Sedimentary facies, clay mineralogy and geochemistry of the Paleogene sediments of ODP Site 105-647

Claystones immediately overlying the early Eocene age ocean-floor basalt, cored at Ocean Drilling Program (ODP) Site 647, underwent hydrothermal and thermal alterations originating from the basalt, which resulted in changes in both the mineralogical and chemical composition of the sediments. Chlorites and higher magnesium and iron concentrations were found in the lowermost sediment sequence. Upcore, changes in the bulk chemical composition of the sediments become smaller, when compensated for variations in the carbonate content originating from biogenic and authigenic components. Chlorite disappears upcore, but still only part of the swelling clay minerals have survived the thermal influence. Thirty meters above the basalt, the clay mineralogy and chemical composition become uniform throughout the Paleogene section. Iron-rich smectites (i.e., nontronitic types), totally dominate the clay mineral assemblage. Biogenic components, responsible for the dominant part of the calcite and cristobalite contents, vary in amount in the upper part, and so do the authigenic carbonate and sulfide contents. Detrital components, such as kaolinite, illite, quartz, and feldspars, make up a very small proportion of the sediment record. The nontronitic smectites are believed to be authigenic, formed by a supply of iron from the continuous formation of ocean-floor basalt in the ridge area that reacted with the detrital and biogenic silicates and alumina silicates.采自大洋钻探计划（Ocean Drilling Program, ODP）647站位的岩芯中，直接覆于始新世早期洋底玄武岩之上的黏土岩，经历了源自该玄武岩的热液与热蚀变作用，使得沉积物的矿物学与化学组成均发生改变。最下部沉积层序中检出绿泥石以及较高的镁铁含量。沿岩芯向上，在校正了生物成因与自生组分引发的碳酸盐含量变化后，沉积物的整体化学组成变化幅度逐渐减小。绿泥石随岩芯深度上移逐渐消失，但仍仅有部分膨胀黏土矿物经受住了热蚀变影响。在距玄武岩上方30米的位置，古近纪剖面内的黏土矿物组成与化学组成趋于均一，富铁蒙脱石（即绿脱石类）成为主导的黏土矿物组合。控制方解石与方英石主要含量的生物成因组分，在剖面上部的丰度存在波动，自生碳酸盐与硫化物的含量亦呈现类似变化。陆源碎屑组分（如高岭石、伊利石、石英与长石）在整个沉积记录中占比极低。研究认为，此类绿脱石类蒙脱石属于自生矿物，其形成源于洋脊区域持续生成的洋底玄武岩所释放的铁，与陆源及生物成因硅酸盐、铝硅酸盐发生反应而成。