CHLORINE STABLE-ISOTOPE COMPOSITION OF THE OCEANIC-CRUST - IMPLICATIONS FOR EARTHS DISTRIBUTION OF CHLORINE

This first investigation of the Cl stable isotope composition of seafl oor rocks reveals systematic variations in Cl-37/Cl-35 of the oceanic crust and its mantle source(s) relative to seawater. Chlorine isotopic measurements (expressed as delta(37)Cl relative to the seawater Cl-37 /Cl-35 ratio) of fresh MORB glass, hydrothermally produced amphibole-c ontaining whole rocks and smectite veins are presented. All the sample s analyzed are isotopically enriched (0.2-7.5 parts per thousand) rela tive to seawater (0 parts per thousand). Uptake of Cl into amphibole d uring axial hydrothermal alteration fractionates Cl relative to seawat er. The isotopic compositions of MORB glasses are between 0.2 and 7.2 parts per thousand. This range appears to be partially due to the assi milation of Cl from hydrothermally altered wallrock into MORB magmas. Samples which are unaffected by assimilation have delta(37)Cl between 3 and 7.2 parts per thousand. Overall, the data imply that Cl is isoto pically fractionated between the surface and mantle reservoirs of Eart h. Calculations based on the degree of fractionation suggest that appr oximately 40% of the Cl originally in the degassed mantle now resides on Earth's surface.