. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography The coating on the zeolites (Fig. 3d) with a percent Co+Cu+Ni content is tran- sitional between hydrothermal and hydro- genous crust compositions; the high Fe content in this coating (18 percent) may be indicative of a hydrogenous origin. Even though no Fe-rich hydrothermal man- ganese deposits have been identified at lat 26°N, Bonatti (1975) showed their existence elsewhere. SEM photographs show the lower crust to have the texture of well-cryst
. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography The coating on the zeolites (Fig. 3d) with a percent Co+Cu+Ni content is tran- sitional between hydrothermal and hydro- genous crust compositions; the high Fe content in this coating (18 percent) may be indicative of a hydrogenous origin. Even though no Fe-rich hydrothermal man- ganese deposits have been identified at lat 26°N, Bonatti (1975) showed their existence elsewhere. SEM photographs show the lower crust to have the texture of well-crystallized birnessite (Fig. 3a); this boxwork of plates is very similar to the texture found in birnessite from widely differing environments (Swanson, 1975; Brown and others, 1971; Fewkes, 1973). In contrast, the upper crust at the same scale (Fig. 3b) shows a smooth featureless surface with no textural indication of crystallinity. The smaller scale view in Figure 3c shows the overall micro- botryoidal form of the lower crust that is similar to columnar zones described by Sorem and Foster (1972) in hydrogenous ferromanganese. DISCUSSION Clearly, from the chemical and physical description and SEM observations, the manganese oxide lower crust 75-1A24 appears to be most similar to other hydro- thermal manganese crusts; the ferro- manganese oxide upper crust 75-1A24 has strong affinities to hydrogenous ferro- manganese crusts and nodules. A compari- son of the compositions shown in Figure 2 with those of Bonatti (1975, Fig. 4) support these conclusions. The dramatic differences in iron and Cu-t-Co+Ni contents between the upper and lower crust samples taken only a few millimetres from one another suggest drastically different mechanisms of formation or different sources of fluids. Several authors (Bonatti and others, 1972; M. R. Scott and others, 1974) have noted an inverse relation between the rate of manganese crust accumulation and the content of trace metals. If data given by M.
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