(Table 1) K-Ar age determinations on biotite from coarse sand within DSDP Sample 72-516F-76-4,107-115

Qualitative petrographic study of selected clastic horizons within the Eocene section of Hole 516F has revealed the presence of abundant fine-grained lithic fragments, probably volcanic, along with coarser fragments of quartz and feldspar apparently derived from a nearby plutonic terrain. In detail, poor sorting, presence of graded bedding, and an abundance of clay suggest these are turbidite horizons locally derived from a mixed volcanic/plutonic terrain, possibly with some direct contribution from contemporary volcanic ash falls. A progressive increase in plutonic versus volcanic components with time is, however, more consistent with an erosional origin for most of this material. Unusual euhedral dark biotite is abundant in several of the lower clastic horizons; it is most easily interpreted as microphenocrysts weathered in situ out of alkalic volcanic ash. Biotite separated from Sample 516F-76-4,107-115 cm, has been dated by the K-Ar method at about 46 Ma. Alkaline volcanoes active on the Rio Grande Rise in the middle Eocene would be the most probable source of this ash and would be consistent with other evidence for potassic, alkaline volcanism along the Rio Grande Rise and at the Tristan da Cunha hot spot.

Geochemistry of secondary carbonates in ODP Leg 115 basalts

This report presents the results of a study of the stable isotopic and chemical composition of secondary carbonate minerals precipitated within basalts at Ocean Drilling Program Sites 707 and 715. At Site 715, the secondary carbonates are all composed of calcite and display a narrow range of carbon and oxygen stable isotope ratios, with values ranging from -2.75 per mil to 1.95 per mil PDB and -0.27 per mil to 2.86 per mil PDB, respectively. Strontium, iron, and manganese values of the samples are generally low. The geochemistry of Site 715 samples indicates that they precipitated from seawater-domi- nated fluids, at low temperatures, as is typical of secondary carbonates from most Deep Sea Drilling Project sites. In contrast, at Site 707, aragonite, siderite, and manganese-rich calcite occur as secondary carbonates in addition to calcite. The carbon isotopes of the Site 707 carbonates of all rock types are depleted in 13C. Values range from -2.79 per mil to -16.43 per mil PDB. Oxygen isotope values do not show a wide variation, ranging from -1.78 per mil to 1.17 per mil. The strontium contents of the samples range from 5200 to 8100 ppm for aragonites, and from 145 to 862 ppm for calcites. Iron and manganese contents are high in calcites and siderites and low in aragonites. Site 707 carbonates precipitated at low temperatures in a fairly closed system, in which basalt-seawater interaction has greatly influenced the chemistry of the pore fluids. The reactions occurring within the system before and in conjunction with secondary carbonate precipita- tion include oxidation of isotopically light methane, derived from fluids circulating within the basalts, and reduction of substantial amounts of iron and manganese oxides from the basalts.

Oligocene plaktonic foraminifera of the Indian Ocean ODP Leg 115 sites

We drilled 13 holes on Ocean Drilling Program Leg 115 in the Indian Ocean and recovered Paleogene sediments that consisted primarily of pelagic components. Planktonic foraminifer assemblages displayed high diversity throughout the Paleogene from the late Paleocene to the Oligocene/Miocene boundary and consist of predominantly warm-water species. Faunas of middle Eocene age are remarkably well represented. Biostratigraphic assignment was, however, very difficult because of the turbiditic character of most of the Paleogene sediments. Reworking is a constant feature of the middle Eocene through early Oligocene planktonic faunas, with reworked faunas frequently overwhelming the younger ones. Preservation within turbidites ranges from excellent to very poor to total destruction of planktonic foraminifers. A major dissolution episode is recorded in the interval that spans most of the late Eocene through the early Oligocene, especially at the deeper sites where the source area was probably well below the lysocline. Redeposition decreases markedly by the mid-Oligocene, but it is only by late Oligocene Zone P22 that normal sedimentation resumes and/or redeposition decreases even at the most affected sites (such as Hole 709C). Comparison with other sites drilled previously in the Indian Ocean reveals that mixed assemblages were already known for sediments from the Mascarene Plateau-Seychelles Bank and surrounding basins during that time span. Because of the disturbances that characterize Paleogene deposits, hiatuses are difficult to detect; nevertheless, a hiatus of less local importance, spanning Subzone P21b, was detected in three holes at different water depths.