Abstract
Hydrothermal sediments (greenish granular-nontronite, black Mn-fragments) were found in a core collected at 29°28.08′S, 131°36.81′W in the South Pacific. The Mnoxides consist of well-crystallized todorokite and birnessite. The nontronite is nearly Al-free. The Mn-oxides as well as the nontronite are characterized by a very low REE and trace-metal contents. Within the hydrothermal sediments, authigenic quartz is common. Oxygen-isotope geothermometry indicates that the quartz was formed under low-temperature (about 20°C) hydrothermal conditions. The mineralogy and chemistry of the hydrothermal sediments reflect varying Eh values of the hydrothermal fluids ranging from slightly oxidizing (Mn-oxides, quartz) to reducing (nontronite). The observed hydrothermal sediments are comparable to those described from mounds in the Galapagos Rift and similar settings on the Mid-Atlantic ridge.
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