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

了解流体溶蚀对硅质岩浆 Mo 稳定同位素组成的影响，这是朝着更好地估计上大陆壳迈出的一步

• 批准号: 471125662
• 负责人: Dr. Rachel Bezard, Ph.D.
• 金额: --
• 依托单位: Institut des Sciences de la Terre dOrléans
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2021
• 资助国家: 德国
• 起止时间: 2020-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/471125662?language=en
• 关键词: Understanding; effect; fluid; exsolution; Mo

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.

陆壳(CC)的形成对硅酸盐地球其余部分和水圈的化学成分产生了重大影响。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集层的稳定同位素组成仍然存在争议。特别是钼稳定同位素系统，这是一种非常有希望的新工具，既可以探索硅酸盐地球的化学演化，也可以探索海洋的古氧化还原条件。与这两类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)。这是因为它高度富含钼，并与水圈直接接触。虽然存在对所谓大氧化事件(GOE；~2.4Ga)之前形成的UCC的钼稳定同位素组成(δ98/95Mo)的估计，但对GOE后UCCδ98/95Mo的限制很少，而且相互矛盾。在GOE后难以限制UCC的δ98/95Mo是由于Mo的氧化还原敏感性和在氧化条件下的流体流动性的结果。限制现代UCC的一种方法是使用主要来自岩浆热液的富钼矿物-辉钼矿(MoS2)的特征作为暴露岩石的代用品。有人认为，MoS2δ98/95Mo的全球平均值可能代表显生界UCC的最大值。然而，这与最近显生界来自火成岩成分的UCC成分是不一致的，因为后者明显比最近的MoS2δ98/95Mo平均值重。显然，显生界UCC的当前约束并不收敛，得出一个可靠的估计将需要更好地了解岩浆-热液系统。一个有可能解决这种差异的地质过程是硅质系统中流体出溶过程中的Mo同位素分馏。UCC中最主要的火成岩类型是深成硅质岩，对这些岩性的钼研究表明，硅质岩浆中高达60%的钼平衡可以转移到出溶流体中。此外，根据流体和熔体中已知的钼物种，流体在出溶过程中优先富集轻钼同位素的可能性很大。考虑到大多数测量和汇编的MoS2δ98/95Mo来自硅质岩浆喷出的流体中高度浓缩的系统，这一过程可以解释MoS2平均δ98/95Mo比硅质岩石轻的原因。因此，这一建议的目的是建立与上地壳硅质岩浆系统有关的温度、流体盐度、熔体成分和氧逸度下钼稳定同位素的流体/熔体平衡分馏因子的首次实验约束。我们的结果将阐明MoS_2和硅质岩石δ98/95Mo的含义，并允许确定更可靠的δ98/95Mo估计。