Smectites diagenetic reactions

For oxygen this means a decrease in from an initial 5-valne very near 0%c (ocean water) to abont -2%o at depths aronnd 200 m (Perry et al. 1976 Lawrence and Gieskes 1981 Brnmsack et al. 1992). Even lower 5 0-values of about -4%c at depths of around 400 m have been observed by Matsumoto (1992). This decrease in 0 is mainly dne to the formation of anthigenic 0-enriched clay minerals such as smectite from alteration of basaltic material and volcanic ash. Other diagenetic reactions inclnde recrystallization of biogenic carbonates, precipitation of... 

The notion that silica is transferred from shales to sandstones during late diagenesis (Towe, 1962) provides a mechanism to link the observed depth trends of at least three (possibly four) of the major diagenetic reactions listed in Table 1 (smectite dissolution, ilhte precipitation, quartz precipitation in sandstones, and quartz pressure solution in shales). Large-scale silica transfer is to be expected as fluids flow from regions of higher to lower temperature (Ferry and Dipple, 1991). 

The first two pathways of formation will not be discussed here as they were found to be only of local/regional importance and are not eonsidered to be of major importance for early diagenetic reactions. Iron bearing clay mineral formation under high-temperatnre conditions near a hydro-thermal system of the Red Sea was studied by Bischoff (1972). A direct precipitation of an iron-rich smectite (nontronite) within the metalliferons sediments was found. This pathway of clay mineral formation was shown to occur at temperatures... 

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