Serpentinization and mass-transfer in modern and ancient forearcs: Implications for chemosynthetic life across space and time

Convergent margins shape the long-term evolution of Earth's lithosphere, driving recycling and mantle re-fertilization through sustained deformation, metamorphism, and fluid release over geologic timescales. In their shallow forearcs, hydration of mantle wedge peridotite—forearc serpentinization—drives water–rock reactions that generate molecular hydrogen, promote abiotic methane formation, and produce alkaline, reducing fluids. These reactions influence forearc redox budgets, mass transfers of volatile components, sequester carbon, and create strong chemical disequilibria that provide metabolic energy for chemosynthetic ecosystems. Yet, the fundamental boundary conditions within subduction systems such as reaction temperatures, the isotopic and chemical evolution of ascending fluids, and the extent to which geochemical energy is expressed in microbial metabolism, remains insufficiently quantified. This dissertation addresses these gaps through an integrated oxygen-isotope, lipid-biomarker, and trace-element investigations of serpentinization processes in modern and fossil forearcs of subduction systems. The Mariana forearc is an active intra-oceanic subduction zone where serpentinite mud volcanism provides direct and rare access to mantle wedge water–rock reactions. Research findings from the past two decades have established a relationship between trench distance and pore fluid chemistry, as well as solid inventory, reflecting progressive changes in depth and dehydration temperature of the subducting plate. This dissertation presents spatially-resolved in-situ oxygen isotope (δ18O, VSMOW) measurements on serpentinites recovered from three mud volcanoes along the forearc. Compiled with published δ18O data for serpentine and pore fluids, the dataset shows a consistent trend across the entire forearc: δ18O in serpentine and pore fluid increases with trench distance, while the serpentine–fluid isotope difference decreases. To quantify this pattern, a new serpentine–fluid oxygen isotope geothermometer (K25) is introduced that harmonizes empirical fractionation relationships at low and high temperatures. Application of the geothermometer reveals a systematic increase in serpentinization temperatures—with increasing slab depth—across the forearc. Furthermore, combining K25 with independent slab thermal constraints allows a first-order estimation of δ18O of the deep formation-fluid at the serpentinization front, which is systematically heavier than pore fluids at the summit regions of the mud volcanoes and maintains the uniform monotonic trend as observed in pore fluid δ18O compositions across the forearc. Taken together, these results provide new quantitative and spatially resolved insights into the thermal structure and fluid evolution of a shallow subduction-channel, with direct implications for molecular hydrogen formation, abiotic carbon reduction, and bioenergetics of forearc ecosystems. Building on this thermal and fluid-geochemical framework, the following part of the dissertation examines how the geochemical energy generated by serpentinization is expressed biologically in the Mariana forearc. In particular, it shows which microbial communities and metabolisms can exist in serpentinite mud volcanoes under harsh conditions of extreme alkalinity and extremely low biomass. Lipid biomarker and stable carbon isotope analyses of serpentinite mud from mud volcanoes in the Mariana forearc provide direct molecular evidence for a chemosynthesis-based biosphere operating under fringes of habitability. This work advances beyond the inference of biological potential from pore fluid chemistry: the lipid and carbon isotopic record provide a direct insight into methane cycling and the metabolic architecture of the system, including evidence of biomarker signatures indicative of sulfate-dependent anaerobic oxidation of methane. Also detected were the signatures attributable to hydrogenotrophic methanogenesis, which probably occur in the deeper regions of the serpentinite mud volcanoes. These findings are supported by bioenergetics calculations that demonstrate the thermodynamic feasibility of these metabolisms under in-situ conditions. To extend this framework back in geological past, sedimentary serpentinites from the Blue Ridge locality within the Great Valley Group, Coast Ranges, California were examined. Blue Ridge is located in close proximity to the well-known Early Cretaceous methane seep locality of Wilbur Springs and also exhibits carbonate seep inclusions in the sedimentary serpentinites. This provides an exceptional opportunity to test a long-standing hypothesis—namely, whether these deposits represent fossil analogues to the processes occurring in the forearc of Mariana subduction zone. Chromium spinel in the serpentinites indicates a high affinity to the supra-subduction zone. The fluid-mobile element contents in serpentine (Li, B, Sr, Cs, Ba) are strongly enriched compared to the peridotites of the Coast Range ophiolites and overlap with the serpentinites of the Mariana forearc, indicating serpentinization by fluids originating from the subducted plate. Analyses of whole rock and mineral compositions show elevated magnetite contents associated with predominantly Mg-rich mineral assemblages, consistent with serpentinization at relatively high temperatures. This indicates that abundant hydrogen was released during serpentinization. Associated carbonate seeps with chemosynthetic fossil assemblages represent an ancient ecosystem that was fed by these hydrogen- and methane-rich fluids. This, in turn, suggests that the biosphere of the forearc serpentinites is not a recent phenomenon, but was also active in the geological past. The Blue Ridge rocks demonstrates that mantle wedge serpentinization and associated serpentinization-related forearc fluid seepage sustained chemosynthetic ecosystems in this Mesozoic convergent margin. In summary, this dissertation establishes forearc serpentinization as an ongoing and geochemically significant process that has enabled chemosynthetic life at convergent plate boundaries over geological time scales.