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

• Applicant: Professor Dr. Raúl Fonseca
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 432220636

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.

• DFG Programme: Research Grants