Geochemical record and fe redox evolution of high-pressure serpentinization

The release of H₂ and CH₄ fluids from subduction-zone-related serpentinization has garnered significant interest for its implications on the deep biosphere, geochemical cycling of elements, and potential as a deep energy resource. However, identifying natural evidence of deep serpentinization in collisional settings remains challenging. This thesis presents new insights into deep serpentinization by studying two natural examples, with a focus on the Monte Maggiore massif in Alpine Corsica. It integrates field and petrographic analyses with high-resolution sampling and geochemical evaluations, including in-situ trace elements, bulk and in-situ boron isotopes (δ11B), and iron isotopes (δ56Fe). Results revealed the presence of multiple serpentine generations along a serpentinization front, indicating various fluid-rock interactions across a wide temperature range, or driven by silica-rich fluids. Geochemical data show a notable decrease in fluid-mobile elements, and a decrease in boron concentrations and δ11B from serpentinites at the basal tectonic contact to the less serpentinized core, reflecting upward migrations of fluids. High Fe3+/FeTot ratios indicate the potential to release reduced fluids. Additionally positive correlation of Fe3+/FeTot with δ56Fe values suggest for input of heavy δ56Fe fluids during serpentinization. The parallel case study Belvidere Mountain Complex (BMC) in Northern Vermont, a high-temperature serpentinized body, shows a decrease in δ56Fe values from partially to fully serpentinized samples, suggesting the influence of lighter iron isotope slab fluids during serpentinization. This thesis emphasizes the use of a cartographic approach and geochemical tracers to identify fluid sources and fluid-rock interaction mechanisms. This provides insights into serpentinization and redox conditions of previously overlooked dry portions of subducted oceanic mantle interacting with slab-derived fluids in subduction. Exploring these themes could enhance our understanding of the magnitude and conditions under which reduced fluids are released in subduction zones, their impact on the deep biosphere, the geochemical cycle of elements, and potential deep energy resources.