Chemical and mineral compositions of basalts from the Pacific Ocean, DSDP data

Mineral and chemical alterations of basalts were studied in the upper part of the ocean crust using data of deep-sea drilling from D/S Glomar Challenger in the main structures of the Pacific floor. Extraction of majority of chemical elements (including heavy metals) from basalts results mainly from their interaction with heated sea water. As a result mineralized hydrothermal solutions are formed. On entering the ocean they influence greatly on ocean sedimentation and ore formation.

Geochemistry of Mid-Atlantic Ridge sediments

Geochemical compositions and Sr and Nd isotopes were measured in two cores collected ~2 and 5 km from the Rainbow hydrothermal vent site on the Mid-Atlantic Ridge. Overall, the cores record enrichments in Fe and other metals from hydrothermal fallout, but sequential dissolution of the sediments allows discrimination between a leach phase (easily leachable) and a residue phase (refractory). The oxy-anion and transition metal distribution combined with rare earth element (REE) patterns suggest that (1) the leach fraction is a mixture of biogenic carbonate and hydrothermal Fe-Mn oxy-hydroxide with no significant contribution from detrital material and (2) >99.5% of the REE content of the leach fraction is of seawater origin. In addition, the leach fraction has an average 87Sr/86Sr ratio indistinguishable from modern seawater at 0.70916. Although we lack the epsilon-Nd value of present-day deep water at the Rainbow vent site, we believe that the REE budget of the leach fraction is predominantly of seawater origin. We suggest therefore that the leach fraction provides a record of local seawater epsilon-Nd values. Nd isotope data from these cores span the period of 4-14 ka (14C ages) and yield epsilon-Nd values for North East Atlantic Deep Water (NEADW) that are higher (-9.3 to -11.1) than those observed in the nearby Madeira Abyssal Plain from the same depth (-12.4 ± 0.9). This observation suggests that either the Iceland-Scotland Overflow Water (ISOW) and Lower Deep Water contributions to the formation of NEADW are higher along the Mid-Atlantic Ridge than in the surrounding basins or that the relative proportion of ISOW was higher during this period than is observed today. This study indicates that hydrothermal sediments have the potential to provide a higher-resolution record of deep water epsilon-Nd values, and hence deepwater circulation patterns in the oceans, than is possible from other types of sediments.

(Table 1, page 146) Iron, manganese and nickel contents of 21 ferromanganese nodules from Combe D'Izame and Mont Peyroux, Montagne Noire

Ferromanganese nodules (equivalent to Recent manganese nodules) are described from the Upper Devonian griotte (red pelagic limestone) of the Montagne Noire (S. France) and the Cephalopodenkalk of the Rheinisches Schiefergebirge, West Germany. They occur as encrustations, commonly exhibiting colloform structures, around skeletal material and limestone clasts. The nodules are associated with encrusting foraminifera and a development in the sublittoral environment is envisaged. Chemically, the ferromanganese nodules are depleted in manganese relative to iron, compared with Recent nodules, a loss which is attributed to diagenetic migration of manganese. Electron probe studies show that manganese covaries positively with calcium, but negatively with iron and silicon. Diagenetic enrichment of hematite occurs in the griotte at hardground horizons where two periods of mineralization can be established. The Devonian ferromanganese nodules show that solution of nodules has not occurred on burial.

Heavy metal concentrations in fauna from hydrothermal fields of the Mid-Atlantic Ridge

Distributions of Fe, Mn, Zn, Cu, Ni, Co, Cr, Pb, As, Ag, Cd, Se, Sb, and Hg in 128 samples of tissues of organisms that inhabit hydrothermal vent fields of the Mid-Atlantic Ridge (Menez Gwen, Snake Pit, and Rainbow) depending on the abiotic environmental parameters were studied. The majority of the elements studied showed direct correlations between their concentrations in fluids released and in tissues of hydrothermal organisms. A higher degree of bioaccumulation of metals was revealed in Bathymodiolus mussels and Rimicaris shrimps from the Rainbow hydrothermal vent field as compared to their analogues from the Menez Gwen and Snake Pit fields. This corresponds to maximal concentrations of the majority of the metals studied in the Rainbow high-temperature hydrothermal fluids. The highest degree of bioaccumulation of heavy metals was found in gills of symbiotrophic mussels Bathymodiolus and in maxillipeds of ectosymbiotic shrimps Rimicaris, i.e., in organs that function in dependence on chemosynthetic bacteria. Within the Rainbow vent field, the shrimps, which inhabit in biotopes with more high-temperature conditions and therefore are more strongly subjected to influence of fluids, were found to contain higher metal contents than mollusks. Fe-Mn hydroxide films that cover mussel shells might serve as important reservoirs of other metals related to Fe and Mn.

(Table 1) Determined and theoretical chemical composition of thaumasite of ODP Hole 135-841B

Hole 841B was drilled in the forearc region of the Lau Basin at a water depth of 4810 m. The hole penetrated a roughly 500-m-thick series of Miocene volcanic sediments with a number of basaltic to andesitic units (sills?) varying in thickness between 7 cm and 17 m. The volcanics are slightly to moderately altered and contain analcite, chabazite, natrolite-thompsonite, heulandite (?), prehnite, and quartz as secondary phases. In addition, thaumasite [Ca3Si(OH)6 * 12H2O](SO4)(CO3) was identified in the altered sequence. Sulfur isotope data of two thaumasite separates (+23.5 per mil and +21.1 per mil d34S) indicate a seawater origin of the sulfate sulfur. It is suggested that thaumasite is a product of low-temperature (<60 °C), seawater-derived CaCl2-rich fluids that were almost identical in composition to those presently circulating in the sub-seafloor.

(Table 3) Reduced inorganic sulphur, d34S and nickel content of ODP Leg 165 sites

Leg 165 of the Ocean Drilling Program afforded a unique opportunity to investigate organic and inorganic geochemistry across a wide gradient of sediment compositions and corresponding chemical pathways. The solid fractions at Sites 998, 999, 1000, and 1001 reveal varying proportions of reactive carbonate species, a labile volcanic ash fraction occurring in discrete layers and as a dispersed component, and detrital fluxes that derive from continental weathering. The relative proportions and reactivities of these end-members strongly dictate the character of the diagenetic profiles observed during the pore-water work of Leg 165. In addition, alteration of the well-characterized basaltic basement at Site 1001 has provided a strong signal that is reflected in many of the dissolved components. The relative effects of basement alteration and diagenesis within the sediment column are discussed in terms of downcore relationships for dissolved calcium and magnesium. With the exception of Site 1002 in the Cariaco Basin, the sediments encountered during Leg 165 were uniformly deficient in organic carbon (typically <0.1 wt%). Consequently, rates of organic oxidation were generally low and dominated by suboxic pathways with subordinate levels of bacterial sulfate reduction and methanogenesis. The low rates of organic remineralization are supported by modeled rates of sulfate reduction. Site 1000 provided an exception to the generally low levels of microbially mediated redox cycling. At this site the sediment is slightly more enriched in organic phases, and externally derived thermogenic hydrocarbons appear to aid in driving enhanced levels of redox diagenesis at great depths below the seafloor. The entrapment of these volatiles corresponds with a permeability seal defined by a pronounced Miocene minimum in calcium carbonate concentration recognized throughout the basin and with a dramatic downcore increase in the magnitude of limestone lithification. The latter has been tentatively linked to increases in alkalinity associated with microbial oxidation of organic matter and gaseous hydrocarbons. Recognition and quantification of previously unconstrained large volumes and frequencies of Eocene and Miocene silicic volcanic ash within the Caribbean Basin is one of the major findings of Leg 165. High frequencies of volcanic ash layers manifest as varied but often dominant controls on pore-water chemistry. Sulfur isotope results are presented that speak to secondary metal and sulfur enrichments observed in ash layers sampled during Leg 165. Ultimately, a better mechanistic understanding of these processes and the extent to which they have varied spatially and temporally may bear on the global mass balances for a range of major and minor dissolved components of seawater.

(Table 2, page 72) Chemical compositin of the two nodule types used in this investigation

Two Pacific Ocean manganese nodules, one from the ocean basin and one from a sea-mount, were examined in transmission electron microscopes at 100 and 650 kV. Of the many specimens examined, ten electron diffraction crystal spot patterns were identified. Sodium birnessite was observed six times and todorokite, Giavanoli's synthetic birnessite, hydrohausmanite and -Fe2O3 one time each. Ferric hydroxide was synthesized in the laboratory and shown to be the same as the primary iron mineral observed in the manganese nodules. The ferric hydroxide had a particle size range from 30 to 450 ?. Manganese oxide particles were frequently embedded in a mass of smaller ferric hydroxide particles.