SYROS, GREECE: A GEOCHEMICAL WINDOW INTO SUBDUCTION ZONE PROCESSES

The oceanic lithosphere represents a significant sink for carbon and sulfur. Hydration that occurs between ridge axes and subduction zones is characterized by unique petrologic and chemical changes associated with incorporation of seawater-derived carbonate and sulfate. Thus, tracking hydration and dehydration conditions within the oceanic lithosphere is crucial to understanding these global biogeochemical budgets. Processes that occur at the ridge and in abyssal plains can be investigated by dredging or drilling of the ocean floor or by studying obducted ophiolite sequences. In contrast, the study of subduction zone hydration is more challenging due to the inaccessibility of the products. The most straightforward way of examining such material is by studying slivers of oceanic crust that were subducted to blueschist/eclogite facies and then exhumed, as in rocks from the island of Syros, Greece. Here we investigate the carbon and sulfur geochemistry of serpentinites and talc chlorite schists from Syros, aiming to (i) decipher the origin/path of metasomatic fluids and (ii) understand the broader role that subduction has on the geochemical cycles of carbon and sulfur.

Total inorganic carbon concentrations range up to 1.8 wt%. Inorganic carbon isotope data, δ¹³C_inorganic, suggests mixing of both seawater-derived carbonate and mantle-derived carbon. Sulfide contents in the studied samples are below the detection limit, but sulfate contents range up to 121 ppm. The sulfate isotope, δ³⁴S_sulfate, composition suggests that its origin is seawater of Cretaceous age. The low abundance of sulfide minerals may be indicative of oxidizing conditions that do not favor sulfide formation, possibly from the phase transition of lizardite to antigorite in serpentinites which accounts for a release of oxygen (Debret et al., 2014). Additionally, the island exhibits geochemical heterogeneity that is evident in both the contents and isotopic values of carbon and sulfur. This variability appears to be lithologically dependent and may reflect primary subducted heterogeneities, local-scale subduction zone processes, or the patchy nature of a retrograde greenschist overprint. Continued geochemical interrogation will help determine the sources of this variation in the subduction zone serpentinites and schists.