observed. Yet, the δ98Mo data form an inverse trend with 206,207,208Pb/204Pb ratios.

Our data suggests limited control of magmatic processes on δ98Mo in this silica-rich system. Similarly, the effect of fluid addition is less evident based on classical geochemical tracers. Yet, fluid addition must have played a crucial role in the formation of the dacites1. As such, the co-variation of δ98Mo with 206Pb/204Pb, two fluid-mobile isotope systems with similar geochemical properties, hints at the presence of multiple fluids from two crustal sources dominating the final Mo and Pb isotope budgets of the dacites, but that had variable Mo and Pb concentrations. Our findings thus imply that the Mo isotope system could be a sensitive tracer for fluid addition processes during intra-crustal differentiation.

1Willbold, M., et al., EPSL, 279, 44-52 (2009). 2Chen, S., et al., Nat Comm, 10, 4773 (2019).