(Table 1) Uranium and Thorium concentrations, activity ratios and calcium carbonate content of DSDP Site 54-424

The geochemical behaviour of uranium and thorium in metalliferous sediments and hydrothermal deposits has been widely studied and the main results have been summarised by Boström and Rydell. These isotopes may be used to clarify how the metal-rich solutions are introduced into sediment cover and seawater. Using radiochemistry followed by alpha spectrometry, we have measured uranium concentrations as high as several hundred p.p.m., which must clearly be associated with ocean ridge thermal activity, in sediments interbedded between the basaltic basement and the green hydrothermal mud at DSDP Site 424. These high uranium concentrations indicate the path followed by the hydrothermal fluid which, debouching at the sediment-water interface, formed the green mud.

Composition and origin of sediments at DSDP Site 54-424

The sediments recovered on Deep Sea Drilling Project Leg 54 appear to be mixtures of the normal pelagic sediments of the area and hydrothermally produced manganese and iron phases. The latter are mineralogically and chemically very similar to phases recovered from surficial sampling of the mounds. The hydrothermal nontronite which is approximately 15 meters thick in the three holes is essentially free of carbonate or detrital contaminants. The basal sediments are similar to the carbonate oozes presently being deposited in the region, but are enriched in Mn and Fe. This enrichment appears to be the result of hydrothermal deposition that took place at or near the spreading center and may not be associated with the mounds formation. Three different hypotheses for the formation of the nontronite layer and the mounds deposits are considered. An initial deposition of a widespread nontronite layer and subsequent diapiric-like movement of the layer into carbonates could account for the observed stratigraphy; however, if this be correct, analogous deposits should be present in other DSDP sites. The second hypothesis - replacement of the normal sediments by nontronite - may be feasible, but the high purity of the nontronite requires dissolution and removal of refractory elements. The third hypothesis, metal deposition in an advancing oxidation gradient, is compatible with submersible observations of the mounds; however, it can account only for the high purity of the nontronite by very rapid deposition of the hydrothermal phases.

Marine barite in sediments of DSDP Site 54-424

The distribution of barite in sediments from D.S.D.P. sites 424 and 424A at the Galapagos hydrothermal mounds field is determined and the process of its formation is deduced. Barite in these deposits is associated with calcareous sediments and is completely absent from the hydrothermal material (manganese crusts and nontronite). Its concentrations tend to increase in the deeper sediments. Since manganese crusts contain significant amounts of Ba, a lack of barite in them is probably due to low concentrations of [SO4]2 in the sediment-seawater interface where they form. The formation of barite occurs within buried sediments, the interstitial waters of which are saturated with [SO4]2. The most probable source of [SO4]2- is the oxidation of H2S which is released from the hydrothermal fluids percolating upwards through the sediments. Although nontronite is formed within buried sediments the environmental conditions occurring during its formation (reducing) prevent barite formation. The association of barite with calcareous sediments is due to the release of Ba by calcareous microorganisms and/or to high concentrations of Ca in the pore waters which maintain a high pH and hence [SO4]2- is stable.

Minerals, geochemistry and x-ray power characteristics at DSDP Sites 54-424, 60-458 and 60-459

Secondary minerals filling veins and vesicles in volcanic basement at Deep Sea Drilling Project Sites 458 and 459 indicate that there were two stages of alteration at each site: an early oxidative, probably hydrothermal, stage and a later, low-temperature, less oxidative stage, probably contemporaneous with faulting in the tectonically active Mariana forearc region. The initial stage is most evident in Hole 459B, where low-Al, high Fe smectites and iron hydroxides formed in vesicles in pillow basalts and low-Al palygorskite formed in fractures. Iron hydroxides and celadonite formed in massive basalts next to quartz-oligoclase micrographic intergrowths. Palygorskite was found in only one sample near the top of basement in Hole 458, but it too is associated with iron hydroxides. Palygorskite has previously been reported only in marine sediments in DSDP and other occurrences. It evidently formed here as a precipitate from fluids in which Si, Mg, Fe, and even some Al were concentrated. Experimental data suggest that the solutions probably had high pH and somewhat elevated temperatures. The compositions of associated smectites resemble those in hydrothermal sediments and in basalts at the Galapagos mounds geothermal field. The second stage of alteration was large-scale replacement of basalt by dioctahedral, trioctahedral, or mixed-layer clays and phillipsite along zones of intense fracturing, especially near the bottom of Holes 458 and 459B. The basalts are commonly slickensided, and there are recemented microfault offsets in overlying sediments. Native copper occurs in one core of Hole 458, but associated smectites are dominantly dioctahedral, unlike Hole 459B, where they are mainly trioctahedral, indicating nonoxidative alteration. The alteration in both holes is more intense than at most DSDP ocean crust sites and may have been augmented by water derived from subducting ocean crust beneath the fore-arc region.