Chemical composition of clay minerals from ODP Leg 168 sites@en

The exchangeable cation compositions of organic-poor terrigenous sediments containing smectite as primary ion exchanger from a series of holes along ODP Leg 168 transect on the eastern flank of the Juan de Fuca Ridge have been examined as a function of distance from the ridge axis and burial depth. The total cation exchange capacity (CEC) values of the sediments ranged from 2 to 59 meq/100 g, increasing with increases in the wt.% smectite. At the seafloor, the exchangeable cation compositions involving Na, K, Mg, and Ca, expressed in terms of equivalent fraction, are nearly constant regardless of the different transect sites: XNa = 0.21 ± 0.04, XK = 0.08 ± 0.01, XMg = 0.33 ± 0.09, and XCa = 0.38 ± 0.09. The calculated selectivity coefficients of the corresponding quaternary exchange reactions, calculated using porewater data, are in log units -5.45 ± 0.39 for Na, 1.97 ± 0.49 for K, 0.42 ± 0.41 for Mg, and 3.06 ± 0.69 for Ca. The exchangeable cation compositions below the seafloor change systematically with distance from the ridge crest and burial depth, conforming to the trends of the same cations in the porewaters. The selectivities for Na and Mg are roughly constant at temperatures from 2 to 66°C, indicating that the equivalent fractions of these two cations are independent of sediment alteration taking place on the ridge flank. Unlike Na and Mg, the temperature influence is significant for K and Ca, with Ca-selectivity decreases being coupled with increases in K-selectivity. Although potentially related to diagenetic and/or hydrothermal mineral precipitation or recrystallization, no evidence of such alteration was detected by XRD and TEM. In sites where upwelling of hydrothermal fluids from basement is occurring, the K-selectivity of the sediment is appreciably higher than at the other sites and corresponds to the formation of (Fe, Mg) rich smectite and zeolites. Our study indicates that local increases in K-selectivity at hydrothermal sites are caused by the formation of these authigenic minerals.本研究针对胡安德富卡海岭（Juan de Fuca Ridge）东侧翼沿大洋钻探计划（Ocean Drilling Program, ODP）168航次断面的多个钻井站位，采集了以蒙脱石（smectite）为主要离子交换剂的贫有机质陆源沉积物，分析了其可交换阳离子组成随距海岭轴部距离及埋藏深度的变化规律。沉积物的总阳离子交换容量（cation exchange capacity, CEC）介于2~59 meq/100 g之间，且随蒙脱石质量分数的升高而增大。在海底表层，以当量分数表示的Na、K、Mg、Ca可交换阳离子组成几乎不受断面站位差异的影响，具体为：XNa=0.21±0.04、XK=0.08±0.01、XMg=0.33±0.09、XCa=0.38±0.09。基于孔隙水（porewater）数据计算得到的四元交换反应对应选择性系数，以对数单位计分别为：Na的-5.45±0.39、K的1.97±0.49、Mg的0.42±0.41以及Ca的3.06±0.69。海底以下沉积物的可交换阳离子组成随距海岭脊部的距离及埋藏深度呈现系统性变化，且与孔隙水中对应阳离子的变化趋势一致。在2~66℃的温度区间内，Na和Mg的选择性系数大致保持恒定，表明这两种阳离子的当量分数不受海岭翼部沉积物蚀变过程的影响。与Na和Mg不同，温度对K和Ca的选择性影响显著：Ca的选择性系数随K选择性系数的升高而降低。尽管该现象可能与成岩作用及/或热液矿物沉淀或重结晶有关，但X射线衍射（X-ray Diffraction, XRD）与透射电子显微镜（Transmission Electron Microscopy, TEM）并未检测到此类蚀变的证据。在发生基底热液流体上涌的站位，沉积物的K选择性系数显著高于其他站位，且对应富(Fe,Mg)蒙脱石与沸石的形成。本研究表明，热液站位处K选择性系数的局部升高，正是由这些自生矿物（authigenic minerals）的形成所导致。