Subduction zone fluxes of halogens and noble gases in seafloor and forearc serpentinites

Serpentinites form by hydration of ultramafic lithologies in a range of seafloor and shallow subduction zone settings. Serpentinites are recognised as major reservoirs of fluid mobile elements and H2O in subducting oceanic lithosphere, and together with forearc serpentinites formed in the mantle wedge, provide critical information about shallow-level volatile fluxes during subduction. The current study provides new Cl, as well as the first comprehensive Br, I and noble gas analyses reported for seafloor and forearc chrysotile-lizardite serpentinites. The samples were recovered from IODP drilling campaigns of mid-ocean ridge, passive margin and forearc settings (n=17), and ophiolites in the Italian Alps and Apennines (n=10). The aims of this study were to determine the compositional variability of noble gases and halogens in serpentinites entering subduction zones and evaluate the efficiency of gas loss during the early stages of serpentinite subduction.The chrysotile-lizardite serpentinites and serpentised peridotites contain 43-2300ppm Cl and 3×10-13-2×10-11molg-1 36Ar, with the concentrations of these elements broadly related to the estimated degree of serpentinisation. The serpentinites have extremely variable Br/Cl and I/Cl ratios with many samples preserving compositions similar to organic-rich sedimentary marine pore fluids. Serpentinites from the Marianas Forearc have very high I concentrations of up to 45ppm I and I/Cl ratios of ~14,000 times the seawater value that is even higher than the maximum I/Cl enrichment observed in sedimentary marine pore fluids. The serpentinites have 130Xe/36Ar and 84Kr/36Ar ratios that are mostly close to or above seawater values, and 20Ne/36Ar ratios that range from seawater to lower values. The serpentinites contain <10-270ppm K and, irrespective of age (0Ma to ~160Ma), are characterised by 40Ar/36Ar ratios of 300-340 that are slightly higher than the seawater value of 296, thus indicating the presence of minor excess 40Ar*. Three of six serpentinites analysed for helium also have measurable excess 4He contents that cannot be explained by in situ production. The data show that serpentinites trap noble gases and halogens that originate from seawater, organic matter and diverse crustal lithologies.Combined with previous analyses of metamorphosed serpentinites, the new data suggest that approximately 60-70% of the 36Ar entering subduction zones in serpentinites is lost from chrysotile and/or antigorite and could potentially escape through the forearc. An additional, ~20-30% of the 36Ar entering subduction zones in serpentinites is lost during antigorite breakdown and may be cycled through the arc or back-arc, and ~1-10% of the 36Ar entering subduction zones in serpentinites may be subducted into the deeper mantle. The data demonstrate decoupling of noble gases, halogens and water during subduction and suggest that subduction-zone fluid fluxes can produce especially high concentrations of noble gases and iodine in newly formed forearc serpentinites. The distinctive I/Cl enrichment of forearc serpentinites suggest that halogen abundance ratios provide a plausible means for inferring the geotectonic setting of serpentinisation in ophiolite samples. The exceptional Cl, Br, I and noble gas concentrations of serpentinites, the potential subduction of the forearc serpentinites and the stability of serpentine minerals to mantle depths of >200km, imply that serpentinites could dominate the deep recycling budgets of both the heavy halogens and atmospheric noble gases.