Isotope compositions and mineralogy of carbonates and barites from sediments of the Deryugin Basin (Sea of Okhotsk)

Results of studies of mineralogy, geochemistry, and isotope parameters (d13C, d34S, d180, and 87Sr/86Sr) of carbonates and barites from sediments of the Deryugin Basin in the Sea of Okhotsk are presented. Diagenetic nature of carbonates and barites formed due to prolonged activity of cold seeps acting along a fracture zone and supplying methane- and barium saturated fluids is determined. Any signs for hydrothermal activity were not observed.

Calcium carbonate contents and oxygen and carbon isotopic compositions of planktic and benthic foraminifera in sediments at DSDP Site 85-575

Data on the composition of benthic foraminiferal faunas at Deep Sea Drilling Project Site 575 in the eastern equatorial Pacific Ocean were combined with benthic and planktonic carbon- and oxygen-isotope records and CaCO3 data. Changes in the composition of the benthic foraminiferal faunas at Site 575 predated the middle Miocene period of growth of the Antarctic ice cap and cooling of the deep ocean waters by about 2 m.y., and thus were not caused by this cooling (as has been proposed). The benthic faunal changes may have been caused by increased variability in corrosivity of the bottom waters, possibly resulting from enhanced productivity in the surface waters.

Chemical and isotope compositions of rocks from altered ocean crust at DSDP/ODP Sites 417/418

Large-scale compositional domains at DSDP/ODP drill sites 417A, 417D and 418A were analyzed for O, Sr and Nd isotope ratios, and REE, U, K, Rb and Sr abundances, to constrain the bulk chemical composition of the oceanic crust that is recycled at subduction zones. The combination of the three sites gives the composition of the upper oceanic crust in this region over a distance of about 8 km. The d18O(SMOW) and 87Sr/86Sr(meas) of compositional domains 10-100 m in size correlate well, with a range of 7.7-19.2 and 0.70364-0.70744, and mean of 9.96 and 0.70475, respectively. The Rb inventory of the upper crust increases by about an order of magnitude, while Sr contents remain constant. U abundances increase moderately under oxidizing alteration conditions and nearly triple in the commonly reducing alteration environments of the upper oceanic crust. REEs are influenced by alteration only to a small extent, and recycled oceanic crust is similar to MORB with respect to 143Nd/144Nd. Even though the average composition of the upper oceanic crust is well defined, the large scale composition varies widely. Highly altered compositional domains may not have a large impact on the average composition of the oceanic crust, but they may preferentially contribute to fluids or partial melts derived from the crust by prograde metamorphic reactions.

Chemical compositions of Fe-Mn nodules and host sediments from the Northwest Pacific

A number of regularities of ore element accumulation in iron-manganese nodules along the profile from the Tsugaru Strait to the Wake Atoll is identified in the paper. It is shown that the ore process is enhanced to the pelagic zone: in nodules content of ore material increases and content of mineral insoluble residue decreases from near-shore areas to central parts of the ocean. Diagenetic redistribution of the elements between host sediments and nodules resulting to enrichment of the latter increases from bottom sediments of the ocean periphery to fine grained pelagic muds. At absolute enrichment by Fe, Mn, Cu, Ni, Co, Mo, W, Ti, Zr, and V (as compared to host sediments) nodules are relatively enriched in Mn group elements (Cu, Ni, Co, Mo, W) and consequently depleted in Fe group elements (Ti, V, Zr) in the direction from the periphery to the center of the ocean. The ratio of reactive forms of Fe and Mn in host sediments is a factor determining the ratio of Fe group and Mn group elements in nodules.

Mineral, chemical and isotope compositions of clay subfractions from formations of the Srednii Peninsula, Murmansk coast of the Barents Sea

Fine-grained clay subfractions (SFs) with particle size of <0.1, 0.1-0.2, 0.2-0.3, 0.3-0.6, 0.6-2.0, and 2-5 µm separated from claystone of Upper Precambrian Pumanskaya and Poropelonskaya formations on the Srednii Peninsula were studied by transmission electron microscopy, X-ray diffraction, and Rb-Sr methods. All subfractions consist of low-temperature illite and chlorite, and contribution of chlorite decreases with diminishing particle size. The crystallinity index and I002/I001 ratio increase from coarse- to fine-grained SFs. Leaching by ammonium acetate solution and Rb-Sr systematics in combination with mineralogical and morphological data indicate that illite in Upper Proterozoic claystone from the Srednii Peninsula formed during three time intervals: 810-830, 610-620, and about 570 Ma ago. The first generation of this mineral with low Rb/Sr ratio dominates in coarse-grained SFs while the second and third generations with a high Rb/Sr ratio prevail in fine-grained SFs. All of three generations are known in Poropelon claystone, whereas Puman claystone contains only illite of the first and second generations. Geological processes responsible for multistage illite evolution in claystones are discussed.

(Table 1) Lithological and organic-geochemical compositions of bottom sediments from the Southwestern Indian Ocean (Mozambique Basin)

Geochemical changes in organic matter of bottom sediments from the Mozambique Basin at the river-sea barrier from the mouths of the Zambezi and Limpopo rivers toward the pelagic zone are discussed. Changes in bitumen, hydrolyzable material, humic acids, amino acids, n-alkanes, and polycyclic aromatic compounds resulting from genetic and diagenetic factors are described. This information is significant for paleoceanology reconstructions and for knowing ways of organic matter transformation into fossil forms.

(Table 1) Concentrations and carbon isotopic compositions of CH4 and CO2 at DSDP Site 76-533

The principal gaseous carbon-containing components identified in the first 400 m of sediment at Deep Sea Drilling Project Site 533, Leg 76, are methane (CH4) and carbon dioxide (CO2). Below a sub-bottom depth of about 25 m, sediment cores commonly contained pockets caused by the expansion of gas upon core recovery. The carbon isotopic composition (d13C per mil relative to PDB standard) of CH4 and CO2 in these gas pockets has been measured, resulting in the following observations: (1) d13C-CH4 values increase with depth from approximately -94 per mil in the uppermost sediment to about -66 per mil in the deepest sediment, reflecting a systematic but nonlinear depletion of 12C with depth. (2) d13C-CO2 values also increase with depth of sediment from about -25 per mil to about -4 per mil, snowing a depletion of 12C that closely parallels the trend of the isotopic composition of CH4. The magnitude and parallel distribution of d13C values for both CH4 and CO2 are consistent with the concept that the formation of the CH4 resulted from the microbiological reduction of CO2 from organic substances. These results imply that CH4 and CO2 incorporated in gas hydrates at this site are biogenic.

Lithologic, mineral, chemical and isotopic compositions of altered rocks with increasing distance across the eastern flank of the Juan de Fuca Ridge, ODP Leg 168 data

During ODP Leg 168, 10 sites were drilled across the eastern flank of the Juan de Fuca Ridge (JdFR), to examine the conditions of fluid-rock interaction in three distinct hydrothermal regimes (referred to as the Hydrothermal Transition (HT), Buried Basement (BB) and Rough Basement (RB) transects), extending over a ~120 km linear transect perpendicular to the spreading ridge. This was carried out in an attempt to constrain the conditions and processes that control the location, style and magnitude of low temperature (<150°C) fluid-rock interaction within this setting. This paper presents new data on the petrology, mineral chemistry and whole rock strontium and oxygen isotopic compositions of basalts from the eastern flank of the JdFR, in order to investigate the extent, style and sequence of low-temperature hydrothermal alteration and to establish how the hydrothermal regime evolved with time. Throughout the flank, a progressive sequence of low-temperature hydrothermal alteration has been identified, marked by changes in the dominant secondary mineral assemblage, changing from: chlorite+chlorite/smectite; to iron oyxhydroxide+celadonite; to saponite+/-pyrite; culminating at present with Ca- to CaMg(+/-Fe,Mn)-carbonate. The changes in secondary mineralogy have been used to infer a series of systematic shifts in the conditions of alteration that occurred as the basement moved off-axis and was progressively buried by sediment. In general, hydrothermal alteration of the uppermost oceanic crust commenced under open, oxidative conditions, with interaction between unmodified to slightly modified seawater and basaltic crust, to a regime in which circulation of a strongly modified seawater-derived fluid was more restricted, and alteration occurred under non-oxidative conditions. Across the flank, petrological observations and microprobe analyses indicate that the observed ranges in secondary mineral composition are directly related to changes in the geochemical and textural characteristics of the basement, as well as to interaction between fluids and phases from the four stages of alteration. This is suggestive of an increase in fluid-rock increased with time. Whole rock 87Sr/86Sr and d18O analyses of basalts from across the eastern flank of the JdFR reinforce petrological observations, with 87Sr/86Sr and d18O values slightly elevated above accepted pristine MORB values for this region. These results are consistent with an increase in the amount of fluid-rock interaction with time. Across the flank, enrichment in the 87Sr/86Sr and d18O relative to MORB, is influenced by a number of factors, including: local and regional variations in the crustal lithology and structure; the age of the crust; the extent of bulk rock alteration; and theoretically, the relative abundance of different isotopically-enriched secondary mineral phases in the crust.