Along-dip variations of subduction fluids: The 30–80 km depth traverse of the Schistes Lustrés complex (Queyras-Monviso, W. Alps)

The Queyras-Monviso traverse of the Schistes Lustrés complex, is a stack underplated nappes oceanic rocks subducted from blueschist- to eclogite-facies conditions. This study reports on salinities and gas contents primary fluid inclusions trapped in high pressure veins metasediments metamafic (445 22 samples). These data provide snapshots compositions fluids present at peak burial conditions, varying ~30 80 km depth illustrate evolution composition with along cold subduction zone. Fluid lawsonite- carpholite-bearing contain moderately saline aqueous (average salinity 4.6 wt% NaCl eq.) subordinate amounts CO2 CH4 vapor phase. observed decrease increasing grade interpreted as reflecting progressive dilution initial seawater-like pore by low-salinity released locally dehydration reactions. Less-frequently measured higher uppermost metasedimentary-dominated tectonic unit suggest brine infiltration embedded blocks margin units containing evaporites. (and scarce potential hydrocarbons) appear be interaction carbonates carbonaceous matter-rich pelitic horizons, respectively. omphacite metagabbros record (mean about 17 small N2 eclogitic only, variety minerals (calcite, white mica, salts) indicative complex chemical systems. are partly inherited seafloor high-temperature hydrothermal alteration gabbros, resulting phase separation formation. Progressive breakdown Cl-rich amphibole glaucophane (blueschist-facies) then (eclogite-facies) release brines could account for fluids. Therefore, metagabbros, Cl grade. signatures metamafics characteristic each rock type. Local redox conditions were preserved within units, possibly due restricted transient circulation (at hm-scale most). Alpine show comparable other fragments lithosphere worldwide, whereas than recorded elsewhere, either (1) higher-temperature formation (compared metavolcanics) or (2) more sediment-derived compared block-in-mélange complexes.