Boron isotope fractionation during fluid exsolution from arc basalts at magmatic temperatures

俯冲带的岩浆作用对地幔与地壳之间流体活动元素的化学循环起着关键作用，这是因为在弧形岩浆作用中引入了不同比例的板条来源的流体和沉积物熔体。这些潜在富水的原始弧形玄武岩在上升过程中和地壳侵位之前应该在深部变得流体饱和，导致含水流体出溶，这应该会对它们的微量元素和稳定同位素库存产生未知的影响。硼及其同位素是流体作用过程的强大示踪剂，因此可能有助于解决这一问题，因为流体往往富含重11B，而熔体和大多数硅酸盐富含10B。此外，变质岩、流体和熔体在其B同位素组成上表现出很大的差异，这使得这个系统特别适合于区分这些成分。此外，尽管俯冲成分的脱水可能释放出富含重B同位素的流体，但板片成分和俯冲混杂岩部分熔融产生的熔体的成分变化很大，取决于被熔融物质的性质和能够容纳B的矿物的存在。因此，有必要考虑B及其同位素在俯冲成分脱水和部分熔融过程中的行为。这些约束是识别和追踪地球内部流体中介过程的关键，也是表征汇聚板块边缘流体活动和重要经济元素循环的关键。我打算开展详细的实验活动，研究岩浆流体出溶过程中的B同位素分馏，以及俯冲成分的部分熔融和脱水。流体释放实验将使用内部加热的压力容器进行，在压力容器中，流体饱和的玄武岩可以以不同的速度和温度进行减压。在这些实验中产生的流体将通过冻结实验在淬火时被回收，从而能够确定它们的B同位素组成。俯冲成分的部分熔化将通过腰带设备实现，使用贵金属胶囊和钻石陷阱来保留流体。在每种类型的实验中，流体和硅酸盐熔体都将使用FS-LA-MC-ICPMS进行现场测量，以确定其B同位素组成。这些新数据将提供工具来区分不同的俯冲成分，并识别由后期过程(如流体-熔体不混溶)产生的任何修改。此外，还将获得必要的限制条件，以跟踪和确定地球地幔和地壳之间的流体循环和化学转移的全面程度。

Magmatism in subduction zones is pivotal to the chemical cycling of fluid-mobile elements between Earth’s mantle and crust, owing to the introduction of variable fractions of slab-derived fluids and sediment melts during arc magmatism. These potentially water-rich primitive arc basalts should become fluid-saturated at depth leading to the exsolution of a water-bearing fluid during their ascent and prior to their crustal emplacement, which should affect their trace element and stable isotope inventory to a yet unknown extent. Boron and its isotopes are a powerful tracer of fluid-mediated processes, and thus potentially useful to address this issue, as fluids tend to be enriched in heavy 11B whereas melts and most silicates are enriched in 10B. Moreover, metasediments, fluids and melts show substantial variations in their B isotope composition, making this system uniquely suited to discriminate between these components. Also, even though dehydration of subduction components likely releases fluids enriched in heavy B isotopes, the composition of melts generated from partial melting of slab components and subduction mélanges is highly variable and depends on the nature of the material being molten and the presence of minerals capable of hosting B. It is thus desirable to consider the behavior of B and its isotopes during the dehydration and partial melting of subduction components. Such constraints are key to identify and track fluid-mediated processes in Earth's interior, and characterize the recycling of fluid-mobile and economically important elements in convergent plate margins.I intend to carry out detailed experimental campaigns to investigate B isotope fractionation during fluid exsolution from a magma, and partial melting and dehydration of subduction components. Fluid exsolution experiments will be carried out using internally heated pressure vessels, where decompression of fluid-saturated basalts can be reproduced at various rates, and temperatures. Fluids generated during these experiments will be recovered by freezing the experiments upon quenching, allowing for the determination of their B isotope composition. The partial melting of subduction components will be achieved with a belt apparatus, using noble metal capsules and diamond traps to retain the fluid. In each type of experiment, both the fluid and silicate melts will be measured in situ using fs-LA-MC-ICP-MS for their B isotope composition. These new data will provide tools to discriminate between different subduction components and identify any modification borne out of late-stage processes like fluid-melt immiscibility. Moreover, essential constraints will be obtained to track and identify the full extent of fluid circulation and chemical transfer between Earth’s mantle and crust.