Sulfur isotopic disequilibrium and fluid-rock interaction during metamorphism of sulfidic black shales from the Waterville-Augusta area, Maine, USA
Sulfur isotope ratios of pyrite (py) and pyrrhotite (po) from regionally metamorphosed graphitic sulfidic schists and related rocks from south-central Maine, USA, were analysed using SO 2 and SF 6 techniques. There is a broad range in δ 34 S values for both pyrite and pyrrhotite at most outcrops, up to 8%. and overall the values are isotopically light, averaging ~ −27‰ for the entire data set, suggesting that the rocks have not been grossly isotopically disturbed by regional metamorphism from their inferred organic-rich sedimentary origins. At all temperatures from chlorite to sillimanite grades, sulfide analysed from veins and blebs within the schists show predominantly disequilibrium fractionations ranging from Δ 34 S py-po −3.0 to +3.5‰ "> Δ 34 S py-po −3.0to+3.5‰ , as do matrix sulfides from rocks that attained temperatures <500°C. Only at T > 500° C do matrix pyrite-pyrrhotite pairs with polygonal or aligned granoblastic microstructures approach isotopic equilibrium at millimeter to centimeter scales, suggesting that the process that favoured equilibration was recrystallization accompanying metamorphism and deformation. This disequilibrium may be a function of preferential interaction of one of the phases with an infiltrating fluid, but the lack of any systematic trends in the data, particularly with both negative and positive Δ 34 S py-po at some outcrops, does not permit ready identification of fluid sources, fluxes, or compositions. By combining published fluid fluxes for the area and a knowledge of the mass of sulfur contained in the rocks and the inferred infiltrating fluid, it appears that sulfur should have been homogenized over at least 10's to 1000's of meters, if equilibrium had been attained between rock sulfides and an infiltrating fluid of constant composition. That this did not occur was probably due to lack of equilibration between sulfides and the fluid but may also have arisen because of channelling of fluid flow along rather than across layers, or a lack of fluid infiltration through this unit.