ALBERT

Description: Sediments from near the basement of a number of Deep Sea Drilling Project (DSDP) sites, from the Bauer Deep, and from the East Pacific Rise have unusually high transition metal-to-aluminum ratios. Similarities in the chemical, isotopic, and mineralogical compositions of these deposits point to a common origin. All the sediments studied have rare-earth-element (REE) patterns strongly resembling the pattern of sea water, implying either that the REE's were coprecipitated with ferromanganese hydroxyoxides (hydroxyoxides denote a mixture of unspecified hydrated oxides and hydroxides), or that they are incorporated in small concentrations of phosphatic fish debris found in all samples. Oxygen isotopic data indicate that the metalliferous sediments are in isotopic equilibrium with sea water and are composed of varying mixtures of two end-member phases with different oxygen isotopic compositions: an iron-manganese hydroxyoxide and an iron-rich montmorillonite. A low-temperature origin for the sediments is supported by mineralogical analyses by x-ray diffraction which show that goethite, iron-rich montmorillonite, and various manganese hydroxyoxides are the dominant phases present. Sr87/Sr86 ratios for the DSDP sediments are indistinguishable from the Sr87/Sr86 ratio in modern sea water. Since these sediments were formed 30 to 90 m.y. ago, when sea water had a lower Sr87/Sr86 value, the strontium in the poorly crystalline hydroxyoxides must be exchanging with interstitial water in open contact with sea water. In contrast, uranium isotopic data indicate that the metalliferous sediments have formed a closed system for this element. The sulfur isotopic compositions suggest that sea-water sulfur dominates these sediments with little or no contribution of magmatic or bacteriologically reduced sulfur. In contrast, ratios of lead isotopes in the metalliferous deposits resemble values for oceanic tholeiite basalt, but are quite different from ratios found in authigenic marine manganese nodules. Thus, lead in the metalliferous sediments appears to be of magmatic origin. The combined mineralogical, isotopic, and chemical data for these sediments suggest that they formed from hydrothermal solutions generated by the interaction of sea water with newly formed basalt crust at mid-ocean ridges. The crystallization of solid phases took place at low temperatures and was strongly influenced by sea water, which was the source for some of the elements found in the sediments.

Description: Ranging in height from gentle hills of less than a meter to steep-sided giants of more than 20 m, the mounds of the Galapagos Rift are spectacular hydrothermal features. Their internal temperatures have been measured at up to 13°C above the bottom water temperature, and total heat flow (conducted plus convected) can be several hundred to several thousand times the normal oceanic values. Fluids, when they discharge from the mound, do so at a very slow rate and at temperatures probably quite near the bottom water temperature. The mounds are principally composed of iron silicates intermixed and incrusted with lesser amounts of manganese oxides. They are generally found in rows, in a uniformly sedimented area above faults or fractures in the crustal rocks which permit fluids to escape from a deep hydrothermal aquifer. The sediment blanket in some way alters the chemistry of the ascending thermal fluids and leads to the development of mounds. The mounds field, covering an area of at least 200 km2 and consisting of thousands of individual mounds, is probably less than 300,000 years old; and many of the mounds may be only a few tens of thousands of years old or less.

Description: The samples analysed were dredged on Pleiades Leg II during a program of heat flow measurements, bottom photography and coring. They are located on topographic mounds in a band 20 km south of the Galapagos Rift on sediments ranging from 20 to 30 m in thickness. The dredge hauls were sampled to represent the diversity of dredged material either Mn encrusted altered basalt or relatively thick Mn coating deposited directly in contact with sediments. Each sample was analysed by atomic absorption spectrophotometry for major elemnts and instrumental neutron activation analysis for minor elements or rare earths.

Description: Using radiochemical methods, the authors have measured the distribution of uranium, thorium, radium, polonium and protactinium isotopes from the top and bottom sides of the a manganese nodule from Station 20 of the PLEIADES Mn7601 campaign. This "apple" shaped nodule (6.5x3.0 cm) was laying on a bed of siliceous ooze (see illustrations). Successive thin layers of the nodule were carefully scraped from the surface of the measured area with a sharp surgical blade. All samples reduced to powder were air-dried at 110°C for at least two hours before analysis. The relative gross alpha activities from each layer were assayed by Alpha spectrometry using a thick-source counting in a scintillation counter.

Description: The samples used for this study are a subset from almost 200 surface samples from the western Nazca plate analyzed during the IDOE Nazca Plate Project. The subset was selected to provide coverage of the major physiographic units north of the Sala y Gomez Ridge and to encompass the major sediment types defined by factor analysis of selected major- and minor-element concentrations. The chemical data used in this study consist of bulk-sediment analyses by atomic absorption spectrometry. Samples were dissolved in Teflon-lined bombs using HF, HNO3, and HC1 heated to 125°C. H3BO3 was added to neutralize the HF and to dissolve insoluble fluorides. The precision of these analyses varies from less than 1% for the major elements and trace elements with concentrations greater than 200 ppm to approximately 20% for trace elements in the CaCO3-rich samples. For Fe,Cu,Ni and Zn the leached and residual parts listed in the original table have been reconciled.