Constraining subduction fluids through non-traditional stable isotopes

Tracing and quantifying fluxes of redox-sensitive elements between the slab, the mantle wedge, the deep mantle, and crust is crucial for understanding Earth chemical evolution, metal enrichment for ore deposits, and habitability. The fluxes can be studied by looking at the oceanic crust and sediment compositions before entering a subduction zone, and the arc magmas that cycle elements back to Earth’s surface. The only direct way to determine what is happening in terms of fluid release from slabs at depth and through time is by studying ophiolites. Ophiolites are remnants of ancient oceanic crust and mantle lithosphere, and record the evolutionary history from seafloor spreading/ocean basins to subduction initiation, and subsequent tectonic emplacement/obduction. It has been recognized that the serpentinised part of the lithosphere is a main carrier of carbon, water, and redox sensitive elements such as sulphur, and a major source of fluids during subduction. Fluids play a crucial role in many geological processes but are extremely difficult to track and constrain since they are unquenchable. Thermodynamic modelling of serpentinite dehydration during subduction has shown that reduced fluids can be released during the brucite-out reaction (e.g. Piccolli et al. 2019), though arc magmas are more oxidized compared to MORB. The proportion of fluids coming from the various parts of a subducting slab (e.g. sediments, oceanic crust, lithosphere) has to be better quantified to understand the redox processes and oxidation state as well as the composition of the fluids metasomatism the mantle wedge. Data from peridotite in ophiolite complexes is limited but such samples can provide new constraints on compositions of fluids released by dehydrating serpentinised lithosphere, by analysing minerals and bulk rock before and after dehydration reactions in serpentinised lithosphere along a prograde path. In this project Mo and Fe isotopes will be used to provide a new (non-traditional stable isotope) perspective. Mo isotopes are a good fluid tracer as the isotopic compositions typically correlate with elemental proxies for aqueous fluid and hydrous melt. Fe isotopes can serve as a tracer of mobility of redox sensitive elements as its fractionation is caused by differences in bonding environment, co-ordination chemistry and oxidation state.