Bulk and clay mineralogy of ODP Leg 110 holes

The mineralogy of both bulk- and clay-sized fractions of samples from Sites 671, 672, and 674 of ODP Leg 110 was determined by X-ray diffraction. The major minerals include quartz, calcite, plagioclase feldspar, and the clay minerals smectite, illite, and kaolinite. The smectite is a dioctahedral montmorillonite and is derived primarily from degradation of volcanic ash. Percentage of smectite varies with sediment age; Miocene and Eocene sediments are the most smectite-rich. High smectite content tends to correlate with elevated porosity, presumably because of the ability of smectite clays to absorb significant amounts of interlayer water. Because of a change in physical properties, the decollement zone at Site 671 formed in sediments immediately subjacent to a section of smectite-rich, high-porosity, Miocene-age sediments. Sediments above the decollement at Site 671, as well as all sediments analyzed from Sites 672 and 674, contain nearly pure smectite characteristic of the alteration of volcanic ash. Within the decollement zone and underthrust sequence, however, the smectite contains up to 65% illite interlayers. Although the illite/smectite could be interpreted as detrital clay derived from South America, its absence in the sediments stratigraphically equivalent to the decollement and underthrust sequences at Sites 672 and 674 favors the interpretation that it originated by diagenetic alteration of pre-existing smectite similar to that in the overlying sediments. A significant percentage of the freshening of the pore waters observed in these zones could be due to the water released during smectite dehydration.本研究采用X射线衍射（X-ray diffraction）对大洋钻探计划（Ocean Drilling Program, ODP）第110航次671、672及674站位样品的全粒级和粘粒级分开展了矿物学分析。所检出的主要矿物包括石英、方解石、斜长石，以及蒙脱石（smectite）、伊利石（illite）和高岭石（kaolinite）等粘土矿物。其中蒙脱石为二八面体蒙脱石（dioctahedral montmorillonite），主要源自火山灰的蚀变作用。蒙脱石含量随沉积物年龄发生变化，中新统与始新统沉积物的蒙脱石丰度最高。高蒙脱石含量通常与较高的孔隙度呈正相关，这可能源于蒙脱石粘土可吸附大量层间水的特性。受物理性质变化影响，671站位的滑脱带（decollement zone）形成于富含蒙脱石、高孔隙度的中新世沉积物的紧邻下伏地层中。 671站位滑脱带之上的沉积物，以及672、674站位的全部分析沉积物，均几乎仅含蒙脱石，这是火山灰蚀变的典型特征。但在滑脱带及逆冲推覆序列（underthrust sequence）中，蒙脱石可含有高达65%的伊利石夹层。尽管该伊利石/蒙脱石组分可被解释为源自南美洲的碎屑粘土，但672、674站位中与滑脱带及逆冲推覆序列地层等效的沉积物中并未检出该组分，因此更支持其起源于上覆沉积物中原有蒙脱石的成岩蚀变作用。在上述区域观测到的孔隙水淡化现象，很大一部分可归因于蒙脱石脱水过程中释放的水。