Evolution of fluid compositions, migration scales, and redox state across an entire subduction complex

Fluids released in subduction zones play a key role in Earth dynamics, yet their compositions and migration scales remain poorly constrained. Here we present the first investigation of primary fluid inclusions across an entire exhumed subduction complex. These inclusions, trapped in high-pressure minerals, preserve small portions of subduction fluids likely unaffected by post-entrapment modifications. Distinct fluid compositions across lithologies point to a dominant closed-system behavior, with fluid transfer at a restricted scale. In blueschist-facies metasediments, a progressive salinity decrease from low to high grade is interpreted as reflecting a dilution of the rock chlorine content by local dehydration reactions. A suspected across-grade evolution of CO 2 CH 4 ratios in metasediment gas mixtures indicates decreasing f O 2 conditions relative to the fayalite-magnetite-quartz buffer, consistent with an internally graphite-carbonate buffered system. In contrast, metamafic rocks preserve high salinity fluids inherited from oceanic hydrothermal alteration, only slightly diluted by devolatilization reactions during burial. Major dehydration reactions occurring at the blueschist- to eclogite-facies transition in metamafic rocks release large volumes of saline fluids, and are likely responsible for the relatively high salinity values observed in the surrounding eclogitic metasediments. Locally, external fluid input is also detected in metasediments near evaporite-rich margin units. Overall, our data reveal restricted fluid circulation (up to the hm-scale) in the downgoing plate, except in zones that localized strain, where subsequent km-scale fluid channelization occurs.