Mineralogy and geochemistry of sulfide minerals and bulk rocks from ODP Hole 176-735B

Ocean Drilling Program (ODP) Leg 176 built upon the work of ODP Leg 118 wherein the 500-m section that was sampled represented the most complete recovery of an intact portion of lower oceanic crust ever described. During Leg 176, we deepened Hole 735B to >1500 m below seafloor in an environment where gabbroic rocks have been tectonically exposed at the Southwest Indian Ridge. This new expedition extended the remarkable recovery (>85%) that allowed unprecedented investigations into the nature of the lower oceanic crust as a result of Leg 118. Sulfide mineral and bulk rock compositions were determined from samples in the 1000-m section of oceanic gabbros recovered during Leg 176. The sulfide assemblage of pyrrhotite, chalcopyrite, pentlandite, and troilite is present throughout this section, as it is throughout the 500-m gabbroic section above that was sampled during Leg 118. Troilite is commonly present as lamellae, and the only interval where troilite was not observed is from the uppermost 150 m of the section sampled during Leg 118, which is intensely metamorphosed. The common presence of troilite indicates that much of the sulfide assemblage from Hole 735B precipitated from a magmatic system and subsequently underwent low-temperature reequilibration. Evaluation of geochemical trends in bulk rock and sulfides indicates that the combined effects of olivine accumulation in troctolites and high pentlandite to pyrrhotite ratios account for the sporadic bulk rock compositions high in Ni. Bulk rock and sulfide mineral geochemical indicators that are spatially coincident with structural and physical properties anomalies indicate a heretofore unrecognized lithologic unit boundary in this section. Platinum-group element (PGE) compositions were also determined for 36 samples from throughout the section that were recovered during Leg 176. Whereas most samples had low (<0.4 ppb) PGE concentrations, rare samples had elevated PGE values, but no unique common trend between these samples is evident.

大洋钻探计划（Ocean Drilling Program, ODP）第176航次的研究基于第118航次的工作成果。第118航次所采集的500米岩芯剖面，是迄今已报道的保存最为完整的下部洋壳完整采样段。第176航次期间，研究团队在西南印度洋中脊（Southwest Indian Ridge）的辉长岩构造出露区域，将735B钻孔加深至海底以下超过1500米处。本次航次延续了第118航次取得的超高岩芯回收率（>85%），为开展前所未有的下部洋壳性质研究提供了支撑。研究人员对第176航次回收的1000米洋壳辉长岩岩芯段样品，开展了硫化物矿物与全岩成分分析。该剖面的硫化物组合以磁黄铁矿、黄铜矿、镍黄铁矿（pentlandite）与陨硫铁（troilite）为主，与第118航次采集的上部500米辉长岩剖面的硫化物组合特征一致。陨硫铁通常以页片状产出，唯一未观测到陨硫铁的层位，是第118航次采集的剖面最上部150米——该层段经历了强烈变质作用。陨硫铁的普遍产出表明，735B钻孔中的大部分硫化物组合均形成于岩浆成因体系，随后经历了低温平衡改造。对全岩与硫化物的地球化学趋势分析显示，橄长岩中橄榄石堆积作用以及高镍黄铁矿/磁黄铁矿比值的共同影响，解释了部分镍含量偏高的异常全岩成分。与构造结构与物理性质异常区域空间重合的全岩及硫化物矿物地球化学指标，揭示了该剖面中此前未被识别的岩性单元边界。研究人员还对第176航次回收的全剖面36件样品，开展了铂族元素（Platinum-Group Element, PGE）组成分析。多数样品的铂族元素浓度较低（<0.4 ppb），仅少数样品呈现铂族元素含量升高的特征，但未发现这些样品间存在统一的共性变化趋势。