Diagenetic dolomite formation in Quaternary anoxic diatomaceous muds at DSDP Leg 64 Holes

During drilling in the Gulf of California, diagenetic carbonate rocks were recovered at 7 out of 8 sites. These are primarily dolomites which record 13C isotopic evidence of the incorporation of carbon derived from the decomposition of organic matter. In Hole 479, drilled to a sub-bottom depth of 440 meters on the Guaymas Slope, under a fertile upwelling belt, we recognized an excellent example of deep sea dolomitization in progress. This Quaternary section of organic-carbon- rich, low-carbonate, hemipelagic diatomaceous oozes contains numerous fine-grained, decimeter-thin, episodic beds of dolomite, which show sedimentologic, geochemical, and isotopic evidence of accretion by precipitation below 40 meters sub-bottom in zones of high alkalinity and low sulfate. The beds preserve original sedimentary structures. Carbon-13 varies from +3 to +14 per mil, indicating biogenic CO2 reservoirs related to active methanogenesis. In single beds, 18O values range outwardly from +5 to -7 per mil, reflecting increasing temperature with progressive accretion of dolomite with depth; the values parallel progressive trends in lithification, texture, mineralogy, and fossil preservation. We estimate slow accretion rates on the order of 0.1-0.7 mm/10**3 yr. with burial. Dolomitization does not proceed merely at the expense of nearby nannofossils. Ca and Mg ions must be derived from interstitial waters. The episodic appearance of beds in the sequence seems partly a reflection of latent climate signals. This process of deep sea dolomitization carries implications for hydrocarbon migration, as well as an interpretation of the presence of dolomite in other modern and ancient pelagic to hemipelagic sediment sequences.

Geochemistry of pelagic and hemipelagic sediments of ODP Leg 126 sites

Chemical analyses were performed on major, minor, and rare-earth elements of pelagic and hemipelagic sediments of the forearc, arc, and backarc sites of the Izu-Bonin Arc, Ocean Drilling Program Leg 126. Analyses of the hemipelagic and pelagic sediments of this area indicate that the chemical composition of this arc is highly affected by the chemical composition of rocks of this arc as a source of sediments. The Oligocene sediments, which are characterized by high MgO contents, reflect the chemical composition of the Paleogene volcanic rocks of the immature arc. Moreover, the late Miocene to Quaternary sediments with low MgO contents are attributed to the composition of the present arc. We also suggest that the sedimentation rates and topography of the sedimentary basin affect the MnO and SiO2 contents of pelagic and hemipelagic sediments.

Recalculated petrographic parameters and normalized microprobe analyses of volcanic glasses for DSDP Sites 63-467, 63-468 and 63-469

The southward passage of the Rivera triple junction and its effect on the North American plate are primary controls on the Miocene tectonic evolution of the outer borderland of California. Detrital modes of sand shed off the Patton Ridge and cored by the Deep Sea Drilling Project provide evidence of progressive tectonic erosion of the Patton accretionary prism and neartrench volcanism. Volcanic glass in the sediment is predominantly calcalkaline rhyolite and andesite, typical of subductionrelated volcanism, but also includes minor low-K2O tholeiitic basalt. We attribute these compositional features to interaction with a spreading ridge associated with a possible trench-ridge-trench triple junction along the Patton Escarpment from 18 to 16 Ma. This study suggests that evidence of ridge-trench interaction may be commonly preserved along submerged plate margins, in contrast to its more limited recognition and discussion in the literature based on exposed examples in Chile, Japan and Alaska.

Abundance of nodules, chemical and mineral compositions of nodules and sediments from the Central Pacific

The monograph considers facial conditions of ore-formation in the Central Equatorial Pacific, as well as lithostratigraphy and local variability of bottom sediments. Mineral composition of nodules, forms of occurrence of chemical elements in sediments and nodules, composition of interstitial waters, age of nodules, regularities and processes of ore formation in the radiolarian belt of the Pacific Ocean zone are also under consideration.

Geochemistry of sediments from the Tyrrhenian Sea

At Holes 650A and 651 A, set respectively in the Marsili Basin and the Vavilov Basin, Pleistocene sediments (turbiditic inputs interbedded with essentially hemipelagic sediments) may show layers of mudrocks with moderate to strong induration. Except in the two samples from Hole 651 A, it seems that zeolite crystallization does not play a role in the induration phenomenon. This latter appears to result from in situ clay authigenesis. Secondary K-Fe beidellite or Fe-Mg beidellite form diagenetic growths and bridges between sedimented particles. Turbidites are rich in volcaniclastics (glass, pumices and other volcanogenic elements) but the induration phenomenon appears to be associated essentially with the occurrence of basaltic detritus. It is proposed that clay authigenesis results from low temperature alteration of basaltic fragments issued from Vavilov and probably Marsili seamounts in sediments isolated from seawater by overlying deposits.

Chemistry of basalts of ODP Hole 111-504B

The effect of oxygen fugacity (fO2) on the partition relationship of Mg and Fe between Plagioclase and sillicate liquid was investigated at 1 atm for basaltic samples recovered during ODP Leg 111 from Hole 504B. Samples 111-504B-143R-2 (Piece 8) and 111-504B-169R-1 (Piece 1) have Plagioclase as the liquidus phase. The distribution coefficient of Mg between Plagioclase and melt is constant at about 0.04 against the variation of fO2, whereas that of Fe (total Fe) varies from 0.3 at f(O2) = 0.2 atm to 0.03 at f(o2) = 10**-11.5 at 1200°C. The distribution coefficient of Mg is slightly higher than that calculated from the phenocryst and bulk-rock compositions, suggesting a kinetic disequilibrium effect on the distribution of Mg in Plagioclase. Because Mg, Fe, and Fe3+ have similar diffusion coefficients in silicate melt, the disequilibrium effect is greatly reduced for the exchange reaction of Mg and total Fe between Plagioclase and liquid. The exchange partition coefficient is highly dependent on fo2, with log fo2 ranging from -0.7 to - 11.5 at approximately 1200°C. Using this relationship, the f(O2) of crystallization of the magmas is estimated to be near the one defined by the fayalite-quartz-magnetite assemblage.

Major and trace elements at DSDP Leg 74 Holes

Deep Sea Drilling Project Leg 74 drilled basement on the Walvis Ridge at Sites 525, 527, and 528. These sites are located on the crest and flanks of the segment of the Ridge about 68 to 70 m.y. old in the central province of the Ridge. Each site has a number of distinct subaqueous flows separated by sediment layers. Although variation in geochemistry among units and sites is related in part to alteration or crystal fractionation, some is caused by small-scale compositional variation in the mantle source of the basalts. Leg 74 basalts are similar to other basalts recovered from the Walvis Ridge and the Rio Grande Rise. They show distinct compositional differences to mid-ocean ridge basalts in general, to those recovered from the South Atlantic at this latitude, and to basalts presently erupting in Tristan da Cunha. The composition of the Walvis Ridge basalts does not suggest simple mixtures of present-day MORB and Tristan da Cunha melts. If the Walvis Ridge represents the trace of the Tristan da Cunha hot spot as the plates separated, then the composition of the mantle source has differed at different times in the past, which suggests mantle heterogeneity.

Chemical composition of volcanic glass of some tephra layers in Northeastern Pacific Ocean

Five widespread upper Cenozoic tephra layers that are found within continental sediments of the western United States have been correlated with tephra layers in marine sediments in the Humboldt and Ventura basins of coastal California by similarities in major-and trace-element abundances; four of these layers have also been identified in deep-ocean sediments at DSDP sites 34, 36, 173, and 470 in the northeastern Pacific Ocean. These layers, erupted from vents in the Yellowstone National Park area of Wyoming and Idaho (Y), the Cascade Range of the Pacific Northwest (C), and the Long Valley area, California (L), are the Huckleberry Ridge ash bed (2.0 Ma, Y), Rio Dell ash bed (ca. 1.5 Ma, C), Bishop ash bed (0.74 Ma, L), Lava Creek B ash bed (0.62 Ma, Y), and Loleta ash bed (ca. 0.4 Ma, C). The isochronous nature of these beds allows direct comparison of chronologic and climatic data in a variety of depositional environments. For example, the widespread Bishop ash bed is correlated from proximal localities near Bishop in east-central California, where it is interbedded with volcanic and glacial deposits, to lacustrine beds near Tecopa, southeastern California, to deformed on-shore marine strata near Ventura, southwestern California, to deep-ocean sediments at site 470 in the eastern Pacific Ocean west of northern Mexico. The correlations allow us to compare isotopic ages determined for the tephra layers with ages of continental and marine biostratigraphic zones determined by magnetostratigraphy and other numerical age control and also provide iterative checks for available age control. Relative age variations of as much as 0.5 m.y. exist between marine biostratigraphic datums [for example, highest occurrence level of Discoaster brouweri and Calcidiscus tropicus (= C. macintyrei)], as determined from sedimentation rate curves derived from other age control available at each of several sites. These discrepancies may be due to several factors, among which are (1) diachronism of the lowest and highest occurrence levels of marine faunal and floral species with latitude because of ecologic thresholds, (2) upward reworking of older forms in hemipelagic sections adjacent to the tectonically active coast of the western United States and other similar analytical problems in identification of biostratigraphic and magnetostratigraphic datums, (3) dissolution of microfossils or selective diagenesis of some taxa, (4) lack of precision in isotopic age calibration of these datums, (5) errors in isotopic ages of tephra beds, and (6) large variations in sedimentation rates or hiatuses in stratigraphic sections that result in age errors of interpolated datums. Correlation of tephra layers between on-land marine and deep-ocean deposits indicates that some biostratigraphic datums (diatom and calcareous nannofossil) may be truly time transgressive because at some sites, they are found above and, at other sites, below the same tephra layers.

Composition of rocks, minerals, water, and gases from uplift areas of the Central Atlantic

Original geological, geophysical, lithological, mineralogical data on uplifts of the Central Atlantic are given in the book based on materials of Cruise 1 of the R/V Akademik Nikolaj Strakhov. Geological and geophysical studies include description of the obtained material and analysis of structural and morphological elements of the ocean floor. Results of lithological, petrochemical and geochemical studies were extremely innovative and develop a conceptual model. The latter include studies of petrochemical evolution of tholeiitic alkaline plate volcanism, large-scale hydrothermal transformation of basement rocks - palygorskitization, phosphatization and ferromanganese mineralization. Showing imposition Superposition of hydrogenic alteration on hydrothermally altered rocks and its role in Cenozoic history of sedimentation is shown.

Geochemistry of serpentenites from DSDP Hole 84-566C

We studied a unique chrysotile-antigorite serpentinite, drilled on Deep Sea Drilling Project Leg 84 (Site 566) in the Guatemala forearc. Our in situ major and trace element data provide new constraints on possible reactions and associated trace element mobilisation during shallow serpentinite subduction. Chrysotile of the studied serpentinite, formed by the hydration of an upper mantle peridotite precursor, is partially replaced by antigorite (alone) which also occurs in 0.5 mm wide unoriented veins crosscutting the rock. Based on textural relationships and the P-T-X stability of the rock forming phases, the replacement of chrysotile by antigorite occurred at T < 300 °C, due to interaction between the chrysotile-serpentinite and an aqueous fluid. A comparison of the chemical compositions of reactant and product phases reveals that about 90% of the Cl, more than 80% of the B and about 50% of the Sr hosted originally by chrysotile was lost during fluid-assisted chrysotile-to-antigorite transformation and accompanying partial dehydration, and documents the much lower affinity of antigorite for trace element uptake than that of chrysotile. The fluid-assisted chrysotile-to-antigorite transformation and associated trace element loss documented here can occur in the shallow (< 30 km) region of subduction zones. This transformation decreases notably the Cl and B inventory of subducting serpentinites, which are regarded as one of the most important carriers of these elements into subduction zones. The evolution of serpentinites during initial subduction stages thus appears to be critical in the recycling of specific trace elements such as B or Cl from forearc to subarc depths.

Chemical and germanium isotopic compositions of sediments and sedimentary rocks from ODP Sites 129-801 and 185-1149

A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal. The analyses were performed using a continuous flow hydride generation system coupled to a MC-ICP-MS. Samples have been purified through anion- and cation-exchange resins to separate Ge from matrix elements and eliminate potential isobaric interferences. Variations of 74Ge/70Ge ratios are expressed as d74Ge values relative to our internal standard and the long-term external reproducibility of the data is better than 0.2? for sample size as low as 15 ng of Ge. Data are presented for igneous and sedimentary rocks, and the overall variation is 2.4? in d74Ge, representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of Ge is not unique. Co-variations of 74Ge/70Ge, 73Ge/70Ge and 72Ge/70Ge ratios follow a mass-dependent behaviour and imply natural isotopic fractionation of Ge by physicochemical processes. The range of d74Ge in igneous rocks is only 0.25? without systematic differences among continental crust, oceanic crust or mantle material. On this basis, a Bulk Silicate Earth reservoir with a d74Ge of 1.3+/-0.2? can be defined. In contrast, modern biogenic opal such as marine sponges and authigenic glauconite displayed higher d74Ge values between 2.0? and 3.0?. This suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the d74Ge of the seawater to +3.0?.This suggests that seawater is isotopically heavy relative to Bulk Silicate Earth and that biogenic opal may be significantly fractionated with respect to seawater. Deep-sea sediments are within the range of the Bulk Silicate Earth while Mesozoic deep-sea cherts (opal and quartz) have d74Ge values ranging from 0.7? to 2.0?. The variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz. Further work is now required to determine Ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification.

Isotopic and chemical compositions of lower crust rocks and minerals from ODP Hole 176-735B

The composition of gabbroic rocks from the drill core of Hole 735B (ODP Leg 176) at the 11 Ma Atlantis II bank close to the slow spreading Southwest Indian Ridge (SWIR) has been analyzed for major and trace elements and Sr, Nd and Pb isotopic composition. The samples are thought to represent much of the mineralogical and geochemical variation in a vertical 1-km section (500-1500 m below the sea floor) of the lower ocean crust. Primitive troctolitic gabbros, olivine gabbros and gabbros that have Mg#=84-70, Ca#>61 and low Na# (Na/(Na+Al)) (8-17) are intruded by patches or veins of more evolved FeTi-oxide rich gabbroic and dioritic rocks with Mg# to 20, Ca# to 32, Na#=14-23, TiO2<7 wt.% and FeOtotal<18 wt.%. All rocks are acdcumulates, and incompatible element concentrations are low, e.g. Pb=0.1-0.7 ppm and U

Geochemistry of selected Ferrar samples from Northern Victoria Land and George V Land during GANOVEX VIII

Newly sampled basaltic andesites and andesites from the tholeiitic Ferrar Supergroup of northern Victoria Land and George V Land, Antarctica, are attributed to the known low-Ti and high-Ti series. Aside from known sparsely distributed high-Ti extrusives, a high-Ti sill was found in the Alamein Range outside the Rennick Graben. Low-Ti lavas, sills and dikes display wide petrographical, mineral and geochemical variations, reflecting extensive in-situ differentiation. High-Ti rocks from Litell Rocks are homogeneous with respect to mineralogy and geochemistry, minor deviations are shown by the sampled sill. Chilled margins of low-Ti sills, dikes and lava flows exhibit nearly constant bulk-rock chemistry (mg# ~60) within the studied area. Compared to chilled margins from Tasmanian sills, the striking uniformity of the pre-emplacement chemistry of Ferrar magmas over large distances supports the magma transport model of Elliot et al. (1999, doi:10.1016/S0012-821X(99)00023-0). In the area investigated, compositional variations within the low-Ti series, caused by in-situ differentiation, increase towards the Wilson-Bowers Terrane boundary, possibly displaying the asymmetrical distribution of outcrops over this area. Absence of Ferrar occurrences east of the Bowers Terrane remains a matter of palaeo-geodynamic discussion. Besides, the secondary mineralogy of extrusives from Litell Rocks and Monument Nunataks exhibits noticeable differences, which indicates an elevated thermal gradient in the vicinity of Litell Rocks compared to Monument Nunataks during the Cretaceous.

Major, trace and rare earth element concentrations of sediments from ODP Leg 207 sites

The Cretaceous and Paleogene sediments recovered during Ocean Drilling Program Leg 207 can be divided into three broad modes of deposition: synrift clastics (lithologic Unit V), organic matter-rich, laminated black shales (Unit IV), and open-marine chalk and calcareous claystones (Units III-I). The aim of this study is to provide a quantitative geochemical characterization of sediments representing these five lithologic units. For this work we used the residues (squeeze cakes) obtained from pore water sampling. Samples were analyzed for bulk parameters (total inorganic carbon, total organic carbon, and S) and by X-ray fluorescence for major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and selected minor (As, Ba, Co, Cr, Cu, Mo, Ni, Pb, Rb, Sr, U, V, Y, Zn, and Zr) elements. Inductively coupled plasma-mass spectrometry analyses for rare earth elements (REEs) were performed on acid digestions of the squeeze cake samples from Site 1258. The major element composition is governed by the mixture of a terrigenous detrital component of roughly average shale (AS) composition with biogenous carbonate and silica. The composition of the terrigenous detritus is close to AS in Units II-IV. For Unit I, a more weathered terrigenous source is suggested. Carbonate contents reach >60 wt% on average in chalks and calcareous claystones of Units II-IV. The SiO2 contribution in excess of the normal terrigenous-detrital background indicates the presence of biogenous silica, with highest amounts in Units II and III. The contents of coarse-grained material (quartz) are enhanced in Unit V, where Ti and Zr contents are also high. This indicates a high-energy depositional environment. REE patterns are generally similar to AS. A more pronounced negative Ce anomaly in Unit IV may indicate low-oxygen conditions in the water column. The Cretaceous black shales of Unit IV are clearly enriched in redox-sensitive and stable sulfide-forming elements (Mo, V, Zn, and As). High phosphate contents point toward enhanced nutrient supply and high bioproductivity. Ba/Al ratios are rather high throughout Unit IV despite the absence of sulfate in the pore water, indicating elevated primary production. Manganese contents are extremely low for most of the interval studied. Such an Mn depletion is only possible in an environment where Mn was mobilized and transported into an expanded oxygen minimum zone ("open system"). The sulfur contents show a complete sulfidation of the reactive iron of Unit IV and a significant excess of sulfur relative to that of iron, which indicates that part of the sulfur was incorporated into organic matter. We suppose extreme paleoenvironmental conditions during black shale deposition: high bioproductivity like in recent coastal upwelling settings together with severe oxygen depletion if not presence of hydrogen sulfide in the water column.

Chemistry of siliciclastic and volcanoclastic sediments of DSDP Site 47-397

The Lower Cretaceous and Miocene sequences of the NW African passive continental margin consist of siliciclastic, volcaniclastic and hybrid sediments. These sediments contain a variety of diagenetic carbonates associated with zeolites, smectite clays and pyrite, reflecting the detrital mineralogical composition and conditions which prevailed during opening of the North Atlantic. In the Lower Cretaceous siliciclastic sediments, siderite (-6 per mil to +0.7per mil d18O PDB, -19.6 per mil to +0.6 per mil d13C PDB) was precipitated as thin layers and nodules from modified marine porewaters with input of dissolved carbon from the alteration of organic matter. Microcrystalline dolomite layers, lenses, nodules and disseminated crystals (-3.0 per mil to +2.5 per mil d18O PDB, -7.2 per mil to +4.9 per mil d13C PDB) predominate in slump and debris-flow deposits within the Lower Miocene sequence. During the opening of the Atlantic, volcanic activity in the Canary Islands area resulted in input of volcaniclastic sediments to the Middle and Upper Miocene sequences. Calcite is the dominant diagenetic carbonate in the siliciclastic-bioclastic-volcaniclastic hybrid and in the volcaniclastic sediments, which commonly contain pore-rimming smectite. Diagenetic calcite (-22 per mil to +1.6 per mil d18O PDB, -35.7 per mil to +0.8 per mil d13C PDB) was precipitated due to the interaction of volcaniclastic and bioclastic grains with marine porewaters. Phillipsite is confined to the alteration of volcaniclastic sediments, whereas clinoptilolite is widely disseminated, occurring essentially within foraminiferal chambers, and formed due to the dissolution of biogenic silica.