Abstract:
This first investigation of the Cl stable isotope composition of seafloor rocks reveals systematic variations in37Cl35Cl of the oceanic crust and its mantle source(s) relative to seawater. Chlorine isotopic measurements (expressed as δ37Cl relative to the seawater 37Cl35Cl ratio) of fresh MORB glass, hydrothermally produced amphibole-containing whole rocks and smectite veins are presented. All the samples analyzed are isotopically enriched (0.2-7.5%%) relative to seawater (0%%). Uptake of Cl into amphibole during axial hydrothermal alteration fractionates Cl relative to seawater. The isotopic compositions of MORB glasses are between 0.2 and 7.2%%. This range appears to be partially due to the assimilation of Cl from hydrothermally altered wallrock into MORB magmas. Samples which are unaffected by assimilation have δ37Cl between 3 and 7.2%%. Overall, the data imply that Cl is isotopically fractionated between the surface and mantle reservoirs of Earth. Calculations based on the degree of fractionation suggest that approximately 40% of the Cl originally in the degassed mantle now resides on Earth's surface.