Mineralogy and geochemistry at DSDP Hole 62-465A

Core recovered from Hess Rise contains concentrations of pyrite, marcasite, and barite in the lowermost meter of limestone (Unit II) and in the brecciated upper part of the underlying volcanic basement (Unit HI). Petrographic and chemical data indicate that the sulfide-barite assemblage in the limestone is mainly a product of low-temperature diagenetic processes. The iron-sulfide phases are biogenic and their concentrations mark the diffusion of sea water sulfate through sedimentary horizons containing abundant organic matter and mafic, glassy volcanogenic detritus. There is some evidence, however, that elevated temperatures augmented or intensified the synsedimentary diagenetic process.

(Table 1.5) Contents of some volatile components in glassy MORB from the north part of the Bougault anomaly, Mid-Atlantic Ridge 15°N

Structure and composition of sub-surface bottom sediments from the southwest Barents Sea have been under study. The study has revealed heterogeneity of sediment structure resulted from temporal irregularity and variability of sedimentation processes. The study of the heavy minerals from 0.1-0.01 mm grain size fraction has shown prevalence of green hornblende, epidote, garnet, and ilmenite in all types of sediments; these minerals are the basis of terrigenous-mineralogical province. At the same time in different areas local terrigenous-mineralogical associations have been identified. Clay mineral composition of in the sediments was quite uniform: biotite, chlorite, hydromica, smectite. Despite this, a number of features indicating initial stages of clay mineral transformation has been identified. Differences in material composition and structure of the studied sediments are associated with rapid change in paleogeographic situation on the land - ice cover melting on the Kola Peninsula and subsequent Holocene climatic situation.

Geochemistry of ODP Site 125-786 volcanic rocks

Eocene-Oligocene volcanic rocks drilled at Site 786 in the Izu-Bonin forearc cover a wide range of compositions from primitive boninites to highly evolved rhyolites. K-Ar dating reveals at least two distinct episodes of magmatism; one at 41 Ma and a later one at 35 Ma. The early episode produced low-Ca boninites and bronzite andesites that form an oceanic basement of pillow lavas and composite intrusive sheets, overlain by flows and intrusive sheets of intermediate-Ca boninites and bronzite-andesites and a fractionated series of andesites, dacites, and rhyolites. The later episode produced high-Ca boninites and intermediate-Ca boninites, exclusively as intrusive sheets.

Composition of carbonaceous sediments from the Late Jurassic Volgian Basin of the East European Platform (Kostroma Region, Russia)

Lithological, geochemical, stratigraphic, and paleoecological features of carbonaceous sediments in the Late Jurassic Volgian Basin of the East European Platform (Kostroma Region) are considered. The shale-bearing sequence studied is characterized by greater sedimentological completeness as compared with its stratotype sections in the Middle Volga region (Gorodishche, Kashpir). Stratigraphic position and stratigraphy of the shale-bearing sequence, as well as distribution of biota in different sedimentation settings are specified. It is shown that Volgian sediments show distinct cyclic structure. The lower and upper elements of cyclites consist of high-carbonaceous shales and clayey-calcareous sediments, respectively, separated by transitional varieties. Bioturbation structures in different rocks are discussed. Microcomponent composition and pyrolytic parameters of organic matter, as well as distribution of chemical elements in lithologically variable sediments are analyzed. Possible reasons responsible for appearance of cyclicity and accumulation of organic-rich sediments are discussed.

Petroleum-generating potential of sediments at DSDP Leg 64 Holes

Sediments from the Gulf of California contain sufficient amounts of thermally reactive organic matter to be considered fair-to-good potential petroleum source rocks. While sediments deposited within the present oxygen-minimum zone have the greatest amounts of organic matter, those deposited below the oxygen-minimum contain sufficient organic matter to be considered potential source rocks. The organic matter in the sediment is almost exclusively marine, Type II kerogen. Different techniques of determining kerogen composition produce generally compatible answers, although pyrolysis gives somewhat misleading results. Elemental analysis of the kerogen and vitrinite reflectance measurements indicate that the organic matter is not buried to sufficiently great depth for significant petroleum generation, despite the high temperature gradients.

Sulfide petrology of basalts at DSDP Holes 69-504B and 69-505B

Primary sulfide mineralization in basalts of the Costa Rica Rift occurs mainly in chrome-spinel-bearing olivine tholeiites. Primary sulfides form both globules, consisting of quenched single-phase solid solutions, and irregular polymineralic segregations of pyrrhotite, chalcopyrite, cubanite, and pentlandite. Two types of sulfide solid solutions - iron-nickel (Mss) and iron-copper (Iss) - were found among sulfide globules. These types appear to have formed because of sulfide-sulfide liquid immiscibility in the host magmas; as proved by the presence of globules with a distinct phase boundary between Mss and Iss. Such two-phase globules are associated with large olivine phenocrysts. Inhomogeneties among the globule composition likewise are caused by sulfide-sulfide immiscibility. Secondary sulfides form irregular segregations and veins consisting of pyrite, marcasite, and chalcopyrite.

Geochemistry of volcanic glass from ODP holes 129-801C and 185-801C in the western Pacific

Major element chemistry of basalt from the southern East Pacific Rise (EPR) is different from that of the EPR at the time of the formation of the Pacific Plate at 170 Ma.Glass recovered from Jurassic age (170 Ma) Pacific ocean crust (Bartolini and Larson, 2001, doi:10.1130/0091-7613(2001)029<0735:PMATPS>2.0.CO;2) at Ocean Drilling Program Hole 801C records higher Fe8 (10.77 wt%) and marginally lower Na8 (2.21 wt%) compared to the modern EPR, suggesting deeper melting and a temperature of initial melting that was 60°C hotter than today.Trace element ratios such as La/Sm and Zr/Y, on the other hand, show remarkable similarities to the modern southern EPR, indicating that Site 801 was not generated on a hotspot-influenced ridge and that mantle composition has changed little in the Pacific over the past 170 Ma. Our results are consistent with the observation that mid-ocean ridge basalts (MORBs) older than 80 Ma were derived by higher temperature melting than are modern MORBs (Humler et al., 1999, doi:10.1016/S0012-821X(99)00218-6), which may have been a consequence of the Cretaceous superplume event in the Pacific.Site 801 predates the formation of Pacific oceanic plateaus and 801C basalt chemistry indicates that higher temperatures of mantle melting beneath Pacific ridges preceded the initiation of the superplume.

Elemental abundance and water content at DSDP Leg 86 Holes

X-ray fluorescence analyses of 1143 samples from Site 576 (32°21.4'N, 164°16.5'E) and 539 samples from Site 578 (33°55.6'N, 151°37.7'E) for the elements Na, Mg, Al, Si, P, K, Ca, Ti, Mn, Fe, Ba, and S show consistent trends from Si-rich surficial deposits to dark brown clays rich in Mn, Fe, P, and Ti in early Cenozoic sections. These data sets form the basis for a detailed paleogeochemical stratigraphy of North Pacific "red" clays.

Minerals and geochemistry of DSDP Hole 81-552A

The acid insoluble coarse fractions of the glacial-interglacial sequence of Hole 552A in the NE Atlantic are made up of varying amounts of terrigenous detritus, biogenic silica, and pyroclastic material, principally volcanic glass. Volcanic ash content varies significantly over the entire interval, and the three North Atlantic ash horizons of Ruddiman and Glover (1972) can be recognized satisfactorily. The terrigenous detritus is of mixed metamorphic-basaltic type and probably originated on the Greenland landmass

Chemical composition of phosphorites from the Peru and Chile shelves

Recent and Late Quaternary shelf phosphorites have low Fe, Ti and Al contents. These elements enter the phosphorites with terrigenous impurities and organic detritus. Ti, Al, and some Fe are removed when the phosphorites are lithified, whereas remaining iron settles in the phosphorites as sulfides. Ti/Fe, Al/Fe and Ti/Al ratios are used as examples of difference between behavior of Fe and that of Ti and Al.

Mineralogy and geochemistry of weathered serpentinites in DSDP Leg 84 sediments

At Sites 566, 567, and 570 of Leg 84, ophiolitic serpentinite basement was covered by a sequence of serpentinitic mud that was formed by weathering of the serpentinites under sea- or pore-water conditions. Several mineralogical processes were observed: (1) The serpentinitic mud that consists mainly of chrysotile was formed from the lizardite component of the serpentinites by alteration. (2) Slightly trioctahedral smectites containing nonexpandable mica layers, trioctahedral smectites containing nonexpandable chlorite layers, and swelling chlorites were presumably formed from detrital chlorite and/or serpentine. (3) The occurrence of tremolite, chlorite, analcime, and talc can be attributed to reworking of gabbroic ophiolite rocks. (4) Dolomite, aragonite, and Mg-calcite, all authigenic, occur in the serpentinitic mud.

Organic geochemistry of deep sea sediments from the Nankai Trough and the Southwest Japan forearc region

Earth's largest reactive carbon pool, marine sedimentary organic matter, becomes increasingly recalcitrant during burial, making it almost inaccessible as a substrate for microorganisms, and thereby limiting metabolic activity in the deep biosphere. Because elevated temperature acting over geological time leads to the massive thermal breakdown of the organic matter into volatiles, including petroleum, the question arises whether microorganisms can directly utilize these maturation products as a substrate. While migrated thermogenic fluids are known to sustain microbial consortia in shallow sediments, an in situ coupling of abiotic generation and microbial utilization has not been demonstrated. Here we show, using a combination of basin modelling, kinetic modelling, geomicrobiology and biogeochemistry, that microorganisms inhabit the active generation zone in the Nankai Trough, offshore Japan. Three sites from ODP Leg 190 have been evaluated, namely 1173, 1174 and 1177, drilled in nearly undeformed Quaternary and Tertiary sedimentary sequences seaward of the Nankai Trough itself. Paleotemperatures were reconstructed based on subsidence profiles, compaction modelling, present-day heat flow, downhole temperature measurements and organic maturity parameters. Today's heat flow distribution can be considered mainly conductive, and is extremely high in places, reaching 180 mW/m**2. The kinetic parameters describing total hydrocarbon generation, determined by laboratory pyrolysis experiments, were utilized by the model in order to predict the timing of generation in time and space. The model predicts that the onset of present day generation lies between 300 and 500 m below sea floor (5100-5300 m below mean sea level), depending on well location. In the case of Site 1174, 5-10% conversion has taken place by a present day temperature of ca. 85 °C. Predictions were largely validated by on-site hydrocarbon gas measurements. Viable organisms in the same depth range have been proven using 14C-radiolabelled substrates for methanogenesis, bacterial cell counts and intact phospholipids. Altogether, these results point to an overlap of abiotic thermal degradation reactions going on in the same part of the sedimentary column as where a deep biosphere exists. The organic matter preserved in Nankai Trough sediments is of the type that generates putative feedstocks for microbial activity, namely oxygenated compounds and hydrocarbons. Furthermore, the rates of thermal degradation calculated from the kinetic model closely resemble rates of respiration and electron donor consumption independently measured in other deep biosphere environments. We deduce that abiotically driven degradation reactions have provided substrates for microbial activity in deep sediments at this convergent continental margin.

Mineralogy and geochemistry of sulfide minerals and bulk rocks from ODP Hole 176-735B

Ocean Drilling Program (ODP) Leg 176 built upon the work of ODP Leg 118 wherein the 500-m section that was sampled represented the most complete recovery of an intact portion of lower oceanic crust ever described. During Leg 176, we deepened Hole 735B to >1500 m below seafloor in an environment where gabbroic rocks have been tectonically exposed at the Southwest Indian Ridge. This new expedition extended the remarkable recovery (>85%) that allowed unprecedented investigations into the nature of the lower oceanic crust as a result of Leg 118. Sulfide mineral and bulk rock compositions were determined from samples in the 1000-m section of oceanic gabbros recovered during Leg 176. The sulfide assemblage of pyrrhotite, chalcopyrite, pentlandite, and troilite is present throughout this section, as it is throughout the 500-m gabbroic section above that was sampled during Leg 118. Troilite is commonly present as lamellae, and the only interval where troilite was not observed is from the uppermost 150 m of the section sampled during Leg 118, which is intensely metamorphosed. The common presence of troilite indicates that much of the sulfide assemblage from Hole 735B precipitated from a magmatic system and subsequently underwent low-temperature reequilibration. Evaluation of geochemical trends in bulk rock and sulfides indicates that the combined effects of olivine accumulation in troctolites and high pentlandite to pyrrhotite ratios account for the sporadic bulk rock compositions high in Ni. Bulk rock and sulfide mineral geochemical indicators that are spatially coincident with structural and physical properties anomalies indicate a heretofore unrecognized lithologic unit boundary in this section. Platinum-group element (PGE) compositions were also determined for 36 samples from throughout the section that were recovered during Leg 176. Whereas most samples had low (<0.4 ppb) PGE concentrations, rare samples had elevated PGE values, but no unique common trend between these samples is evident.