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About 150 basalt samples from Hole 504B, near the Costa Rica Rift were analyzed for sulfur content and sulfur-isotope composition. The basement in Hole 504B can be divided into an upper part, which has oxidative alteration (274.5-550 m below sea floor), and a lower part, which has nonoxidative alteration (550-835 m below sea floor) (the interval from 540 to 585 meters actually is transitional). This division is reflected in both the sulfur content and the sulfurisotope composition. Oxidative alteration of basalts by sea water at low temperatures has resulted in a depletion in sulfur in the upper part of the hole (mostly less than 600 ppm S) as compared to fresh sulfur-saturated oceanic tholeiites (900-1200 ppm S). High amounts of sulfur in the lower part of the hole are a result of precipitation of secondary pyrite under non-oxidative or weakly oxidative conditions from solutions which dissolved igneous sulfides. The average sulfur-isotope composition of the primary igneous sulfides is δS = -0.01%0, which is close to the assumed mantle sulfur composition (δS = 0%0. Pyrite and sulfate sulfur extracted together in a separate preparation step (as "pyrite-sulfate" sulfur) indicate addition of sea-water sulfate to the upper part of the basalts. The δS of secondary pyrite isolated by hand-picking varies between -8.0 and +5.8%O; the "pyrite-sulfate" sulfur (δ S -4.8 to +10.5%0), as well as that of the isolated pyrite, may have originated in the precipitation of pyrite from solutions containing sulfur from the dissolution of igneous sulfides, but addition of sulfur transported by hydrothermal solutions cannot be excluded.

Sulfur Content and Sulfur Isotopes of Basalts from the Costa Rica Rift (Hole 504B, Deep Sea Drilling Project Legs 69 and 70)