Total CEC and equivalent fractions of Na, K, Mg, and Ca in the bulk sediment are listed in Table 1. Total CEC values of studied samples ranged from 2 to 59 meq/100 g. Preliminary plots of the CEC and smectite percentage showed a parallel variation through the sediment column, although the CEC-measured samples in the present study are not the same as those used for mineral quantification by XRD.
For illustrative purpose, relations of equivalent fractions vs. depth at each site are plotted in Figure 2. The equivalent fractions of Na, K, Mg, and Ca are roughly constant at the seafloor, and the average values calculated from data from 0 to 4 m below seafloor (mbsf) at nine sites, excluding Site 1032, are XNa = 0.21 ± 0.04, XK = 0.08 ± 0.01, XMg = 0.33 ± 0.09, and XCa = 0.38 ± 0.09. The values change systematically with depth at each drilling site and distance from the ridge axis, mirroring variations in pore-water composition. In general, the equivalent fractions of K and Mg decrease monotonously with increasing depth, and the degree of decrease is much more remarked with increasing distance from the ridge axis. There seems to be a mirror image between variations of Na and Ca. Distinct variations observed at Sites 1030 and 1031 are related to upwelling hydrothermal fluids from the basement.
In summary, analysis of exchangeable cation composition in bulk sediments where smectite is assumed to be the primary ion exchanger, together with analysis of pore-water composition data, may provide insight into understanding of the factors controlling the composition of circulating fluids in ridge flanks. Further discussion of this topic and the theoretical model for calculating ion selectivity coefficients from the data in Table 1 will be presented elsewhere.