Hydrothermal Deposits of the Galapagos Rift Zone, Leg 70: Mineralogy and Geochemistry of Major Components

Abstract

The hydrothermal deposits that we analyzed from Leg 70 are composed of ferruginous green clays and fragments of manganese-hydroxide crust.Data from X-ray diffraction, IR-spectroscopy, electron diffraction, and chemical analyses indicate that the hydrothermal green clays are composed of disordered mixed-layer phases of celadonite-nontronite.Electron diffraction shows that the parameters of the unit cells and the degree of three-dimensional ordering of mixed-layer phases with 80% celadonite interlayers are very close to Fe-micas of polymorphic modification IMceladonite. In some sections, there is a tendency for the number of celadonite layers to increase with depth.The manganese-hydroxide crust fragments are predominantly composed of todorokite (buserite). An essential feature of hydrothermal accumulation is the sharp separation of Fe and Mn. Ba/Ti and Ba/Sr ratios are typical indicators of hydrothermal deposits. Sediments composing the hydrothermal mounds were deposited from moderately heated waters, which had extracted the components from solid basalts in environments where there were considerable gradients of temperature, eH, and pH. The main masses of Fe and Mn were deposited in the late Pleistocene. Postsedimentary alteration of deposited hydrothermal sediments led to their slight recrystallization and, in the green clays, to celadonitization.Further, factor analysis (by Varentsov) of chemical components from these hydrothermal deposits revealed paragenetic assemblages. Green clays corresponding to a definite factor assemblage were formed during the main stage of hydrothermal mineral formation. Manganese hydroxide and associated components were largely accumulated during an early stage and at the end of the main stage.

Fe-Smectite-Glauconite Transition in Hydrothermal Green Clays from the Galapagos Spreading Center

“…The mounds are located in areas of high heat flow (8 to 10 heat flow units) above small, vertical basement faults (Williams et al 1974). Clay minerals in the Galapagos mounds were described by Hoffert et al (1980), Honnorez et al (1981Honnorez et al ( , 1983, Kurnosov et al (1983), and Varentsov et aL (1983) as Fe-rich smectite and Fe-rich mica, i.e., nontronite and celadonite, respectively. The origin and subsequent precipitation of these minerals from solutions exiting from a highly permeable ocean basement in the vent area was ascribed by Honnorrez eta[.…”