Chemical composition of upper Cenozoic at DSDP Leg 70 Holes

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 IM-celadonite. 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.

Chemiscal composition of basalts from DSDP Legs 69 and 70 Holes

Chemical compositions and 1-atm. phase relations were determined for basalts drilled from Holes 501, 504A, 504B, 505, and 505B on Legs 68, 69, and 70 of the Deep Sea Drilling Project. Chemical, experimental, and petrographic data indicate that these basalts are moderately evolved (Mg' values from 0.60 to 0.70), with olivine plus Plagioclase and often clinopyroxene on the liquidus. Chemical stratigraphy was used to infer that sequential influxes of magma into a differentiating magma chamber or separate flows from different magma chambers or both had occurred. Two major types of basalt were found to be inter layered: Group M, a rarely occurring type with major element chemistry and magmaphile element abundances within the range of the majority of ocean-floor basalts (TiO2 = 1.3%, Na2O 2.5%, Zr = 103 ppm, Nb = 2.5 ppm, and Y = 31 ppm); and Group D, a highly unusual series of basalt compositions that exhibit much lower magmaphile element abundances (TiO2 = 0.75-1.2%, Na2O = 1.7-2.3%, Zr = 34-60 ppm, Nb = 0.5-1.2 ppm, and Y = 16-27 ppm). The liquidus temperatures of the Group D basalts are high (1230- 1260°C) compared with those of other ocean-floor basalts of similar Mg' values. They have high CaO/Na2O ratios (5-8) and are calculated to be in equilibrium with unusually calcic Plagioclase (An78-84). The two basalt groups cannot be related by fractionation processes. However, constant Zr/Nb ratios (>40) for the two groups suggest a single mantle source, with differences in magmaphile element abundances and other element ratios (e.g., Zr/Ti, Zr/Y, Ce/Yb) arising through sequential melting of the same source. Magmas similar to Group D, if mixed with more typical mid-ocean-ridge basalt (MORB) magmas in shallow magma chambers, could provide a source for the highly calcic Plagioclase phenocrysts that appear in more common (i.e., less depleted) phyric ocean-floor basalts.

Geochemistry at DSDP Leg 70 Holes

The hydrothermal mounds on the southern flank of the Galapagos Spreading Center are characterized by the following main features: 1) They are located over a young basement (0.5 to 0.85 m.y. of age) in a region known for its high sedimentation rate (about 5 cm/10**3 y.) because it is part of the equatorial high biological productivity zone. 2) They are located in a region with generally high heat flow (8 to 10 HFU). The highest heat-flow measurements (up to 10**3 HFU) correspond to mound peaks (Williams et al., 1979), where temperatures up to 15°C were measured during a dive of the submersible Alvin (Corliss et al., 1978). 3) They are often located on small vertical faults which displace the basement by a few meters (Lonsdale, 1977) and affect the 25- to 50-meter-thick sediment cover. Most of these characteristics have also been observed in the other three known cases of hydrothermal deposits with mineral parageneses similar to that of the Galapagos mounds. However, the case of the hydrothermal mounds south of the Galapagos Spreading Center is unique because of the unusual thickness of the hydrothermal deposits present. The mounds are composed of several, up to 4.5-meter-thick, layers of green clays which, in one case (Hole 509B), are overlain by about 1.4 meters of Mn-oxide crust. We suspect that such a large accumulation of hydrothermal products results from the "funnelling" of the hydrothermal solutions exiting from a highly permeable basement along the faults. This chapter reports a preliminary study of those green clays collected by hydraulic piston coring of the Galapagos mounds during Deep Sea Drilling Project (DSDP) Leg 70 of the D/V Glomar Challenger. Green clays have also been reported from three presently or recently active hydrothermal areas in or close to spreading centers.