P-T PATHS FROM SYROS, GREECE, AND CONSTRAINTS ON SUBDUCTION ZONE FLUID GENERATION

Understanding the P-T evolution of high pressure metamorphic rocks is important for constraining the composition and flux of fluids in subduction zones by revealing the sequence of mineral–fluid equilibria that exist during prograde and retrograde conditions. Many studies show that the island of Syros, Greece, provides a natural laboratory for investigating the evolution of such lithologies, as it preserves the interface between the subducted slab and mantle wedge. Most rock units on Syros experienced prograde dehydration and were variably rehydrated during exhumation, with previous work yielding a range of pressures (1.7-2.1GPa) and temperatures (500-550^oC) of peak metamorphism. This implies that different units of the island experienced variable peak metamorphic conditions, that differing approaches yield inconsistent results, or both.

The purpose of this work is to build detailed P-T paths for blueschist and eclogite facies rocks from Syros, using P-T pseudosections and quartz in garnet Raman barometry when applicable. Phase fractionation models constructed stepwise along these calculated P-T paths are then used to predict the volume and composition of fluid evolved during subduction, subject to uncertainties about the thermodynamic properties of high-P fluids. We constructed P-T paths for (i) a garnet bearing blueschist with abundant lawsonite pseudomorphs and (ii) a micaceous eclogite with millimeter sized garnet crystals. Results suggest that garnet first grew in the blueschist at ~ 500˚C and 1.8–2.2 GPa, with peak metamorphism at ~ 550˚C and 2.4 GPa. In the eclogite, garnets grew from ~550˚C and 2.2GPa with peak metamorphism at about 600˚C and above 2.4GPa. Models that progressively fractionate garnet and fluid during prograde evolution predict substantial fluid release with chlorite breakdown in the blueschist and small changes in the fluid H₂O/CO₂ ratio over the prograde history. In the eclogite, fluid release is dominated by the breakdown of lawsonite and amphibole. Differences between the location of thermodynamically-predicted garnet-in reactions and the P-T of initial garnet growth implied by preserved crystal compositions suggest reaction overstepping of 40-100˚C, implying that these samples may have spent a sizable part of their history removed from sample-wide equilibrium.