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

岩浆温度下弧玄武岩流体出溶过程中的硼同位素分馏

• 批准号: 432220636
• 负责人: Professor Dr. Raúl Fonseca
• 金额: --
• 依托单位: Institut für Geologie, Mineralogie und Geophysik
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2019
• 资助国家: 德国
• 起止时间: 2018-12-31 至 2023-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/432220636?language=en
• 关键词: Boron; isotope; fractionation; during; fluid

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.

俯冲带的岩浆活动是地球地幔和地壳之间流体流动元素化学循环的关键，这是由于弧岩浆活动期间引入了不同比例的板片衍生流体和沉积物熔体。这些可能富含水的原始弧玄武岩应成为流体饱和的深度，导致出溶的含水流体在其上升过程中，并在其地壳侵位，这应该会影响其微量元素和稳定同位素库存到一个未知的程度。硼及其同位素是流体介导过程的强有力示踪剂，因此可能有助于解决这一问题，因为流体往往富含重11 B，而熔体和大多数硅酸盐富含10 B。此外，变质沉积物，流体和熔体显示其B同位素组成的实质性变化，使该系统独特地适合于区分这些组件。此外，尽管俯冲成分的脱水可能会释放出富含重B同位素的流体，但板块成分和俯冲混合物部分熔融产生的熔体成分变化很大，取决于熔融物质的性质和是否存在能够容纳B的矿物。因此，需要考虑B及其同位素在俯冲组分脱水和部分熔融过程中的行为。这种限制是关键，以确定和跟踪流体介导的过程在地球的内部，和特点的回收流体流动和经济上重要的元素在收敛板margins.I打算进行详细的实验活动，以调查B同位素分馏流体从岩浆出溶过程中，部分熔融和俯冲成分的脱水。将使用内部加热的压力容器进行流体出溶实验，其中流体饱和玄武岩的减压可以在各种速率和温度下再现。在这些实验过程中产生的流体将通过在淬灭时冷冻实验来回收，从而允许测定它们的B同位素组成。俯冲成分的部分熔化将通过带式设备实现，使用贵金属囊和金刚石捕集器来保留流体。在每种类型的实验中，流体和硅酸盐熔体都将使用fs-LA-MC-ICP-MS原位测量其B同位素组成。这些新的数据将提供工具来区分不同的俯冲成分，并确定任何后期过程（如流体熔融不相容性）所产生的修改。此外，将获得必要的限制，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部范围。