Stable-Isotope Evidence for the Origin of Secondary Carbonate Veins in Deep Sea Drilling Project Leg 58 Basalts

The stable-isotope composition of carbonate minerals is a function of the temperature and isotopic composition of the materials from which they were precipitated or recrystallized. Because carbonates are among the most abundant secondary phases in oceanic volcanic rocks, information derived from their isotopic composition is useful in determining the environment(s) of seafloor alteration. Isotopic analyses of secondary carbonates in basalt recovered from numerous DSDP sites have been reported previously (Anderson and Lawrence, 1976; Brenneke, 1977; Lawrence et al., 1977; Seyfried et al., 1976; among others). These results are consistent with the formation of most secondary carbonates with sea water at low temperatures. The good recovery of basalts during DSDP Leg 58 provided the opportunity to extend the isotopic study of low-temperature alteration and vein formation to the crust of marginal ocean basins. The evidence for complex off-ridge volcanism and intrusive emplacement encountered at Leg 58 sites (Klein et al., 1978) suggested that modes of alteration at these sites might differ from those previously observed and described. We have isotopically analyzed carbonates in veins from 17 samples of volcanic rocks from Leg 58. These samples are representative of most of the common types of volcanic rocks encountered in Leg 58 (sills, subsediment flows, pillow lavas, etc.). Thin sections of each sample were examined and described to determine the extent to which petrographic characteristics of the vein material and adjacent host rock are correlated to the isotopic data. Oxygen-isotope analyses of several quartz and smectite samples from the veins were provided by R. Houghton.