Physical properties, grain size distribution and clay mineralogy of ODP Leg 117 sites

The fabric of sediments recovered at sites drilled on the Indus Fan, Owen Ridge, and Oman margin during Ocean Drilling Program Leg 117 was examined by scanning electron microscopy to document changes that accompany sediment burial. Two sediment types were studied: (1) biogenic sediments consisting of a variety of marly nannofossil and nannofossil oozes and chalks and (2) terrigenous sediments consisting of fine-grained turbidites deposited in association with the Indus Fan. Biogenic sediments were examined with samples from the seafloor to depths of 306 m below seafloor (mbsf) on the Owen Ridge (Site 722) and 368 mbsf on the Oman margin (Sites 723 and 728). Over these depth ranges the biogenic sediments are characterized by a random arrangement of microfossils and display little chemical diagenetic alteration. The microfossils are dispersed within a fine-grained matrix that is predominantly microcrystalline carbonate particles on the Owen Ridge and clay and organic matter on the Oman margin. Sediments with abundant siliceous microfossils display distinct, open fabrics with high porosity. Porosity reduction resulting from gravitational compaction appears to be the primary process affecting fabric change in the biogenic sediment sections. Fabric of illite-rich clayey silts and silty claystones from the Indus Fan (Site 720) and Owen Ridge (Sites 722 and 731) was examined for a composite section extending from 45 to 985 mbsf. In this section fabric of the fine-grained turbidites changes from one with small flocculated clay domains, random particle arrangement, and high porosity to a fabric with larger domains, strong preferred particle orientation roughly parallel to bedding, and lower porosity. These changes are accomplished by a growth in domain size, primarily through increasing face-to-face contacts, and by particle reorientation which is characterized by a sharp increase in alignment with bedding between 200 and 400 mbsf. Despite extensive particle reorientation, flocculated clay fabric persists in the deepest samples examined, particularly adjacent to silt grains, and the sediments lack fissility. Fabric changes over the 45-985 mbsf interval occur in response to gravitational compaction. Porosity reduction and development of preferred particle orientation in the Indus Fan and Owen Ridge sections occur at greater depths than outlined in previous fabric models for terrigenous sediments as a consequence of a greater abundance of silt and a greater abundance of illite and chlorite clays.

Lipid analyses of ODP Leg 117 samples

The solvent-extractable organic fractions of sediment samples from six Ocean Drilling Program Leg 117 sites were investigated by gas chromatography and gas chromatography-mass spectrometry. Sediments deposited in the Indus Fan (Site 720) as well as Miocene sediments from the Owen Ridge (Sites 722 and 731) contain almost exclusively organic matter of terrigenous origin. The organic matter in sediments from the Oman Margin (Sites 723, 725, and 728) and in the Pliocene/Pleistocene sections from the Owen Ridge is mainly of a marine origin with variable admixtures of terrigenous material. In these latter samples strong variations of the lipid composition and distribution are noted. However, the interpretation of the relation to potential biological sources is hampered by a lack of information on the possible lipid composition of appropriate organisms.

Concentrations of urea nitrogen in water of the northwestern Indian Ocean and Red Sea

Distribution of urea in different water layers of the northwestern Indian Ocean is described. It was found to be non-uniformly distributed. High concentration was found in waters close to the bottom. Urea may serve as an indicator of state of the water environment.

Geochemical composition of olivine, spinel and amphibole in ODP Hole 209-1271B

Dunite and gabbroic materials recovered from Hole 1271B, Ocean Drilling Program (ODP) Leg 209, were examined for mineral chemistry to understand melt flow and melt-mantle reactions in the shallowest upper mantle of the Mid-Atlantic Ridge near the 15°20' Fracture Zone. Hole 1271B was drilled to 103.8 meters below seafloor on the inner corner high along the south wall of the 15°20' Fracture Zone. The total length of core collected was 15.9 m (recovery = ~15%). The dominant rock type in Hole 1271B is dunite, followed by brown amphibole gabbro, olivine gabbro, and troctolite, along with minor amounts of harzburgite and olivine gabbronorite. A large proportion of the dunite is associated with gabbroic rocks in Hole 1271B, similar to those observed in the Mohorovicic (Moho) transition zone of the Oman ophiolite, indicating significant magmatic activity in this region near the 15°20' Fracture Zone. Olivine Fo content varies from 89.2 to 91.2 in impregnated dunite and from 85.6 to 88.6 in troctolite, olivine gabbro, and olivine gabbronorite. Spinel Cr# (= 100 x Cr/[Cr + Al] molar ratio) ranges from 38.9 to 62.7 in dunite and from 46.3 to 57.6 in troctolites, olivine gabbro, and olivine gabbronorite. Compositional trends for spinel from dunite through troctolite toward olivine gabbro/gabbronorite are characterized by increases in TiO2, Cr#, and Fe3+#, very similar to those reported from Hess Deep Site 895. Olivine gabbro, olivine gabbronorite, and troctolite in Hole 1271B are considered to have formed as hybrid rocks between dunite and an evolved melt in the walls of a melt channel in the shallowest upper mantle that is tens of meters wide. The melt trapped in the wall rock crystallized plagioclase and clinopyroxene. On the other hand, dunite in the center of the melt channel became more refractory by melt-mantle reactions, increasing spinel Cr# to 62.5.