Lawsonite and Garnet Oxygen Isotope Record of Fluid-Rock Interaction During Subduction

During the subduction of an oceanic plate, fluids are released from metabasaltic crust, metasediment, and serpentinite under high-pressure/low-temperature conditions. Although some fluids may eventually leave the slab, some participate in metamorphic reactions within the slab during subduction and exhumation. To identify fluid sources and other controls influencing mineral composition, we report the in situ-measured delta O-18 of lawsonite and garnet in blueschist- to eclogite-facies rocks from 10 subduction zones that represent various field settings, including melanges, structurally coherent terranes, and an eclogite xenolith derived from a subducted plate. Lawsonite records distinct delta O-18 depending on the host rock type and other rock types that were fluid sources during lawsonite growth. In general, lawsonite in metabasalt (7.6 +/- 0.2-14.8 +/- 1.1 parts per thousand ) is isotopically lighter than in metasediment (20.6 +/- 1.4-24.1 +/- 1.3 parts per thousand ) but heavier than in metagabbro (4.0 +/- 0.4-7.9 +/- 0.3 parts per thousand ). The extent of delta O-18 fractionation was evaluated for lawsonite-fluid and lawsonite-garnet pairs as a function of temperature (T). Results demonstrate that variations of >1.7 parts per thousand in lawsonite and >0.9 parts per thousand in garnet are not related to changing T. More likely, the relative contributions of fluids derived from isotopically heavier lithologies (e.g., sediments) versus lighter lithologies (e.g., ultramafic rocks) are the major control. Monte Carlo simulations were performed to investigate the sources of metasomatic fluids and the water/rock ratio that formed lawsonite-bearing metasomatite. Results indicate that delta(18)OLws and delta(18)OGrt record interactions with fluids sourced from diverse lithologies (sediment, serpentinite), further supporting that delta O-18(Lws) is a useful indicator of subduction fluid-rock interactions.