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）的形成对硅酸盐地球和水圈的化学组成产生了重大影响。然而，由于缺乏可靠的CC估计，许多系统仍然存在争议。特别是Mo稳定同位素体系，这是一个非常有前途的新工具，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程序相关联的质量平衡模型强烈依赖于CC的组成，特别是其上层（UCC）。这是因为它高度富集钼并且与水圈直接接触。虽然在大氧化事件（GOE; ~2.4-2.2 Ga）之前，对UCC的Mo稳定同位素组成（δ98/95 Mo）进行了估计，但对GOE后UCC δ98/95 Mo的限制很少且相互矛盾。在GOE之后难以限制UCC的δ98/95 Mo是Mo的氧化还原敏感性及其在氧化条件下的流体流动性的结果。限制现代UCC的一种方法是使用富含Mo的矿物的特征，主要来自岩浆热液流体的钼铅矿（MoS 2），作为暴露岩石的代理。有人认为，全球平均值的MoS 2 δ98/95 Mo可以代表一个最大值的中生代UCC。然而，这与最近的中生代UCC成分来自火成岩成分不一致，因为后者明显比最近的MoS 2 δ98/95 Mo平均值重。显然，目前的制约因素对中生代UCC不收敛，并得出一个强大的估计将需要更好地了解岩浆热液system.One地质过程有可能解决这种差异是Mo同位素分馏流体出溶过程中的岩浆热液系统。在UCC最主要的火成岩类型是深成岩，钼调查这些岩性表明，高达60%的钼预算的岩浆可以转移到出溶流体。此外，根据已知的钼物种在流体和熔体中，轻钼同位素在流体的出溶过程中的优先富集是一个很大的可能性。鉴于大多数测量和汇编的MoS 2 δ98/95 Mo来自于从岩浆中喷出的流体中高度富集的系统，这个过程可以解释MoS 2平均值的δ98/95 Mo与岩浆岩相比较轻。因此，本建议的目的是建立在温度，流体盐度，熔体成分和氧逸度有关的上地壳岩浆系统的Mo稳定同位素的流体/熔体平衡分馏因子的第一个实验约束。我们的研究结果将阐明MoS 2和岩石δ98/95 Mo的意义，并允许确定更可靠的UCC δ98/95 Mo估计值。

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.