Fluid Capture During Exhumation of Subducted Lithologies: A Fluid Inclusion Study from the Cycladic Blueschist Unit (Sifnos, Greece)

Fluids at the subduction interface can serve as agents of mass transfer - leaching trace and major elements from the slab and carrying them as solutes into the mantle wedge. Detailed sampling and geochemistry of devolatilization veins or mélange block-and-matrix systems have expanded our understanding of fluid-mediated mass transfer at the subduction interface. However, fluid composition and abundance is often inferred. Measurements of the chemical compositions of fluid inclusions can provide a more direct archive of fluid-related processes. We present phase equilibria modeling, combined with fluid inclusion microthermometry and textural observations, to constrain the composition and relative timing of subduction zone fluid-rock interaction during exhumation of two garnet-bearing blueschists from the northern part of the island of Sifnos, Greece. Fluids initially incorporated in mafic lithologies by low-temperature interaction with seawater ( 4 wt.% NaCl equivalent) are later trapped as fluid inclusions along fractures in quartz grains. Coupled phase equilibria modeling and textural observations suggest that fluid release resulted from the breakdown of the hydrous phases epidote and glaucophane. Homogenization temperatures of measured fluid inclusion assemblages corroborate petrographic observations. Constructed isochores cross the inferred P-T path for Sifnos exhumation at depths of 0.7-1.1 GPa ( 25-45 km depth) and at temperatures of 470-520˚C, suggesting fluid entrapment during the early stages of exhumation and cooling from peak subduction depths of 80 km. The fluid inclusions have high measured salinities ( 30-36 wt.% NaCl equivalent), likely resulting from a two-stage process that a) releases low salinity fluids upon breakdown of prograde metamorphic phases, and b) preferentially incorporates water into newly grown retrograde phases, generating a higher salinity fluid that is subsequently trapped as inclusions in quartz. This implies that fluid-rock interaction in the northern part of Sifnos occurred in a closed system, with a low water to rock mass ratio ( 0.03 to 0.05) and little to no infiltration of externally-derived fluids. This is likely due to armoring of the unit by bounding marble layers, limiting penetration of exotic fluids during burial and/or exhumation.