Molybdenum isotope fractionation at upper-crustal magmatic-hydrothermal conditions

Abstract

Molybdenum isotopes are an established proxy for paleoredox conditions in low-temperature surface systems. However, the mechanisms behind demonstrated Mo isotope fractionation during igneous and hydrothermal processes at elevated temperatures are still controversial. This study focusses on a comprehensive dataset documenting the late stage magmatic-hydrothermal evolution of Mo isotope systematics in miarolitic cavities and their host granite from a shallow arc-related intrusive system, the Torres del Paine laccolith in Chile. Molybdenum isotopic compositions (δ98MoSRM3134) were measured for (i) granitic bulk with or without petrographic signs of fluid loss, (ii) magmatic-hydrothermal fluids, and (iii) successively crystallised hydrothermal minerals and range from −1.6 to +1.8‰. The observed variability in δ98MoSRM3134 for individual miarolitic cavities approaching closed system conditions are smaller than the overall range in our dataset but still exceed 1.5‰. The Mo isotopic signature of magmatic fluids was directly measured for the first time by bulk dissolution of magmatic fluid inclusion bearing quartz. Absolute values for magmatic-hydrothermal fluids vary between +0.6 to +1.8‰ δ98MoSRM3134, which is significantly heavier than the granitic bulk rock signatures of −0.1 to +0.46‰ δ98MoSRM3134. Hydrothermal minerals in contrast exhibit variably light δ98MoSRM3134 between −1.6 and +0.6‰. Isotopic differences Δ98Mofluid-mineral between fluid and hydrothermal minerals coexisting in the sampled cavities are largest for plagioclase with 1.9–2.2‰ Δ98Mo, and amount to 1.6–1.9‰ Δ98Mo and 1.5–1.9‰ Δ98Mo for alkali feldspar and biotite, respectively. Smaller values of 1.2–1.5‰ Δ98Mofluid-siderite, 0.8–1.9‰ Δ98Mofluid-molybdenite, 0.4–1.2‰ Δ98Mofluid-titanite and 0.4–1.3‰ Δ98Mofluid-allanite are obtained for higher Mo concentration minerals. Given that fluid-mineral pairs coexisted in equilibrium the ranges in Δ98Mofluid-mineral values we report offer first constraints on the extent of hydrothermal Mo isotope fractionation. The magnitude and direction of these values agrees well with fractionation factors calculated based on an ionic bond-strength model for the incorporation of Mo6+ in hydrothermal minerals for crystallisation temperatures in miarolitic cavities (650–450 °C). This implies that significant fractionation effects can arise during hydrothermal processes even without changes in Mo redox state from oxidised fluid.We can summarise the Mo isotope evolution during magmatic-hydrothermal processes as follows: First, Mo is transferred into the fluid phase exsolving from solidifying magma during late stage igneous evolution. The exsolved fluid subsequently precipitates hydrothermal minerals upon cooling, which dominantly incorporate light Mo isotopes (at variable KDMo(fluid-mineral)). With progressive hydrothermal crystallisation, the remaining fluid evolves to increasingly higher δ98MoSRM3134 along with decreasing Mo concentration.Our data demonstrate that large variability in Mo isotopic signatures can be produced solely by primary magmatic-hydrothermal isotope fractionation processes at elevated temperatures. The generated large range in δ98Mo signatures implies that (i) Mo isotopic signatures of evolved samples cannot be employed for tracing sources or precursor processes unless isotopic fractionation during magmatic-hydrothermal stages is quantified and can be corrected for; and (ii) mass balance models for the global Mo cycle used in paleoredox reconstruction need to account for potentially heterogeneous and isotopically fractionated continental contributions from evolved or hydrothermally overprinted rocks.