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Manganese crusts recovered from sediment sections at Ocean Drilling Programme (ODP) Sites 835 and 838 in the Lau Basin have been studied mineralogically and by bulk and partition geochemical techniques. Results show they are of hydrothermal rather than hydrogenous origin and are composed principally of stable 10 Å-manganite. The stratigraphic location of the crusts in the sediment sections would suggest they range between 0.2 and 3.5 million years old. The crusts were most likely formed at the sediment water interface before being incorporated into the sediment section by slumping and subsequent burial. Basal Site 835 crusts were most likely formed at or close to the Eastern Lau Spreading Center (ELSC), and those in the younger part of the sediment section were probably formed at a site of ''off-axis'' volcanism. Site 838 crusts were most likely formed on an intrabasin seamount. The crusts are thus the fossil equivalents of modern hydrothermal Mn crusts. Although the crusts show geochemical characteristics of their modern hydrothermal counterparts, they contain elevated concentrations of Ba, Cu, Ni, V, and Zn compared to their modern counterparts, and these cannot be accounted for by either included sedimentary material or hydrogenous inputs. The elevated concentrations of these elements in the crusts is thought to result from reaction with interstitial waters soon after burial, a phenomenon which has profound implications for our understanding of marine ferromanganese oxides in general, as it indicates that they can undergo postdepositional compositional changes over time. [ABSTRACT FROM AUTHOR]

Hydrothermal Mn-oxide deposits from the Tonga-Kermadec Ridge and the Valu Fa Ridge back-arc spreading center in the Lau Basin have been studied mineralogically and by bulk and partition geochemical techniques. The Mn-oxides are present as three main morphological types; of these, a dense, grey submetallic material is most common. Common lamination of this material suggests an intermittent rate of formation. Mineralogically, the crusts comprise variable proportions of 10Å and 7Å manganite. Concentrations of some trace elements, including Li, Mo, and Zn are generally significantly higher than those observed in Mn-crusts of hydrogenous origin, suggesting that the observed trace element enrichments are due to a hydrothermal source. Both temporal and geographical controls are seen to affect these trace element concentrations. Although most of the hydrothermally enriched elements are associated with the Mn-oxide phases, Li appears to reflect either a surficial adsorption or the presence of a separate Li-rich phase. Observed variations in deposit composition could reflect differences in the geochemistry of the basement rocks from which the elements have been leached, the proximity of high-temperature sulphide deposits and temporal variability in the hydrothermal systems. [ABSTRACT FROM AUTHOR]

Seven sediment cores from the Palaea and Nea Kameni hydrothermal embayments (PK and NK) on Santorini have been studied by using sediment smear slides, X-ray diffraction, and radiocarbon techniques to determine their lithology, mineralogy, and age, respectively. The cores have been analyzed chemically for a suite of elements, including selective chemical leaching, to determine the partition of elements between coexisting phases. Lithologically and chemically, the sediments from the two embayments are very different. Sediments from PK are oxidized in the upper parts of the cores and reduced at depth; they comprise mainly hydrothermal pyritiferous diatomaceous ooze with minor Fe oxyhydroxide, volcanic debris, gypsum, and siderite. Sediments from NK are oxidized throughout and comprise predominantly amorphous iron oxyhydroxides and goethite; they are thought to have formed by direct precipitation from solution. In PK, biogenic, detrital seawater evaporate, and hydrothermal sediment phases have been identified but only the detrital inputs can be correlated between cores. Mn shows a pronounced enrichment towards the outer part of the embayment because of its dispersion down the embayment and precipitation in the more-oxidizing sediments towards the open sea. In NK, the sediments contain detrital, seawater evaporate, and hydrothermally precipitated and scavenged phases. The presence of substantial amounts of organic matter in the PK sediments but not in the NK sediments is probably the main reason why geochemical processes in the two embayments are different. Age dates obtained from the cores by using the [sup 14]C technique indicate that sedimentation in the PK embayment commenced at approximately 50 AD, whereas in the NK embayment it commenced at approximately 1900 AD, which is consistent with the known history of Santorini. Based on these dates, calculated metal accumulation rates in both embayments are higher than in any other dated hydrothermal metalliferous sediments with the exception of those in the Atlantis II Deep of the Red Sea. Sediment ponding in the embayments is the principal cause of these results. [ABSTRACT FROM AUTHOR]