Microscale oxygen isotope tracing of fluid production and circulation in subducted serpentinites

Dehydration of serpentinites in subduction zones is a major process that releases water at mantle depth. Tracking the dynamics and composition of the fluid produced is a critical step in constructing robust models of subduction processes. We analysed fluid-mobile elements and oxygen isotopes at the microscale as geochemical proxies for fluid production and circulation in high-pressure serpentinites in the Erro Tobbio and Zermatt-Saas units (Western Alps).

In the Erro Tobbio ultramafic rocks, oceanic lizardite is metastable at the peak metamorphic conditions of ~2 GPa, 550–600°C, suggesting that dehydration reactions may be delayed in low strain zones of the subducted slabs. In high strain zones, lizardite is completely replaced by antigorite. The oxygen isotopic composition of antigorite (δ¹⁸O of +6 to +8 ‰) is relatively uniform compared to the large variations observed in lizardite (0 to +12 ‰), indicating homogenisation at the sample scale during prograde metamorphism. In the Zermatt-Saas serpentinites, antigorite is the only serpentine phase and its fluid mobile elements (As, Sb and B) and O isotopic compositions still preserve evidence of different conditions during oceanic serpentinisation.

In both units, metamorphic olivine is formed by the brucite + antigorite dehydration reaction and generally shows isotopic equilibrium with antigorite, with ∆18O_Atg-Ol of 1.5–2.5 ‰ at 550­–600 °C. In the Erro Tobbio serpentinites, metamorphic olivine has homogeneous δ¹⁸O values of +4 to +5 ‰, and shows isotopic equilibrium with antigorite, regardless of the degree of deformation. However, in the Zermatt-Saas samples, metamorphic olivine and antigorite show different degrees of equilibration depending on the texture (Ulrich et al. 2024). Olivine in structures associated with fluid flow is either (i) in isotopic equilibrium with antigorite when the fluid responsible for olivine crystallisation is internally derived, or (ii) in isotopic disequilibrium with antigorite when olivine is formed by infiltration of an externally derived fluid released from serpentinite with a different isotopic composition. The occurrence of non-equilibrated olivine only in shear bands, shear zones and olivine veins suggests that these structures act as channels for large-scale fluid mobilisation. Channelling of the fluid flow is expected because the replacement of antigorite by olivine leads to a reduction in volume and thus changes the porosity and the fluid pressure at the microscale, while deformation increases permeability. The interaction of such fluids, which have a low δ¹⁸O of 4–5 ‰, with the overlying altered metabasalts and metasediments can result in a significant lowering of the oxygen isotopic composition and can be used to trace the pathways of serpentinite-derived fluids in the subducted slab (Bovay et al. 2021, Rubatto et al. 2023). Our study shows that the combined study of structures, textures, trace elements and oxygen isotopes at the microscale allows the reconstruction of fluid production and transport in subduction zone environments.

 

References

Bovay T, Rubatto D, Lanari P (2021) doi:10.1007/s00410-021-01806-4

Rubatto D, Williams M, Markmann T, Hermann J, Lanari P (2023) doi:10.1007/s00410-023-02060-6

Ulrich M, Rubatto D, Hermann J, Markmann T, Bouvier A-S, Deloule E (2024) doi:10.1016/j.chemgeo.2024.121978