Olivine formation processes and fluid pathways in subducted serpentinites revealed by in-situ oxygen isotope analysis (Zermatt-Saas, Switzerland)

Ulrich, Michelle; Rubatto, Daniela; Hermann, Jörg; Markmann, Thorsten A.; Bouvier, Anne Sophie; Deloule, Etienne (2024). Olivine formation processes and fluid pathways in subducted serpentinites revealed by in-situ oxygen isotope analysis (Zermatt-Saas, Switzerland). Chemical geology, 649 Elsevier 10.1016/j.chemgeo.2024.121978

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Dehydration of serpentinites plays a crucial role in mass transfer into the Earth's interior by releasing aqueous fluids and forming new minerals. These minerals, such as metamorphic olivine, can serve as tracers of fluid-related processes. High-pressure (HP) antigorite, metamorphic olivine, and coexisting magnetite in serpentinites from a continuous, km-scale outcrop within the Zermatt-Saas HP ophiolite were analyzed in situ for trace elements and oxygen isotopes to identify differences in the initial serpentinization conditions and to investigate fluid pathways during subduction-related metamorphism. The oxygen isotopic composition, and As and Sb concentrations in antigorite reveal two distinct serpentinization conditions within the studied region: i) high As and Sb (1–25 μg/g and 0.5–5 μg/g, respectively), coupled with δ18O of +6 to +7 ‰, suggesting serpentinization at relatively low temperatures near the seafloor, and ii) low As and Sb (0.03–5 μg/g and ≤ 0.1 μg/g, respectively), coupled with mostly lower δ18O of +4 to +6 ‰, suggesting serpentinization at higher temperatures by interaction with fluids deeper below the seafloor.

Olivine produced in situ by the brucite + antigorite dehydration reaction during subduction shows isotopic equilibrium with antigorite, and coexisting magnetite with ∆18OAtg-Ol of +1.5–2.5 ‰ and ∆18OOl-Mt of ∼+3 ‰ at reaction temperature conditions of 550–600 °C. The obtained isotopic signatures of metamorphic olivine with δ18O values of +1 to +2 ‰ and + 4 to +5 ‰ correspond to two different isotopic compositions of the released fluid of +5 to +6 ‰ and + 8 to +9 ‰ at these temperature conditions. This suggests that fluids released from subducted serpentinites may have variable δ18O under forearc conditions. The presence of fluids with variable δ18O can cause olivine in structures associated with fluid flow (e.g., shear bands, shear zones and veins) to be in isotopic equilibrium with magnetite, but in either isotopic equilibrium or disequilibrium with antigorite. Isotopic equilibrium with antigorite is achieved when the fluid responsible for olivine crystallization is internally derived. Isotopic disequilibrium is due to an externally derived fluid released by dehydration of serpentinite with a different isotopic composition than the serpentinite with which the fluid interacts. The restricted occurrence of non-equilibrated olivine only in shear bands and nearly pure Ol-veins indicates channelized fluid flow in subduction zone settings and demonstrates that isotopic disequilibrium can be used as a tracer for fluid infiltration.

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