THE NATURE AND ORIG IN OF AUTHIGENIC SMECTITES IN SOME RECENT MARINE SEDIMENTS

Recent marine sediments: alteration of volcanic rocks and glass, low-temperature combination of biogenic silica and Fe oxyhydroxides, and direct precipitation from hydrothermat fluids. The latter two mechanisms are discussed with reference to new evidence from studies of sediments from the Bauer Deep of the equatorial eastern Pacific and the Atlantis II Deep in the Red Sea. In the Atlantis II Deep sediments, three sub-environments of smectite formation from hydro-thermal fluids are recognized. In two of them nontronites are formed, whilst in the third an Fe-poor smectite, intermediate in composition between beidellite and montmorillonite, occurs. In general, clay mineral assemblages in Recent marine sediments are largely detrital in origin and reflect the nature of soil-clay minerals formed by weathering on the adjacent continents. The distributions of the main clay mineral groups in the oceans are well-documented (Biscaye, 1965; Griffin et al., 1968; Rateev et aL, 1969; Windom, 1976). Smectites, which are the predominant clay minerals in large areas of the Pacific, Indian and South Atlantic Oceans (Fig. 1), present an important exception to the pattern of detrital supply. Although a significant proportion of the smectites are detrital, the majority form authigenically in the ocean sediments where a variety of formation mechanisms are evident. This paper presents a summary of the nature and origin of authigenic smectites in Recent marine sediments and singles out two particular environments of authigenic smectite formation which have been the subject of detailed analytical study (Cole, 1982) and which represent wo dominant sources of authigenic smectite in Recent marine sediments