Petrography and geochemistry of organic matter at DSDP Holes 80-549 and 80-550B

Three lower Barremian to middle/upper Cenomanian samples from DSDP Hole 549 and three lower Cenomanian to lower Maestrichtian samples from DSDP Hole 550B were investigated by organic geochemical and organic petrographic methods. The samples came from wells drilled in the area of the Goban Spur in the northeastern Atlantic; they represent gray to greenish gray carbonaceous mud or siltstones from the deeper parts of the Cretaceous sequences penetrated and light-colored chalks from the shallower ones. The total amount of organic carbon is below 1% in all samples; it is especially low in the Cenomanian to Maestrichtian chalks. Terrigenous organic matter predominates; only the Barremian sample shows a moderate number of marine phytoclasts. As indicated by several parameters, the maturity of the organic matter is low, corresponding to about 0.4% vitrinite reflectance.

Magnetic properties and oxide petrography of ODP Hole 111-504B

Magnetic properties measurements were performed on 47 samples drilled during Leg 111 of the Ocean Drilling Program and oxide petrography was studied in 32 samples taken at depths throughout the sheeted dike complex in Hole 504B. Integration of these data with results from previous DSDP legs shows that while natural remanent magnetization is constant with depth, magnetic susceptibility increases and median demagnetizing field and the Q ratio decrease with depth in the section. These trends appear to be a result of an increase in deuteric oxidation and a decrease in hydrothermal alteration of primary titanomagnetite with depth. A distinct change in stable magnetic inclination occurs between the extrusive basalts and the sheeted dikes and may be a result of tectonic rotation of the upper extrusive basalts.

Petrography, microprobe analyses and isotope composition of ultramafic rock from the northern Haskard Highlands, Shackleton Range, East Antarctica

In the Shackleton Range of East Antarctica, garnet-bearing ultramafic rocks occur as lenses in supracrustal high-grade gneisses. In the presence of olivine, garnet is an unmistakable indicator of eclogite facies metamorphic conditions. The eclogite facies assemblages are only present in ultramafic rocks, particularly in pyroxenites, whereas other lithologies - including metabasites - lack such assemblages. We conclude that under high-temperature conditions, pyroxenites preserve high-pressure assemblages better than isofacial metabasites, provided the pressure is high enough to stabilize garnet-olivine assemblages (i.e. >=18-20 kbar). The Shackleton Range ultramafic rocks experienced a clockwise P-T path and peak conditions of 800-850 °C and 23-25 kbar. These conditions correspond to ~70 km depth of burial and a metamorphic gradient of 11-12 °C/km that is typical of a convergent plate-margin setting. The age of metamorphism is defined by two garnet-whole-rock Sm-Nd isochrons that give ages of 525 ± 5 and 520 ± 14 Ma corresponding to the time of the Pan-African orogeny. These results are evidence of a Pan-African suture zone within the northern Shackleton Range. This suture marks the site of a palaeo-subduction zone that likely continues to the Herbert Mountains, where ophiolitic rocks of Neoproterozoic age testify to an ocean basin that was closed during Pan-African collision. The garnet-bearing ultramafic rocks in the Shackleton Range are the first known example of eclogite facies metamorphism in Antarctica that is related to the collision of East and West Gondwana and the first example of Pan-African eclogite facies ultramafic rocks worldwide. Eclogites in the Lanterman Range of the Transantarctic Mountains formed during subduction of the palaeo-Pacific beneath the East Antarctic craton.

Organic petrography and extractable hydrocarbons of sediments at DSDP Leg 58 Holes

The quantity, type, and maturity of organic matter of Quaternary and Tertiary sediments from the Philippine Sea (DSDP Leg 58; Sites 442-446) were determined. Hydrocarbons in lipid extracts were analyzed by capillary-column gas chromatography. Kerogen concentrates were investigated by microscopy for vitrinite reflectance values and maceral composition. In the Shikoku Basin sediments (Sites 442, 443, and 444), organic carbon values range between 0.03 and 0.44 per cent. The higher values in the younger sediments are interpreted as an indication of increasing deposition of eroded organic particles during the past 4 m.y. Microscopic analyses revealed a dominance of reworked organic matter. Primary material could not be distinguished readily; thus, no maturation trend could be established. Extract yields were low. TV-alkane distributions mostly show maxima at n-C29 and n-C31 and high odd-over-even predominances, typical of material which originated in terrigenous higher plants. The organic-carbon values of sediments of the Daito Ridge and Basin region (Sites 444 and 445) range from less than 0.01 to 0.05 per cent. TV-alkanes exhibit varying marine and terrigenous influences. Some carbonate-rich samples show a pronounced even-over-odd predominance. At least the older sediments contained less recycled organic matter than the Shikoku Basin samples. The maturity, where measurable, was low. None of the Philippine Sea samples indicates a significant hydrocarbon-generation potential.

Texture, geochemistry, isotope composition, petrography and epoch at DSDP Site 93-603

Abundant Fe-Mn carbonate concretions (mainly siderite, manganosiderite, and rhodochrosite) were found in the hemipelagic claystones of Site 603 on the eastern North American continental rise. They occur as nodules, micronodules, or carbonate-replaced burrow fills and layers at a subbottom depth of between ~ 120 (Pliocene) and 1160 m (Albian-Cenomanian). In general, the Fe-Mn carbonate concretions form from CO3- produced by the microbiological degradation of organic matter in the presence of abundant Fe + or Mn + and very low S- concentrations. However, there is also some evidence for diagenetic replacement of preexisting calcite by siderite. The carbon isotope composition of diagenetic Fe-Mn carbonate nodules is determined by CO2 reduction during methanogenesis. Carbonate nodules in Cretaceous sediments at sub-bottom depths of 1085 and 1160 m have distinctly lower d13C values (- 12.2 and - 12.9 per mil) than Neogene siderites, associated with abundant biogenic methane in the pore space (-8.9 to 1.7 per mil between 330 and 780 m depth). Since no isotopic zonation could be detected within individual nodules, we assume that the isotopic composition reflects more or less geochemical conditions at the present burial depth of the carbonate nodules. Carbonates did not precipitate within the zone of sulfate reduction (approximately 0.01 to 10 m), where all of the pyrite was formed. The oxygen isotope composition indicates precipitation from seawater-derived interstitial waters. The d18O values decrease with increasing burial depth from + 5.1 to - 1.2 per mil, suggesting successively higher temperatures during carbonate formation.

Petrography and geochemistry of basement rocks from five DSDP Leg 60 Sites

Bulk chemistry and trace elements data were measured in 72 samples, selected from 5 basement sections, which have been recovered by Leg 60 drilling (Sites 453, 454, 456, 458, and 459). According to analytical results a metagabbro- metabasalt breccia, deposited about 5 Ma at the westernmost flank of the Mariana Trough (Site 453), was derived from an island arc source. Basalts from the Mariana Trough (Sites 454 and 456) are similar in many respects to midoceanic ridge basalts (MORB). Yet rocks of unusual geochemistry, reflecting the possible influence of arc volcanism, were found among the pillow lavas at the easternmost trough (Site 456). The acoustic basement in the Mariana fore-arc region was formed by submarine eruptions of arc tholeiites (Sites 458 and 459) and peculiar high-MgO andesites related to the boninite suite.

Petrography and geochemistry of lithic clasts in sediments obtained from ODP Leg 180 sites

This report includes the petrographic description and reviews the distribution of lithic clasts in sediments drilled during Leg 180 in the Woodlark Basin (southwest Pacific). The lithic clasts include (1) metamorphic rocks; (2) granites; (3) serpentinites, gabbros, dolerites, and basalts likely derived from the Papuan ophiolite belt; (4) rare alkaline volcanites reworked in middle Miocene sediments; (5) medium- to high-K calc-alkaline island arc volcanites, in part as reworked clasts, and explosive products deposited by fallout or reworked by turbiditic currents; and (6) rare sedimentary fragments. At the footwall sites the clast assemblage evidences the association of dolerites and evolved gabbroic rocks; the serpentinite likely pertaining to the same ophiolitic complex are likely derived from onland outcrops and transported by means of turbidity currents. On the whole, extensional tectonics active at least since the middle Pliocene can be inferred. The calc-alkaline volcanism is in continuity with the arc-related products from the Papua Peninsula and D'Entrecasteaux Islands and with the latest volcanics of the Miocene Trobrian arc. However, the medium- to high-K and shoshonitic products do not display a significant temporal evolution within the stratigraphic setting. Lava clasts, volcanogenic grains, and glass shards are associated with turbidity currents, whereas in the Pliocene of northern margin the increasing frequency of tephra (glass shards and vesicular silicic fragments) suggests more explosive activity and increasing contribution to the sediments from aerial fallout materials. Evidence of localized alkalic volcanism of presumable early to middle Miocene age is a new finding. It could represent a rift phase earlier than or coeval to the first opening of the Woodlark Basin or, less probably, could derive from depositional trajectories diverted from an adjacent basin.

Petrography, minerals and isotopic composition at DSDP Leg 66 Holes

We collected 20 carbonate nodules from the inner trench slope deposits of the Middle America Trench area off Mexico. Carbonate nodules are found only within the methane-rich layer beneath the mixed layer of methane and hydrogen sulfide. They have been investigated by microscopic, scanning electron microscopic (SEM), X-ray diffraction, and stable isotopic analytical methods. Calcite, magnesian calcite, dolomite, and rhodochrosite were recognized as carbonate minerals. Each carbonate nodule is usually represented by single species of carbonate minerals. Carbonate nodules are subdivided into micrite nodules and recrystallized nodules according to textural features. The carbonate crystallites in each micrite nodule are equidimensional. Their sizes range from several to 30 µm, as revealed by SEM micrographs. The chemical composition of calcite is changed from pure calcite to high magnesian calcite, as shown by the shift of the (104) reflection in X-ray diffraction patterns. Fe substitution for Ca in dolomite was also observed. Carbon isotopic composition shows an unusually wide range - from -42.9 to +13.5 per mil - in PDB scale, whereas oxygen isotopic compositions of almost all the carbonate nodules are constantly enriched in 18O from +3.4 to +7.60 per mil in PDB scale. These wide variations in carbon isotopic composition indicate several sources for the carbon in carbonate nodules. Carbon with a negative d13C value was derived from biochemical oxidation of methane with a negative d13C value. On the other hand, carbon with positive d13C value was probably formed during methane production in an anoxic condition.

Geochemistry and petrography of organic matter at DSDP Hole 77-535

Organic geochemical and organic petrographic methods were used to study three Lower to middle Cretaceous sediment samples from Hole 535 in the southeastern Gulf of Mexico for organic matter contents and origin and level of maturation. All three samples contain mixed kerogen Type II/III organic matter with a maturity corresponding to about 0.4% vitrinite reflectance. The marine component increases with stratigraphic age, and microbial reworking of the organic matter is significant in each age. The lower two samples of Hauterivian to Valanginian age appear to be impregnated (or contaminated) with soluble polar organic compounds, but there is only a weak indication for the presence of more mature, nonindigenous hydrocarbons.

Petrography and geochemistry of organic matter in Triassic and Cretaceous sediments from the Wombat and Exmouth Plateau

Triassic (Carnian-Rhaetian) continental margin sediments from the Wombat Plateau off northwest Australia (Sites 759, 760, 761, and 764) contain mainly detrital organic matter of terrestrial higher plant origin. Although deposited in a nearshore deltaic environment, little liptinitic material was preserved. The dominant vitrinites and inertinites are hydrogen-lean, and the small quantities of extractable bitumen contain w-alkanes and bacterial hopanoid hydrocarbons as the most dominant single gas-chromatography-amenable compounds. Lower Cretaceous sediments on the central Exmouth Plateau (Sites 762 and 763) farther south in general have an organic matter composition similar to that in the Wombat Plateau sediments with the exception of a smaller particle size of vitrinites and inertinites, indicating more distal transport and probably deposition in deeper water. Nevertheless, organic matter preservation is slightly better than in the Triassic sediments. Long-chain fatty acids, as well as aliphatic ketones and alcohols, are common constituents in the Lower Cretaceous sediments in addition to n-alkanes and hopanoid hydrocarbons. Thin, black shale layers at the Cenomanian/Turonian boundary, although present at several sites (Sites 762 and 763 on the Exmouth Plateau, Site 765 in the Argo Abyssal Plain, and Site 766 on the continental margin of the Gascoyne Abyssal Plain), are particularly enriched in organic matter only at Site 763 (up to 26%). These organic-matter-rich layers contain mainly bituminite of probable fecal-pellet origin. Considering the high organic carbon content, the moderate hydrogen indices of 350-450 milligrams of hydrocarbon-type material per gram of Corg, the maceral composition, and the low sedimentation rates in the middle Cretaceous, we suggest that these black shales were accumulated in an area of oxygen-depleted bottom-water mass (oceanwide reduced circulation?) underlying an oxygen-rich water column (in which most of the primary biomass other than fecal pellets is destroyed) and a zone of relatively high bioproductivity. Differences in organic matter accumulation at the Cenomanian/Turonian boundary at different sites off northwest Australia are ascribed to regional variations in primary bioproductivity.

Petrography and mineral compositions of gabbros at DSDP Leg 60 Holes

The compositions, mineralogies, and textures of gabbros recovered in polymict breccias in Hole 453 indicate that they are the cumulus assemblages of calc-alkalic crystal fractional on that occurred beneath the West Mariana Ridge. They are among a class of gabbros known only from other calc-alkalic associations (e.g., the Lesser Antilles and the Peninsular Ranges batholith of Southern California) and differ from gabbros of stratiform complexes, ophiolites, and the ocean crust. Particularly abundant in the Hole 453 breccias are olivine-bearing gabbros with extremely calcic Plagioclase (An94-97) but with fairly iron-rich olivines (Fo76-77). Other gabbros contain biotite and amphibole and occur in breccias with fairly high-grade greenschist facies (amphibole-chlorite-stilpnomelane) metabasalts. One unusual gabbro has experienced almost complete subsolidus recrystallization to an assemblage of aluminous magnesio-hornblende, anorthite, and green hercynitic spinel. This reaction, the extremely calcic Plagioclase, the occurrence of biotite and amphibole, and the association with greenschist facies metamorphic rocks suggest that crystallization of the gabbros occurred at elevated P(H2O). Comparisons with other calc-alkalic gabbro suites suggest pressures in excess of 4 kbar (about 12 km depth). The gabbros were exposed by the early stages of opening of the Mariana Trough and imply that considerable uplift may have attended rifting. They were also subjected to hydrothermal alteration after breccia formation, resulting in formation of chlorite, epidote, actinolite, and prehnite. Temperatures of at least 200°C - and probably 350°C - were reached, and most likely could not have been attained without extrusion or intrusion of magmas nearby, even though no such rocks were cored.

Zeolite and silica diagenesis and sandstone petrography at DSDP Sites 57-438 and 57-439

We detected authigenic clinoptilolites in two core samples of tuffaceous, siliceous mudstone in the lower Miocene section of Hole 439. They occur as prismatic and tabular crystals as long as 0.03 mm in various voids of dissolved glass shards, radiolarian shells, calcareous foraminifers, and calcareous algae. They are high in alkalies, especially Na, and in silica varieties. There is a slight difference in composition among them. The Si : (Al+ Fe3+) ratio is highest (4.65) in radiolarian voids, intermediate (4.34) in dissolved glass voids, and lowest (4.26) in voids of calcareous organisms. This difference corresponds to the association of authigenic silica minerals revealed by the scanning electron microscope: There are abundant opal-CT lepispheres in radiolarian voids, low cristobalite and some lepispheres in dissolved glass voids, and a lack of silica minerals in the voids of calcareous organisms. Although it contains some silica from biogenic opal and alkalies from trapped sea water, clinoptilolite derives principally from dissolved glass. Although they are scattered in core samples of Quaternary through lower Miocene diatomaceous and siliceous deposits, acidic glass fragments react with interstitial water to form clinoptilolite only at a sub-bottom depth of 935 meters at approximately 25°C. Analcimes occur in sand-sized clasts of altered acidic vitric tuff in the uppermost Oligocene sandstones. The analcimic tuff clasts were probably reworked from the Upper Cretaceous terrain adjacent to Site 439. Low cristobalite and opal-CT are found in tuffaceous, siliceous mudstone of the middle and lower Miocene sections at Sites 438 and 439. Low cristobalite derives from acidic volcanic glass and opal-CT from biogenic silica. Both siliceous organic remains and acidic glass fragments occur in sediments from the Quaternary through lower Miocene sections. However, the shallowest occurrence is at 700 meters subbottom in Hole 438A, where temperature is estimated to be 21°C. The d(101) spacing of opal-CT varies from 4.09 to 4.11 Å and that of low cristobalite from 4.04 to 4.06 Å. Some opal-CT lepispheres are precipitated onto clinoptilolites in the voids of radiolarian shells at a sub-bottom depth of 950 meters in Hole 439. Sandstone interlaminated with Upper Cretaceous shale is chlorite- calcite cemented and feldspathic. Sandstones in the uppermost Oligocene section are lithic graywacke and consist of large amounts of lithic clasts grouped into older sedimentary and weakly metamorphosed rocks, younger sedimentary rocks, and acidic volcanic rocks. The acidic volcanic clasts probably originated from the volcanic high, which supplied the basal conglomerate with dacite gravels. The older sedimentary and weakly metamorphosed rocks and green rock correspond to the lithologies of the lower Mesozoic to upper Paleozoic Sorachi Group, including the chert, limestone, and slate in south-central Hokkaido. However, the angular shape and coarseness of the clasts and the abundance of carbonate rock fragments indicate a nearby provenance, which is probably the southern offshore extension of the Sorachi Group. The younger sedimentary rocks, including mudstone, carbonaceous shale, and analcime-bearing tuff, correspond to the lithologies of the Upper Cretaceous strata in south-central Hokkaido. Their clasts were reworked from the southern offshore extension of the strata. Because of the discontinuity of the zeolite zoning due to burial diagenesis, an overburden several kilometers thick must have been denuded before the deposition of sediments in the early Oligocene.