THE RATE AND MECHANISM OF DEEP-SEA GLAUCONITE FORMATION AT THE IVORY COAST-GHANA MARGINAL RIDGE

The environmental conditions and reaction paths of shallow-water glauconitization (<500 m water depth, similar to 15 degrees C) close to the sediment-seawater interface are generally considered to be well understood. In contrast, the key factors controlling deep-sea glauconite formation are still poorly constrained. In the present study, green grains formed in the recent deep-sea environment of the ODP Site 959, Ivory Coast Ghana Marginal Ridge, (similar to 2100 m water depth, 3-6 degrees C) were investigated by X-ray diffraction and electron microscopic methods in order to determine the rate and mechanism of glauconitization. Green clay authigenesis at Hole 959C occurred mainly in the tests of calcareous foraminifera which provided post-depositional conditions ideal for glauconitization. Within this organic-rich micro-environment, Fe-smectite developed <10 ky after deposition of the sediments by precipitation from precursor gels containing Fe, Mg, Al, and silica. This gel formation was supported by microbial activity and cation supply from the interstitial solution by diffusion. At a later stage of early marine diagenesis (900 ky), the Fe-smectites reacted to form mixed-layer glauconite-smectite. Further down (similar to 2500 ky), almost pure glauconite with no compositional gaps between the Fe-smectite and glauconite end members formed. This burial-related Fe-smectite-to-glauconite reaction indicates that the glauconitization process was controlled mainly by the chemistry of the interstitial solutions. The composition of the interstitial solution depends heavily on micro-environmental changes related to early diagenetic oxidation of biodegradable (marine) organic matter, microbial sulfate reduction, silicate mineral alteration, carbonate dissolution, and Fe redox reactions. The availability of Fe is suggested as the probable limiting factor for glauconitization, explaining the various states of green-grain maturity within the samples, and this cation may be the most important rate-determining element. The rate of glauconite formation at ODP Site 959 is given by %Gl(Sed) = 22.6.log(age(Sed)) + 1.6 (R-2 = 0.97) where %Gl(Sed) is the state of glauconitization in the sediment and age(Sed) is the sediment age (in ky). This glauconitization rate depends mainly on continuous cation supply (in particular Fe) and is about five times less than that in shallow-shelf regions, suggesting significantly slower reaction at the lower temperature of deep-sea environments.