Transmission electron microscopy study of conversion of smectite to illitein mudstones of the Nankai Trough: Contrast with coeval bentonites

Clay minerals in shales from cores at Site 808, Nankai Trough, have been st udied using X-ray diffraction (XRD), scanning transmission electron microsc opy (STEM), and analytical electron microscopy (AEM) to compare the rates a nd mechanisms of illitization with those of coeval bentonites, which were d escribed previously. Authigenic K-rich smectite having a high Fe content (s imilar to7 wt. %) was observed to form directly as an alteration product of volcanic glass at a depth of similar to 500 meters below seafloor (mbsf) w ith no intermediate precursor. Smectite is then largely replaced by Reichwe ite, R, (R = I) illitesmectite (I-S) and minor illite and chlorite over dep ths from similar to 550 to similar to 700 mbsf. No further mineralogical ch anges occur to the maximum depth cored, similar to 1300 m. Most smectite an d IS in shales are derived from alteration of glass, rather than being detr ital, as is usually assumed. Discrete layer sequences of smectite, I-S, or illite coexist, indicating discontinuities of the transformation from smect ite to (R = 1) I-S to illite. Authigenic Fe-rich chlorite forms concomitant ly with I-S and illite, with the source of Fe from reactant smectite. Smectite forms from glass with an intermediate precursor in coeval bentonit es at approximately the same depth as in shales, but the smectite remains l argely unchanged, with the exception of exchange of interlayer cations (K - -> Na --> Ca) in response to formation of zeolites, to the bottom of the co re. Differences in rates of illitization reflect the metastability of the c lays. Temperature, structure-state, and composition of reactant smectite ar e ruled out as determining factors that increase reaction rates here, where as differences in water/rock ratio (porosity/permeability), Si and K activi ties, and organic acid content are likely candidates.