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 (δ⁹⁸Mo_SRM3134) 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 δ⁹⁸Mo_SRM3134 for individual miarolitic cavities approaching closed system conditions are smaller than the overall range in our data set 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‰ δ⁹⁸Mo_SRM3134, which is significantly heavier than the granitic bulk rock signatures of −0.1 to +0.46‰ δ⁹⁸Mo_SRM3134. Hydrothermal minerals in contrast exhibit variably light δ⁹⁸Mo_SRM3134 between −1.6 and + 0.6‰. Isotopic differences Δ⁹⁸Mo_{fluid-mineral} between fluid and hydrothermal minerals coexisting in the sampled cavities are largest for plagioclase with 1.9–2.2‰ Δ⁹⁸Mo, 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‰ Δ⁹⁸Mo_{fluid-siderite}, 0.8–1.9‰ Δ⁹⁸Mo_{fluid-molybdenite}, 0.4–1.2‰ Δ⁹⁸Mo_{fluid-titanite} and 0.4–1.3‰ Δ⁹⁸Mo _{fluid-allanite} are obtained for higher Mo concentration minerals. Given that fluid-mineral pairs coexisted in equilibrium the ranges in Δ⁹⁸Mo_{fluid-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 KD_Mo(fluid-mineral)). With progressive hydrothermal crystallisation, the remaining fluid evolves to increasingly higher δ⁹⁸Mo_SRM3134 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.

钼同位素是低温表面系统中古氧化还原条件的公认代表。然而，在高温下火成岩和热液过程中，Mo同位素分馏的背后机理仍存在争议。这项研究集中在一个综合的数据集上，该数据集记录了从浅弧相关的侵入系统智利的托雷斯德尔潘恩漆岩中，含镁同位素系统在微孔洞及其宿主花岗岩中的晚期岩浆-水热演化。钼同位素组成（δ ⁹⁸沫_SRM3134）的测量值（i）有或没有流体损失的岩石学迹象的花岗岩块体；（ii）岩浆热液流体；以及（iii）连续结晶的热液矿物，范围为-1.6至+ 1.8‰。在δ观察到的变化⁹⁸莫_SRM3134对于接近封闭的系统环境个别晶洞腔是在我们的数据集比整体范围较小，但仍然超过1.5‰。第一次通过大量溶解岩浆流体包裹体石英直接测量了岩浆流体的Mo同位素特征。为岩浆热液绝对值变化0.6之间，以+ 1.8‰δ ⁹⁸沫_SRM3134，这比-0.1至+ 0.46‰δ花岗岩散装签名显著较重⁹⁸Mo _SRM3134。相反展览热液矿物可变光δ ⁹⁸沫_SRM3134 -1.6和+ 0.6‰之间。同位素差Δ ⁹⁸沫_流体矿物流体和热液矿物在采样腔共存之间是最大与斜长石1.9-2.2‰Δ ⁹⁸沫，达1.6-1.9‰Δ ⁹⁸ Mo和1.5-1.9‰Δ ⁹⁸沫为碱长石和黑云母分别。的1.2-1.5值越小‰Δ ⁹⁸沫_{流体菱铁矿}，0.8-1.9‰Δ ⁹⁸沫_{流体辉钼矿}，0.4-1.2‰Δ ⁹⁸沫_流体榍和0.4-1.3‰Δ ⁹⁸沫_流体褐帘是对于较高的Mo浓度矿物质获得。鉴于在平衡共存流体矿物对在Δ范围⁹⁸沫_流体矿物值，我们报告提供关于水热沫同位素分馏程度第一约束。这些值的大小和方向与基于离子键强度模型计算出的分馏因子非常吻合，该模型用于将Mo ⁶⁺掺入热液矿物中，以改善微晶石腔中的结晶温度（650-450°C）。这意味着即使在热氧化过程中Mo氧化还原状态没有变化的情况下，在水热过程中也会产生明显的分馏效果。

我们可以总结出岩浆热液过程中的Mo同位素演化过程：首先，在晚期火成岩演化过程中，Mo被转移到固相岩浆溶解的液相中。溶解的流体随后在冷却时沉淀出热液矿物，这些矿物主要掺入了轻的Mo同位素（可变KD _Mo（流体矿物））。具有渐进热液结晶，剩余的流体演变到越来越高的δ ⁹⁸沫_SRM3134随Mo浓度沿。

我们的数据表明，仅在高温下岩浆-水热同位素分馏过程才能产生Mo同位素特征的较大变化。^δ98 Mo标记的大范围生成意味着：（i）除非对岩浆-热液阶段的同位素分馏进行了量化并可以对其进行校正，否则演化样品的Mo同位素标记不能用于追踪源或前体过程。（ii）古氧化还原重建中使用的全球Mo周期的质量平衡模型，必须考虑到来自演化或热液叠印岩石的潜在非均质和同位素分馏的大陆贡献。