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.

连续地壳（CC）的形成显着影响了硅胶土和水圈的其余化学组成。然而，由于缺乏强大的CC估计值，它如何影响这些储层的稳定同位素组成，但对于许多系统而言。 MO稳定的同位素系统尤其如此，这是一种非常有希望的新工具，可以探索硅胶地球的化学演化和海洋的古雷克斯条件。与两种应用相关的质量平衡模型强烈依赖于CC的组成，尤其是其上层（UCC）。这是因为它高度富集在MO中，并与水圈直接接触。尽管存在于所谓的大氧化事件（GOE; 〜2.4-2.2 GA）之前创建的UCC的Mo稳定同位素组成（Δ98/95MO）的估计值，但对后GOE UCCΔ98/95MO的限制却很少且稀缺且相互矛盾。在GOE之后，难以限制UCC的Δ98/95MO是MO氧化还原敏感性及其在氧化条件下的流体动力的结果。限制现代UCC的一种方法是使用富含Mo-rich矿物质，钼矿（MOS2）的特征，主要源自岩浆 - 热热流体，作为暴露岩石的代理。有人认为，MOS2Δ98/95MO的全球平均值可能代表Phanerozoic UCC的最大值。然而，这与最新的源自源自火成岩的岩石组成的植物学UCC组成不一致，因为后者比最近的MOS2Δ98/95MO平均值更重。显然，当前对Phanerokoic UCC的限制不会融合，并且得出强大的估计值将需要更好地了解岩浆 - 氢热系统。一种地质过程有可能解决这种差异的潜力，即在硅系统中流体溶液中的Mo同位素分馏。 UCC中最主要的火成岩类型是五硅岩石，对这些岩性的研究表明，硅岩浆的MO预算中有60％可以转移到溶解的液体中。此外，基于在流体和熔体中已知的MO物种，在液体中，液体中的光Mo同位素在其溶液中的优先富集是一种强大的可能性。鉴于大多数测量和编译的MOS2Δ98/95MO是从高度富含从硅岩浆中溶解的流体中的系统得出的，因此与硅岩相比，此过程可以解释MOS2平均值较轻的Δ98/95MO。因此，该提案的目的是在温度，流体盐分，熔体成分和与上层甲壳硅胶岩浆系统相关的流体/熔体平衡分级因子的第一个实验约束。我们的结果将阐明MOS2和硅岩Δ98/95MO的含义，并允许确定更强大的UCCΔ98/95MO估计值。