VAPOR TRANSPORT AND RAYLEIGH FRACTIONATION OF MO ISOTOPES DURING MAGMATIC/HYDROTHERMAL PROCESSES: IMPLICATIONS FOR ORE GENESIS

A growing body of molybdenum (Mo) isotope data documents fractionation of up to 1 per mil per atomic mass unit (‰/amu) during redox reactions in marine environments, leaving geochemical fingerprints that mark variations in the oxidation state of the oceans. It is commonly assumed, however, that Mo isotopes are not significantly fractionated during high-temperature magmatic/hydrothermal processes. Here we show that Mo isotopes are indeed fractionated during magmatic/hydrothermal processes.

Analyses of Mo isotopes in twenty molybdenite samples, representing a range of mineralized geologic settings, each precisely dated by the Re-Os method, show mass-dependent fractionation spanning 0.63‰/amu (Hannah et al., 2007, Geology). Analyses were done by MC-ICP-MS, following introduction of a ⁹⁴Mo-¹⁰⁰Mo double spike and purification by ion exchange. The Mo isotopic compositions of molybdenites are not correlated with crystallization temperature, age, or geologic conditions. Rather, kinetic fractionation may explain variations of up to 0.34‰/amu within a single molybdenite occurrence or deposit, exceeding the suggested variability in typical continental crust (~0.2‰/amu; Siebert et al., 2003, EPSL). Vapor transport of Mo as molybdate species and precipitation of molybdenite in rapidly propagating fractures may be responsible for isotope fractionation of Mo (and perhaps other metals) at very small scales.

Combined data from previous experimental and petrographic studies suggest that molybdenite crystallizes from a vapor phase by reaction of hydrated molybdate species with H₂S as oxygen fugacity falls below the SO₂-H₂S buffer with declining temperature. If so, then measurable (and spatially variable) Mo isotope fractionation may occur even at elevated temperatures because of the very different bond strengths of the two Mo species. Furthermore, expansion of vapor into propagating fracture systems is a highly energetic event that almost certainly amplifies isotopic fractionation by distillation. A Rayleigh fractionation model assuming a molybdenite-molybdate fractionation factor of 0.999 (i.e., Mo in crystalline MoS₂ is isotopically lighter than Mo in MoO₃•nH₂O by ~1‰/amu) shows that this mechanism can result in the observed fractionation of Mo isotopes in hydrothermal systems.