Understanding the effect of fluid exsolution on the Mo stable isotopic composition of silicic magmas, a step toward a better upper continental crust estimate

大陆地壳（CC）的形成显著地影响了硅酸盐土和水圈其余部分的化学成分。然而，由于缺乏可靠的CC估计，对于许多系统来说，它究竟如何影响这些储层的稳定同位素组成仍然存在争议。Mo稳定同位素系统尤其如此，这是一个非常有前途的新工具，可以探索硅酸盐地球的化学演化和海洋的古氧化还原条件。与这两种类型的应用相关的质量平衡模型强烈依赖于CC的组成，特别是其上层（UCC）。这是因为它富含钼，并与水圈直接接触。虽然在所谓的大氧化事件（GOE; ~2.4-2.2 Ga）之前存在对UCC中Mo稳定同位素组成（δ98/95Mo）的估计，但对GOE后UCC δ98/95Mo的限制很少且相互矛盾。由于Mo的氧化敏感性及其在氧化条件下的流体迁移性，在GOE后难以约束UCC的δ98/95Mo。限制现代UCC的一种方法是使用富钼矿物的特征，辉钼矿（MoS2），主要来自岩浆热液流体，作为暴露岩石的代用物。认为全球平均的MoS2 δ98/95Mo可以代表显生宙UCC的最大值。然而，这与最近由火成岩组成的显生宙UCC组成不一致，因为后者明显比最近的MoS2 δ98/95Mo平均值重。显然，目前对显生宙UCC的限制并不收敛，要得到一个可靠的估计，将需要对岩浆-热液系统有更好的了解。一种有可能解决这种差异的地质过程是硅体系中流体溶出过程中的Mo同位素分馏。UCC中最主要的火成岩类型是深成型硅质岩，对这些岩性的Mo研究表明，高达60%的硅质岩浆的Mo平衡可以转移到溶解流体中。此外，根据流体和熔体中已知的Mo种类，在流体析出过程中，轻Mo同位素在流体中优先富集的可能性很大。考虑到大多数测量和编译的MoS2 δ98/95Mo来自于从硅质岩浆中溶解的高度富集流体的系统，这一过程可以解释与硅质岩石相比，MoS2平均δ98/95Mo较轻。因此，本文的目的是建立与上地壳硅质岩浆系统有关的Mo稳定同位素在温度、流体盐度、熔体成分和氧逸度条件下的流体/熔体平衡分馏因子的第一个实验约束。我们的结果将阐明MoS2和硅质岩石δ98/95Mo的意义，并允许确定更可靠的UCC δ98/95Mo估计。

The formation of the continental crust (CC) has significantly impacted the chemical composition of the rest of the silicate Earth and the hydrosphere. Exactly how it impacted the stable isotopic composition of these reservoirs, however, remains debated for many systems due to the lack of robust CC estimates. It is particularly the case for the Mo stable isotopic system, a very promising new tool to explore both the chemical evolution of the silicate Earth and the paleo-redox conditions of oceans. Mass balance models associated with both types of applications strongly rely on the composition of the CC, especially its upper layer (UCC). This is because it is highly enriched in Mo and in direct contact with the hydrosphere. While an estimate for the Mo stable isotope composition (δ98/95Mo) of UCC created prior to the so-called Great Oxidation event (GOE; ~2.4-2.2 Ga) exists, constraints on post-GOE UCC δ98/95Mo are scarce and conflicting. The difficulty to constrain the δ98/95Mo of the UCC after the GOE is a consequence of the redox-sensitivity of Mo and its fluid-mobility in oxidizing conditions. One approach to constrain modern UCC has been to use the signatures of Mo-rich minerals, molybdenites (MoS2), mostly derived from magmatic-hydrothermal fluids, as proxies for exposed rocks. It was argued that a global average for MoS2 δ98/95Mo could represent a maximum value for Phanerozoic UCC. This, however, is at odds with a recent Phanerozoic UCC composition derived from igneous rock compositions, since the latter is visibly heavier than the most recent MoS2 δ98/95Mo averages. Clearly, current constraints on Phanerozoic UCC do not converge, and deriving a robust estimate will require a better understanding of magmatic-hydrothermal systems.One geological process having the potential to solve this discrepancy is Mo isotopic fractionation during fluid exsolution in silicic systems. The most dominant igneous rock types in the UCC are plutonic silicic rocks, and Mo investigations of these lithologies suggest that up to 60% of the Mo budget of silicic magmas could be transferred to exsolved fluids. Furthermore, based on known Mo species in fluids and melts, a preferential enrichment of light Mo isotopes in fluids during their exsolution is a strong possibility. Given that most measured and compiled MoS2 δ98/95Mo derive from systems highly enriched in fluids exsolved from silicic magmas, this process could explain the lighter δ98/95Mo of MoS2 averages, compared to silicic rocks. It is therefore the aim of this proposal to establish the first experimental constraints of the fluid/melt equilibrium fractionation factor of Mo stable isotopes at temperatures, fluid salinities, melt compositions and oxygen fugacities relevant to upper crustal silicic magmatic systems. Our results will shed light on the meaning of both MoS2 and silicic rock δ98/95Mo and allow the determination of a more robust UCC δ98/95Mo estimate.