(Table 1) Sulfur and oxygen isotopes from two anhydrite samples recovered from DSDP Hole 70-504B

The first anhydrite reported from oceanic basalts occurs in altered basalts drilled during DSDP Leg 70 from Hole 504B. Anhydrite has been identified in several samples, two of which were studied in detail. Anhydrite in Sample 504B-40-3 (130-135 cm), which was acquired at 310 meters sub-basement, occurs in a dolerite at the center of a vug rimmed by saponite and calcite. Red iron-hydroxide-rich alteration halos occur from 0 to 310 meters sub-basement; primary sulfides in these halos are oxidized, and the rocks have lost large amounts of sulfur. The anhydrite in this sample has a d34S value of 18.5 per mil, and it is interpreted to have formed from a fluid containing a mixture of seawater sulfate (20.9 per mil) and basaltic sulfur (0 per mil) released during the oxidation of primary sulfides. Anhydrite in Sample 504B-48-3 (14-18 cm), which was found at 376 meters sub-basement, occurs intergrown with gyrolite at the center of a 1-cm-wide vein that is rimmed by saponite and quartz. At sub-basement depths below 310 meters to the bottom of the Leg 70 section (562 m sub-basement), the rocks exhibit the effects of anoxic alteration with common secondary pyrite. Anhydrite in Sample 504B-48-3 (14-18 cm) has a d34S value of 36.7 per mil, and it is interpreted to have formed from seawater-derived fluids enriched in 34S through sulfate reduction. Temperatures of alteration calculated from oxygen isotope data range from 60 to 100°C. Sulfate reduction may have occurred in situ, or elsewhere at higher temperature, possibly deeper in the crust. The secondary mineral paragenetic sequence indicates a progressive decrease in Mg and increase in Ca in the circulating fluids. This eventually led to anhydrite formation late in the alteration process.本次报道的首例产于大洋玄武岩中的硬石膏（anhydrite），发现于深海钻探计划（Deep Sea Drilling Project，DSDP）第70航次504B钻孔所钻取的蚀变玄武岩中。目前已在多个样品中鉴定出硬石膏，其中两件样品得到了详细研究。 采自基底以下310米深度的504B-40-3（130-135 cm）样品中的硬石膏，赋存于以皂石（saponite）和方解石（calcite）为边壁的溶蚀孔洞中心的辉绿岩（dolerite）内。在0至310米基底深度范围内，发育富红色氢氧化铁（iron-hydroxide）的蚀变晕带；该带内的原生硫化物已发生氧化，围岩损失了大量硫元素。该样品中的硬石膏δ³⁴S值为18.5‰，其被认为由混合流体形成：该流体包含千分比为20.9的海水硫酸盐（seawater sulfate）与原生硫化物氧化过程中释放的千分比为0的玄武岩来源硫。 采自基底以下376米深度的504B-48-3（14-18 cm）样品中的硬石膏，与水硅钙石（gyrolite）共生，赋存于宽1厘米、以皂石和石英（quartz）为边壁的矿脉中心。在基底以下310米至第70航次岩心段底部（基底以下562米）的深度范围内，围岩呈现缺氧蚀变（anoxic alteration）特征，普遍发育次生黄铁矿（pyrite）。该样品中的硬石膏δ³⁴S值为36.7‰，其被认为由经硫酸盐还原（sulfate reduction）作用而富集³⁴S的海水源流体形成。通过氧同位素（oxygen isotope）数据计算得到的蚀变温度区间为60至100℃。硫酸盐还原作用既可能发生在原位，也可能在地壳更深部的更高温环境中发生。 次生矿物共生序列（paragenetic sequence）表明，循环流体中的镁元素含量逐渐降低、钙元素含量逐渐升高，这一过程最终使得硬石膏在蚀变作用晚期形成。