(Table 1) Sulfur isotopes, total sulfur, and degree of oxidation of basalt samples from DSDP Hole 69-504B and 70-504B

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 d34S = -0.01 per mil, which is close to the assumed mantle sulfur composition (d34S = 0 per mil. 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 d34S of secondary pyrite isolated by hand-picking varies between -8.0 and +5.8 per mil; the "pyrite-sulfate" sulfur (d34S = -4.8 to +10.5 per mil), 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.对采自哥斯达黎加裂谷（Costa Rica Rift）附近504B钻孔（Hole 504B）的约150件玄武岩样品（basalt samples），开展了硫含量（sulfur content）与硫同位素组成（sulfur-isotope composition）分析。该钻孔的洋壳基底可划分为两段：上段为氧化蚀变（oxidative alteration）带（海底以下274.5~550米），下段为非氧化蚀变（nonoxidative alteration）带（海底以下550~835米），其中540~585米区间实际为过渡带。这一划分在硫含量与硫同位素组成特征中均有所体现。低温条件下海水对玄武岩的氧化蚀变作用，使得钻孔上段的硫发生贫化，其硫含量多低于600 ppm S，相较于新鲜硫饱和大洋拉斑玄武岩（oceanic tholeiites）（900~1200 ppm S）而言。钻孔下段的高硫含量源于非氧化或弱氧化条件下，溶解了火成硫化物（igneous sulfides）的流体中次生黄铁矿（secondary pyrite）的沉淀作用。原生火成硫化物的平均硫同位素组成为δ³⁴S = -0.01‰，与假定的幔源硫组成（mantle sulfur composition）δ³⁴S = 0‰十分接近。通过单独制样步骤提取的黄铁矿与硫酸盐硫，即"黄铁矿-硫酸盐硫（pyrite-sulfate sulfur）"，表明海水硫酸盐已加入至上段玄武岩中。经手选分离得到的次生黄铁矿的δ³⁴S值介于-8.0‰~+5.8‰之间；"黄铁矿-硫酸盐硫"的δ³⁴S值为-4.8‰~+10.5‰，其与分离得到的黄铁矿的同位素组成相似，成因可能为溶解了火成硫化物的流体沉淀黄铁矿所致，但热液溶液（hydrothermal solutions）搬运的硫加入的可能性亦无法排除。