Tracing retrograde fluid sources in serpentinite and greenschist facies rocks from Syros, Greece

Abstract:

The Cyclades are a prime locality for studying a fossil subduction zone and volatile transfer that occurs within convergent margin and back-arc settings. We have begun to constrain the composition and timing of influx for fluids that caused retrograde metamorphism in southern Syros lithologies. Most of the southern part of the island has experienced greenschist facies metamorphism, and as a reconnaissance study, we have analyzed carbonate veins from metabasalt and serpentinite lithologies. Calcite veins from serpentinites and one carbonate vein from metabasite average low δ¹³C values: δ¹³C = -8.98‰ and δ¹³C = -8.33‰, respectively. Relative to the preceding samples, other carbonate veins from metabasites [avg. δ¹³C = -0.27‰] and a dolomite vein from serpentinite (δ¹³C = -2.90‰) are enriched in ¹³C. In contrast, δ¹⁸O values are homogenous and range from +24.2‰ to +28.1‰. Carbonates with low δ¹³C values are consistent with derivation of carbon from organic matter, and have δ¹⁸O values that reflect interaction and precipitation from meteoric water. Based on an estimated mean annual value of Syros meteoric water (δ¹⁸O = -5.4‰), calcite/dolomite-water equilibrium fractionation estimate reaction temperatures of ~25-44 °C. Samples with higher δ¹³C values likely reflect C contribution from dissolved limestone, wherein CO₂-bearing fluids retain constant δ¹⁸O values because of the high abundance of water in the metamorphic fluid, but constantly change δ¹³C because of the abundant carbonate in the region that is readily dissolved. To help constrain fluid sources, we will determine oxygen isotope values of silicate minerals formed during various stages of retrogression. Petrography on thin sections will also assist in determining if various stages of retrogression exist. Ar-Ar dating of micas will be carried-out to determine the timing of fluid-assisted retrogression.