A total of 32 holes at five sites near 1°N, 86°W drilled on Deep Sea Drilling Project (DSDP) Leg 70 (November-December 1979) provide unique data on the origin of the hydrothermal mounds on the southern flank of the Galapagos Spreading Center. Hydrothermal sediments, primarily Mn-oxide and nontronite, are restricted to the immediate vicinity of the mounds (< 100 m) and are probably formed by the interaction of upward-percolating hydrothermal solutions with seawater and pelagic sediments above locally permeable zones of ocean crust. Mounds as high as 25 meters form in less than a few hundred thousand years, and geothermal and geochemical gradients indicate that they are actively forming today. The lack of alteration of upper basement rocks directly below the mounds and throughout the Galapagos region indicates that the source of the hydrothermal solutions is deeper in the crust.

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Ferromanganese oxide concretions from the Indian Ocean exhibit considerable mineralogical and chemical variation, often differentiated on a regional basis. Nodules richest in Mn, Ni, Cu and Zn are found in sub-equatorial basin areas, but enrichments of some of these metals also occur in nodules elsewhere, particularly where there is evidence of major biogenic contributions to their underlying sediments. Fe, CO and Pb are most enriched in concretions from sea-mounts and the mid-ocean ridge system, but also attain high values in some basin areas. Local variations in concretion composition have been found off southern Africa, and are due to both biological and continental influences. There are several major controls on the compositions of the concretions. Source variations influence composition in that Mn, Ni and Cu show evidence of diagenetic enrichment in deposits after being cycled through the biogenic component of sediments, whereas, Fe, CO and Pb appear to be supplied mainly by normal seawater. Environmental influences, including bottom water characteristics, affect the degree of dissolution of biogenic remains and this affects the sediment type and the supply of metals to the deposits and also their mineralogy, which in turn affects trace metal uptake. In the Central Indian Basin, source and environmental influences combine to produce ‘ore grade’ nodules.

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Five cores were recovered on a traverse in the Lau Basra at 18°30'S crossing a supposed active spreading center. The sediments were subjected to selective chemical leaching for Mn. F'e, Cu, Zn, and As. Accumulation rates were determined using J~C. These rates increase from west to east, reflecting the influence of volcanielastic inputs from the Tonga-Kermadec Ridge. All elements display highest non-detrital accumulation rates closest to the supposed spreading center, suggesting a hydrothermal input to the sediments there. Variable hydrothermal inputs also influence the other cores.