Molybdenum isotope fractionation at upper-crustal magmatic-hydrothermal conditions

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 still controversial. This study focusses on a comprehensive dataset documenting late stage magmatic-hydrothermal evolution of systematics miarolitic cavities their host granite from shallow arc-related intrusive system, Torres del Paine laccolith Chile. isotopic compositions (?98MoSRM3134) were measured (i) granitic bulk with or without petrographic signs fluid loss, (ii) fluids, (iii) successively crystallised minerals range ?1.6 to +1.8‰. The observed variability ?98MoSRM3134 individual approaching closed system smaller than overall our but exceed 1.5‰. signature magmatic fluids was directly first time by dissolution inclusion bearing quartz. Absolute values vary between +0.6 +1.8‰ ?98MoSRM3134, which is significantly heavier rock signatures ?0.1 +0.46‰ ?98MoSRM3134. Hydrothermal contrast exhibit variably light +0.6‰. Isotopic differences ?98Mofluid-mineral coexisting sampled largest plagioclase 1.9–2.2‰ ?98Mo, amount 1.6–1.9‰ ?98Mo 1.5–1.9‰ alkali feldspar biotite, respectively. Smaller 1.2–1.5‰ ?98Mofluid-siderite, 0.8–1.9‰ ?98Mofluid-molybdenite, 0.4–1.2‰ ?98Mofluid-titanite 0.4–1.3‰ ?98Mofluid-allanite obtained higher concentration minerals. Given that fluid-mineral pairs coexisted equilibrium ranges we report offer constraints extent fractionation. magnitude direction these agrees well factors calculated based ionic bond-strength model incorporation Mo6+ crystallisation (650–450 °C). implies significant effects can arise even changes redox state oxidised fluid. We summarise as follows: First, transferred into phase exsolving solidifying magma evolution. exsolved subsequently precipitates upon cooling, dominantly incorporate (at variable KDMo(fluid-mineral)). With progressive crystallisation, remaining evolves increasingly along decreasing concentration. Our data demonstrate large be produced solely primary temperatures. generated ?98Mo evolved samples cannot employed tracing sources precursor unless stages quantified corrected for; mass balance models global cycle used reconstruction need account potentially heterogeneous isotopically fractionated continental contributions hydrothermally overprinted rocks.