The evolution of stable S-isotopes in poly-metamorphosed sulfide deposits from the Italian Western Alps

Devolatilization reactions in subducting slabs produce H2O- and CO2- dominated fluids which take part in metamorphic reactions and enhance element mobility. Although sulfur is a minor component of subduction-related fluids, it can play an important role in mobilizing normally un-reactive calcophile and siderophile elements (potentially to be released into mantle wedge) and in taking part in oxidation-reduction reactions. Evidence of fluid-rock interaction, mineral replacements and/or re-crystallization during metamorphism can be recorded by sulfides and give insights on the mobilization of sulfur and metals in subduction related processes. On this basis, mineralized samples from two ocean floor-related sulfide deposits (Servette and Beth-Ghinivert in the Italian Western Alps), with different HP to greenschists facies poly-metamorphic evolutions, were investigated with a detailed micro-textural and isotopic study by means of in situ analyses of S-isotopes ratios. In situ δ34S values within individual pyrite and chalcopyrite grains were interpreted combining micro-textural, mineralogical and geochemical analyses which allowed to assess the effectiveness of metamorphism in modifying the isotopic record. Evidence of fractionation of stable sulfur isotopes between sulfides and fluids was utilized as a tool to identify fluid-rock interaction processes and mobilization of sulfur during metamorphism. Ocean-floor related features are well preserved at Beth-Ghinivert even at the micro-scale and very little metamorphic overprint is recorded by the sulfides mineralization. At Servette evidence of metamorphic textures is widespread and sub-millimeter scale modifications of δ34S values occur, indicating interactions with an infiltrating hydrothermal fluid. However, none of the deposits record diffusive metamorphic re-equilibration at the deposit scale and largely preserve isotopic records and mineralogical features ascribable to ocean-floor metasomatism. The preservation of pre-Alpine isotopic features is here interpreted as a consequence of negligible fluid/rock interaction (i.e. limited fluid release) and negligible sulfur release during subduction-exhumation metamorphism. As a consequence, the contribution of sulfur to the mobility of chalcophile and siderophile elements is minimized to the conditions experienced by the two deposits. This study evidences the key-role of the amount of circulating fluids and of deformation in determining the isotopic evolution of subducted sulfide deposits and in favoring sulfur release during metamorphism.