Middle Eocene to early Oligocene planktonic and benthic foraminifers from ODP Hole 125-786A

Drilling at Site 786, located in the center of the Izu-Bonin forearc basin, penetrated an apparently continuous section of middle Eocene/lower Oligocene volcaniclastic breccias and nannofossil oozes. Planktonic foraminiferal faunas underwent a gradual transition from relatively high-diversity middle Eocene through late Eocene tropical or warm-water assemblages to a cooler-water, less diverse assemblage during the early Oligocene. In the cosmopolitan benthic foraminiferal faunas, the major transition occurred during the early late Eocene. Middle Eocene benthic assemblages resembling the bathyal 'Lenticulina' fauna (characterized by Osangularia mexicana, Cibicidoides eocaenus, and several buliminid species) changed to an upper Eocene abyssal 'Globocassidulina subglobosa' fauna (characterized by Cibicidoides praemundulus, Globocassidulina subglobosa, Gyroidinoides girardanus, Oridorsalis umbonatus, and Siphonodosaria aculeata). Even though no large, abrupt faunal changes appear to have been associated with the assumed Eocene/Oligocene boundary, benthic species turnover continued through the late Eocene and into the early Oligocene. This resulted in a slightly lower diversity early Oligocene fauna dominated by three species: Laevidentalina sp., Bulimina jarvisi, and Gyroidinoides girardanus. The progression from a middle Eocene bathyal 'Lenticulina' fauna, rather than an abyssal 'Nuttallides truempyi' fauna, to an abyssal 'Globocassidulina subglobosa' fauna during the early late Eocene, suggests that a bathymetric deepening occurred at Site 786. Increased water depths may have resulted from tectonic subsidence.

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.

C1-C8 hydrocarbons in DSDP Leg 64 Holes sediments

Sediments from the Baja California Continental Margin Transect - Sites 474 and 476 - showed small amounts of C2-C8 hydrocarbons and functionalized compounds (alkenes) typical of organic-rich, Recent, cold (<30°C) marine sediments. In contrast, some samples from Sites 477, 478, 479, and Hole 481A in the Guaymas Basin, an active spreading center, showed the characteristics of thermally generated hydrocarbons. These include an increase (sometimes exponential) in amount and diversity of C2-C8 hydrocarbons and a decrease in alkenes in more thermally mature sediments. The results indicate that the injection of basaltic sills has minimal effect on C2-C8 hydrocarbon generation except in the immediate vicinity of the sill. The absence of light hydrocarbons close to the hottest sills suggests that the compounds distill away as they are formed in these areas of very active hydrothermal circulation. A sample of young sediment exposed to very high temperatures (>300°C) from deeper thermal sources at the hottest site, 477, showed a very limited hydrocarbon distribution, including primarily ethane, benzene, and toluene, together with smaller amounts of propane and butane.

Petrology and geochemistry of the basement boninite series of ODP Site 125-786

The 720 m of igneous basement that was penetrated at Site 786 of Ocean Drilling Program Leg 125 consists of boninite-series volcanics. Bronzite andesites dominate the lithology and primitive magmas of high-Ca, intermediate-Ca, and low-Ca boninite are present in subordinate amounts. Sparsely phyric boninites typically contain olivine and orthopyroxene phenocrysts with Mg numbers [= Mg/(Mg + Fe) in moles] between 86% and 87%. Their high whole-rock Mg numbers, and the absence of zonation in the phenocrysts, imply equilibration at temperatures probably between 1200° and 1250°C, and 20° to 50°C below their liquidus. Equilibrium olivine and orthopyroxene have identical Mg numbers, and Mg/Fe partitioning between these minerals and the melt thus can be described with a single Kd. The invariably phenocryst-rich bronzite andesites contain Plagioclase that has spectacular zoning and mafic phases that can be as magnesian as those of the boninite parent. The most evolved melts are rhyolites with hypersthene, Plagioclase (An50), and magnetite. Eruption temperatures for the rhyolites are estimated at about 1000°C. Some magmas contain ferroactinolite in the groundmass, which is most likely a secondary, low-temperature phase. The locally large contrasts in degree of alteration are consistent with multiple episodes of magmatic activity. However, all igneous events produced boninite volcanics. Only the first, the edifice-building episode, gave rise to differentiated magmas. Differentiation of parental boninites took place by limited fractional crystallization, producing bronzite andesites. The erupted andesites, dacites and rhyolites are filter pressed extracts from these bronzite andesite magmas, which, as a result, have accumulated crystals. Subsequent younger igneous events produced high-Ca and intermediate-Ca boninites which intruded as dikes and sills throughout the basement sequence. The mineralogy of the dikes and sills reflects variable degrees of subliquidus cooling of the magma before emplacement.

Geochemistry of fallout tephra from ODP Hole 125-782A

Very rare, halogen-rich andesite melt inclusions (HRA) in bytownitic plagioclase phenocrysts (An89-90) from tephra fallout of the Izu arc volcanic front (Izu VF) provide new insights into the processes of fluid release from slab trenchward to the volcanic front in a cool subduction zone. These HRA are markedly enriched in Cl, F and Li - by factors of up to 8 (Cl, F) and 1.5 (Li) - but indistinguishable with respect to the fluid-mobile large-ion lithophile elements (LILE; K, Sr, Rb, Cs, Ba, Pb, U), rare earths (REE) or high field strength elements (HFSE) from the low-K tholeiitic magmas of the Izu VF. We suggest that the chemical signature of the HRA reflects the presence of a fluid in the mantle source that originated from the serpentinized mantle peridotite above the metacrust. This "wedge serpentinite" presumably formed by fluid infiltration beneath the forearc and was subsequently down-dragged with the slab to arc front depths. The combined evidence from the Izu VF (?110 km above slab) and the outer forearc serpentinite seamounts (~25 to 30 km above slab) suggests that the slab flux of B and Cl is highest beneath the forearc, and decreases with increasing slab depths. In contrast, the slab flux of Li is minor beneath the forearc, but increases with depth. Fluorine may behave similarly to Li, whereas the fluid-mobile LILE appear to be largely retained in the slab trenchward from the Izu VF. Consequently, the chemical signatures of both Izu trench sediments and basaltic rocks appear preserved until arc front depths.