Mineralogy, chemistry and stable isotope composition of hydrothermal altered sheeted dikes of ODP Hole 111-504B

During ODP Leg 111 Hole 504B was extended 212 m deeper into the sheeted dikes of oceanic Layer 2, for a total penetration of 1288 m within basement. Study of the mineralogy, chemistry, and stable isotopic compositions of the rocks recovered on Leg 111 has confirmed and extended the previous model for hydrothermal alteration at the site: axial greenschist hydrothermal metamorphism was followed by seawater recharge and subsequent off-axis alteration. The dikes are depleted in 18O (mean delta18O = +5.1 per mil +/- 0.6 per mil ) relative to fresh mid-ocean ridge basalt. Oxygen isotopic data on whole rocks and isolated secondary minerals indicate temperatures during axial metamorphism of 250°-350°C and water/rock ratios about one. Increasing amounts of actinolite with depth in the dike section, however, suggest that temperatures increased downward in the dikes.
Pyrite + pyrrhotite + chalcopyrite + magnetite was the stable sulfide + oxide mineral assemblage during axial alteration, but these minerals partly re-equilibrated later at temperatures less than 200°C. The dikes sampled on Leg 111 contain an average of 500 ppm sulfur, slightly lower than igneous values. The delta34S values of sulfide average 0 per mil , which indicates the presence of basaltic sulfide and incorporation of little or no seawater-derived sulfide into the rocks. These data are consistent with models for the presence of rock-dominated sulfur in deep hydrothermal fluids. The presence of anhydrite at 1176 m within basement indicates that unaltered seawater can penetrate to significant depths in the crust during recharge.

在大洋钻探计划（Ocean Drilling Program, ODP）第111航次中，科研人员将504B钻孔向洋壳第二层的席状岩墙群加深了212米，最终在基底内的总钻进穿透深度达1288米。对本航次回收岩石的矿物学、化学组成及稳定同位素特征开展的研究，验证并拓展了该点位此前提出的热液蚀变模型：轴部绿片岩相热液变质作用之后，伴随海水补给过程，继而发生轴外蚀变。与新鲜洋中脊玄武岩相比，该席状岩墙群的18O同位素发生亏损（平均δ18O = +5.1‰ ± 0.6‰）。全岩及分离得到的次生矿物的氧同位素数据显示，轴部变质作用的温度区间为250℃至350℃，水岩比约为1。但岩墙剖面内阳起石的含量随深度递增，这表明岩墙内的温度随向下深度逐渐升高。轴部蚀变阶段的稳定硫化物-氧化物矿物组合为黄铁矿+磁黄铁矿+黄铜矿+磁铁矿，但这些矿物后续在低于200℃的条件下发生了部分再平衡。本航次采集的岩墙样品的平均硫含量为500ppm，略低于火成岩硫含量基准值。硫化物的δ34S平均值为0‰，这表明岩石中的硫化物以玄武岩成因硫化物为主，几乎未混入海水来源的硫化物。上述数据与深部热液流体以岩石主导硫为主要硫源的模型相一致。在基底内1176米处发现硬石膏，说明未发生蚀变的海水可在补给过程中穿透至地壳内较深的深度。