(Table 1) Pore-water composition of ODP Leg 126 sites

The Ocean Drilling Program Leg 126 sites may be classified into two categories depending on the presence (Group I: Sites 787, 792, and 793) or absence (Group II: Sites 788, 790, and 791) of steep concentration gradients. Shipboard X-ray diffraction analyses of bulk sediments from Group I sites revealed the presence of a number of diagenetic minerals (some of which are incompatible), but no systematic diagenetic zonation. The results of the chemical analyses of the pore waters from Group I have been used to estimate the activities of dissolved species. Thermodynamic analyses of the composition of the pore waters and the stability of authigenic minerals (gypsum, zeolites, feldspars, smectites, chlorites, and micas) show that the pore waters are close to equilibrium with most of the observed phases. Thus, only a small perturbation of the system (substitution in minerals and fluctuations in pore-water composition, in particular, in pH and SiO2 activity) will cause any of these phases to precipitate. Therefore, one would not expect mineralogical observations to show systematic vertical zonations at these sites. It is suggested that chlorites and high-temperature zeolites are not diagenetic sensu stricto, but were eroded from volcaniclastic highs. The absence of concentration gradients at the Group II sites has been analyzed in terms of reaction kinetics, hydrothermal advection, and temperature distribution. The absence of diagenetic imprints on the pore-water concentration profiles at these sites is probably caused by the slow nucleation of silica phases.

根据陡峭浓度梯度的有无，海洋钻探计划（Ocean Drilling Program）第126航次的站位可分为两类：存在陡峭浓度梯度的为第一组（站位787、792和793），不存在的为第二组（站位788、790和791）。对第一组站位的全岩沉积物（bulk sediments）开展船载X射线衍射（X-ray diffraction）分析，结果显示存在多种成岩矿物（部分矿物间存在不相容性），但未观测到系统性的成岩分带现象。针对第一组站位孔隙水（pore water）的化学分析结果被用于估算溶解组分的活度。对孔隙水组分及自生矿物（authigenic minerals，包括石膏、沸石、长石、蒙脱石、绿泥石与云母）稳定性的热力学分析表明，孔隙水与绝大多数观测到的矿物相均处于近平衡状态。因此，该系统仅需微小扰动（矿物中的类质同象置换，以及孔隙水组分的波动——尤其是pH与二氧化硅活度的变化），即可引发上述任一矿物相的沉淀。据此可推测，在这些站位的矿物学观测中不会出现系统性的垂直分带现象。研究认为，绿泥石与高温沸石并非严格意义上的成岩成因矿物，而是从火山碎屑高地侵蚀而来的碎屑物质。针对第二组站位不存在浓度梯度这一现象，已从反应动力学、热液平流（hydrothermal advection）与温度分布三个维度展开分析。这些站位的孔隙水浓度剖面未出现成岩改造痕迹，其成因大概率为二氧化硅矿物相的成核速率过慢。