QUANTIFYING THE INFLUENCE OF LAWSONITE ON MICROSTRUCTURAL EVOLUTION AND HYDRATION PATHWAYS IN SUBDUCTED CRUST AND THE ECLOGITE-BLUESCHIST FACIES TRANSITION

Lawsonite bearing blueschists lie adjacent to eclogite on Syros, Greece, suggesting the two metamorphic facies occurred at the same P-T conditions of ca. 22 kbar and 550 °C but record different hydration histories. In one outcrop, we show that small variations in bulk composition, lead to significantly different modal abundances of garnet, quartz, and lawsonite, with higher values of Ca, Al, Na and Si in the lawsonite bearing blueschists compared to the non-lawsonite bearing eclogite. We find that increases in garnet mode does not correlate with larger garnet crystal size distributions (CDS’s). In contrast samples with matrix lawsonite contain less garnet and more quartz and have smaller garnet CSD’s. In eclogite, lawsonite occurs as small pseudomorphs preserved only as inclusions within garnet, but not the external matrix, suggesting lawsonite was present at peak pressures and was followed by breakdown to clinozoisite and paragonite during decompression. The preserved total eclogite assemblage is therefore less hydrated compared to when at peak conditions. In contrast, the blueschists with matrix lawsonite contain less or no garnet, suggesting either: i) the presence of lawsonite inhibited garnet growth during prograde metamorphism due to a consuming the components required for garnet growth or ii) lawsonite grew on the retrograde path due to the input of external water during exhumation beyond the stability of garnet. Equilibrium phase diagram modelling over a range of P-T-X(H₂O) conditions, consistently show that theoretical lawsonite stability is primarily dependant on high X(H₂O) rather than bulk composition and between 3–6% molar H₂O becomes available upon crossing the lawsonite-out reaction during the initial stages of exhumation following peak pressures. However, the adjacent nature of eclogite and lawsonite bearing blueschists suggest that the initial presence of prograde lawsonite is controlled by the small preferences in initial bulk rock composition, which acts as a sink of water causes a positive feedback controlling subsequent fluid pathways during prograde and retrograde metamorphism and influences garnet growth and CSD’s. In either case, lawsonite breakdown must hydrate the adjacent rocks, enhancing weakening, influencing strain localisation and facilitating the growth of secondary zoned amphiboles associated with smaller values of Na in the B site occupancy (Na_B) and increasing barroisite composition from core to rim, indicative of continuous fluid release during exhumation.