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.

大陆地壳（CC）的形成极大地影响了硅酸盐地球和水圈其余部分的化学成分，然而，由于缺乏科学证据，对于许多系统来说，它到底如何影响这些储层的稳定同位素组成仍然存在争议。钼稳定同位素系统尤其如此，它是探索硅酸盐地球的化学演化和与海洋相关的古氧化还原条件的非常有前途的新工具。两种类型的应用都强烈依赖于 CC 的成分，尤其是其上层 (UCC)，这是因为它富含 Mo 并且与水圈直接接触，同时对 Mo 稳定同位素成分 (δ98/) 进行了估计。在所谓的大氧化事件（GOE；~2.4-2.2 Ga）之前创建的 UCC δ98/95Mo）存在，对后 GOE UCC δ98/95Mo 的限制很少且相互矛盾。限制 GOE 后 UCC 的 δ98/95Mo 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钼矿物辉钼矿的特征。 (MoS2)，主要源自岩浆热液，作为暴露岩石的代表。有人认为，MoS2 δ98/95Mo 的全球平均值可以。然而，这与最近源自火成岩成分的显生宙 UCC 成分不一致，因为后者明显比最新的 MoS2 δ98/95Mo 平均值更重。显然，当前对显生宙 UCC 的限制并不收敛，并且得出了稳健的结论。估计需要更好地了解岩浆-热液系统。一个地质过程有可能解决这个问题差异在于硅质系统中流体出溶过程中的钼同位素分馏。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% 的钼预算可以转移到出溶中。此外，根据流体和熔体中已知的钼种类，轻钼同位素在流体中的出溶过程中优先富集是一种现象。鉴于大多数测量和编制的 MoS2 δ98/95Mo 来自于硅质岩浆中高度富集的流体系统，该过程可以解释与硅质岩石相比，MoS2 的平均 δ98/95Mo 较轻。建议建立 Mo 稳定同位素在温度、流体盐度、熔体成分和氧逸度相关条件下的流体/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 的含义，并允许确定更可靠的 UCC δ98/95Mo 估计。