Petrologic description and values of resistivity, CEC, and porosity of samples from ODP Hole 111-504B (Table 1)

The resistivity, porosity, and cation exchange capacity of 36 basaltic samples recovered in Hole 504B during four DSDP and ODP legs have been measured in the laboratory at room temperature and atmospheric pressure. The presence of chlorites and particularly smectites as alteration products of basalt phases is reflected by high values of cation exchange capacity (CEC). Whereas the massive units of Layers 2A and 2B are defined by high and uniform CEC values, the more fractured and altered pillows are characterized in the entire hole by even higher values of CEC and a large variability. The lowest CEC values, measured for the massive units of Layer 2C, are due to changes of basalt alteration fades with depth and the related decreasing abundance of smectites with increasing depth in the oceanic crust. The porosity and the apparent formation factor (computed from resistivity measurements made with a fluid salinity similar to that of seawater) are related by an inverse power law similar to Archie's formula, with m close to 1.0 and a as large as 9.1. A review of the literature shows that such a low m value equates to current conduction in cracks and microcracks present throughout the rock. The presence of these microstructures reflects the extensional regime under which the rock formed at the ridge axis, and they were conserved by precipitation of clay minerals attributable to intense hydrothermal circulation. The comparison of these results to similar studies of mid-ocean ridge basalt physical properties indicates that m tends to increase to values close to 2.0 with age. Such values are, in fact, similar to those found for sedimentary rocks and probably reflect an increased tortuosity of the conducting pore space with increasing age. An inverse relationship also relates CEC and apparent formation factor, indicating that surface conduction of clay minerals plays an important role during downhole electrical experiments. This provides a plausible key to the paradox of low permeability and high apparent porosity obtained from comparing the in-situ experiments conducted in Hole 504B.本数据集针对深海钻探计划（Deep Sea Drilling Project, DSDP）与大洋钻探计划（Ocean Drilling Program, ODP）四个航次在504B钻孔中采集的36块玄武岩样品，在室温与常压的实验室条件下完成了电阻率、孔隙度与阳离子交换容量（cation exchange capacity, CEC）的测试。玄武岩相蚀变产物中的绿泥石，尤其是蒙脱石的存在，会对应较高的CEC数值。2A、2B层的块状单元以高且均一的CEC值为典型特征，而整个钻孔中更为破碎且蚀变程度更高的枕状熔岩，则呈现出更高的CEC值与更大的数值离散度。2C层块状单元的CEC值最低，这源于洋壳深部玄武岩蚀变相带随深度的变化，以及蒙脱石丰度随洋壳深度增加而逐步降低的分布规律。 孔隙度与视地层因数（apparent formation factor）——该参数由与海水盐度相似的流体中测得的电阻率数据计算得到——遵循类似阿尔奇公式（Archie's formula）的逆幂律关系，其中胶结指数m接近1.0，系数a可达9.1。文献综述表明，如此低的m值对应着贯穿岩石的裂缝与微裂缝中的电流传导机制。这些微结构反映了岩石在洋脊轴部形成时所处的伸展构造环境，并通过强烈热液循环形成的黏土矿物沉淀作用得以保存。 将本研究结果与洋中脊玄武岩物理性质的同类研究对比后可知，胶结指数m会随岩石年龄增长逐步升高至接近2.0的水平。该数值实际上与沉积岩中测得的数值相近，这大概率反映了导电孔隙空间的迂曲度（tortuosity）随岩石年龄增加而增大的现象。CEC与视地层因数之间同样存在负相关关系，表明黏土矿物的表面传导在井下电学实验中发挥着重要作用。这为解释504B钻孔原位实验中出现的低渗透率与高视孔隙度这一矛盾现象提供了合理的关键依据。