Fluid Capture During Exhumation of Subducted Lithologies: A Fluid Inclusion Study from Sifnos, Greece

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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. Water (H2O) from seawater was incorporated into the mafic lithologies by low-temperature water-rock interaction to produce hydrous phases, and was subsequently released during the subduction and exhumation cycle. Coupled phase equilibria modeling and textural observations suggest that fluid release resulted from the breakdown of the hydrous phases epidote and glaucophane, and some of the fluid was trapped as fluid inclusions in quartz. Homogenization temperatures and isochores of fluid inclusions are consistent with petrographic observations that suggest fluid inclusions were trapped during exhumation. Constructed isochores cross the inferred P-T path for Sifnos exhumation at pressures of ~0.3–1.0 GPa (~10–30 km depth) and temperatures of ~440–540 °C, suggesting fluid entrapment during the late stages of exhumation and cooling from peak subduction depths of ~70 km. The fluid inclusions have salinities of ~30–35 wt% NaCl equivalent, likely resulting from a multi-stage process that involves an increase in salinity during formation of hydrous minerals during subduction, leaving high salinity fluids and/or halide salts sequestered in pores. This is followed by the partial release of H2O during subsequent breakdown of high-pressure metamorphic phases, with later incorporation of H2O into newly forming retrograde phases. Portions of this fluid that interacted with earlier-formed saline fluids are now preserved as inclusions trapped in quartz. This interpretation suggests 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 lower salinity fluids. The closed system behavior is consistent with the observation that the blueschist unit is bounded by impermeable marble layers that limited exchange with external fluids during burial and/or exhumation.