Chemistry of magmatic samples of ODP Holes 134-832B and 134-833B

New petrographic and compositional data were reported for 143 samples of core recovered from Sites 832 and 833 during Ocean Drilling Program (ODP) Leg 134. Site 832 is located in the center and Site 833 is on the eastern edge of the North Aoba Basin, in the central part of the New Hebrides Island Arc. This basin is bounded on the east (Espiritu Santo and Malakula islands) and west (Pentecost and Maewo islands) by uplifted volcano-sedimentary ridges associated with collision of the d'Entrecasteaux Zone west of the arc. The currently active Central Belt volcanic front extends through the center of this basin and includes the shield volcanoes of Aoba, Ambrym, and Santa Maria islands. The oldest rocks recovered by drilling are the lithostratigraphic Unit VII Middle Miocene volcanic breccias in Hole 832B. Lava clasts are basaltic to andesitic, and the dominant phenocryst assemblage is plagioclase + augite + orthopyroxene + olivine. These clasts characteristically contain orthopyroxene, and show a low to medium K calc-alkaline differentiation trend. They are tentatively correlated with poorly documented Miocene calc-alkaline lavas and intrusives on adjacent Espiritu Santo Island, although this correlation demands that the measured K-Ar of 5.66 Ma for one clast is too young, due to alteration and Ar loss. Lava clasts in the Hole 832B Pliocene-Pleistocene sequence are mainly ankaramite or augite-rich basalt and basaltic andesite; two of the most evolved andesites have hornblende phenocrysts. These lavas vary from medium- to high-K compositions and are derived from a spectrum of parental magmas for which their LILE and HFSE contents show a broad inverse correlation with SiO2 contents. We hypothesize that this spectrum results from partial melting of an essentially similar mantle source, with the low-SiO2 high HFSE melts derived by lower degrees of partial melting probably at higher pressures than the high SiO2, low HFSE magmas. This same spectrum of compositions occurs on the adjacent Central Chain volcanoes of Aoba and Santa Maria, although the relatively high-HFSE series is known only from Aoba. Late Pliocene to Pleistocene lava breccias in Hole 833B contain volcanic clasts including ankaramite and augite + olivine + plagioclase-phyric basalt and rare hornblende andesite. These clasts are low-K compositions with flat REE patterns and have geochemical affinities quite different from those recovered from the central part of the basin (Hole 832B). Compositionally very similar lavas occur on Merelava volcano, 80 km north of Site 833, which sits on the edge of the juvenile Northern (Jean Charcot) Trough backarc basin that has been rifting the northern part of the New Hebrides Island Arc since 2-3 Ma. The basal sedimentary rocks in Hole 833B are intruded by a series of Middle Pliocene plagioclase + augite +/- olivine-phyric sills with characteristically high-K evolved basalt to andesite compositions, transitional to shoshonite. These are compositionally correlated with, though ~3 m.y. older than, the high-HFSE series described from Aoba. The calc-alkaline clasts in Unit VII of Hole 832B, correlated with similar lavas of Espiritu Santo Island further west, presumably were erupted before subduction polarity reversal perhaps 6-10 Ma. All other samples are younger than subduction reversal and were generated above the currently subduction slab. The preponderance in the North Aoba Basin and adjacent Central Chain islands of relatively high-K basaltic samples, some with transitional alkaline compositions, may reflect a response to collision of the d'Entrecasteaux Zone with the arc some 2-4 Ma. This may have modified the thermal structure of the subduction zone, driving magma generation processes to deeper levels than are present normally along the reminder of the New Hebrides Island Arc.

Thecosomata in late Quternary deposits of ODP Holes 134-830A and 134-831A

We undertook a quantitative study of Thecosomata shells (pelagic gastropods) and their remains in Quaternary foraminiferal oozes deposited on the tilted calcareous platform of the Bougainville Guyot (Hole 831 A), and in the late Quaternary volcanic siltstones, claystones and sandy interbeds on the upper forearc slope of the central New Hebrides Island Arc (Hole 830A). The distribution of the species is based on the identification of adult shells, juvenile stages, protoconchs, and characteristic shell fragments. By studying thecosomatous shells using a scanning electron microscope (SEM), we were able to specify the fine microstructure of the coiled Limacina inflata and compare it with the rod-type crossed-lamellar structure of some other Limacina species, as well as with the helical structure of the Cavoliniidae.

Cenozoic benthic foraminifers from ODP Leg 134 holes

This paper discusses the Paleobathymetric and paleoenvironmental history of the New Hebrides Island Arc and North d'Entrecasteaux Ridge during Cenozoic time based on benthic foraminiferal and sedimentological data. Oligocene and Pliocene to Pleistocene benthic foraminiferal assemblages from Sites 827, 828, 829, and 832 of Ocean Drilling Program (ODP) Leg 134 (Vanuatu) are examined by means of Q-mode factor analysis. The results of this analysis recognize the following bathymetrically significant benthic foraminiferal biofacies: (1) Globocassidulina subglobosa biofacies and Bulimina aculeata-Bolivinita quadrilatera biofacies representing the upper bathyal zone (600-1500 m); (2) Gavelinopsis praegeri-Cibicides wuellerstorfi biofacies, indicating the Pacific Intermediate Water (water depth between 1500 and 2400 m); (3) Tosaia hanzawai-Globocassidulina muloccensis biofacies, Valvulineria gunjii biofacies, and the Melonis barleeanus-Melonis sphaeroides biofacies, which characterize the lower bathyal zone; (4) the Nuttallides umbonifera biofacies, which characterizes the interval between the lysocline (approximately 3500 m) and the carbonate compensation depth (approximately 4500 m); and (5) the Rhabdammina abyssorum biofacies representing the abyssal zone below the carbonate compensation depth. Benthic foraminiferal patterns are used to construct Paleobathymetric and paleogeographic profiles of the New Hebrides Island Arc and North d'Entrecasteaux Ridge for the following age boundaries: late Miocene/Pliocene, early/late Pliocene, Pliocene/Pleistocene, and Pleistocene/Holocene.

Geochemistry of pore fluids from ODP Leg 134 sites

Depending on the temperature and the extent of diagenetic alteration of fluid chemistry, fluid flow at convergent margins may transfer important quantities of heat and mass between the crust and seawater, thereby influencing global mass, isotopic and heat budgets. In the North Aoba Basin, an intra-arc basin located at the New Hebrides Island Arc, alteration of volcanic ash to clay minerals and zeolites forms a CaCl2 brine, perhaps in less than 1 to 3 m.y. The brine results from an exchange of Ca for Na, K, and Mg, and an increase in Cl concentrations to a maximum of 1241 mM. The Cl increase is partly due to the transfer of H2O from the pore fluid into authigenic minerals, but water mass balances, d18O-Cl correlations, and Br/Cl ratios suggest that there is a source of Cl in the sediments. Concentration profiles indicate that Li is transferred from the fluid to solid phase at depths <300 meters below seafloor (mbsf), but at greater depths it is transferred from the solid to fluid phase, at temperatures possibly as low as 25°C. In the accretionary wedge extensive fluid flow appears to be confined to highly faulted regions. Although Cl concentrations less than seawater value are common at convergent margins, the New Hebrides margin contains little low-Cl fluid. Br/Cl ratios suggest the low-Cl fluid is from dilution, and d18O values indicate the water may be derived from mineral dehydration and mixing with meteoric water. The New Hebrides margin exhibits few surface manifestations of venting (e.g., sulfide-oxidizing benthic biological communities, carbonate crusts, mud volcanoes) and thus fluid fluxes may be smaller than at many other margins.

Geochemistry of neritic carbonates of ODP Hole 134-831B

Drilling at Bougainville Guyot (Ocean Drilling Program Site 831), New Hebrides Island Arc during Leg 134 revealed that 727.5 m of carbonate overlies an andesite basement. The carbonate cap at Site 831 consists of 20 m of pelagic carbonate overlying 707.5 m of neritic carbonates. The neritic section consists of ~230 m of largely unaltered aragonite sediment that overlies ~497 m of totally calcitized limestone. The deeper portion of the calcitized interval has been pervasively altered by diagenesis. Prior to this study the age distribution of sediments at Bougainville Guyot was poorly known because age diagnostic fossils are sparsely and discontinuously distributed in the sequence. We have used Sr isotopes to provide temporal constraints on the deposition of carbonates at Site 831; these constraints are critical in reconstructing the vertical movement of Bougainville Guyot before its collision with the New Hebrides Island Arc. Overall, the chronostratigraphy of Bougainville Guyot can be subdivided into three intervals: (1) a Pleistocene interval (102.4 to 391.11 meters below sea floor [mbsf]); (2) a Miocene interval (410.31 to 669.53 mbsf); and (3) an Oligocene interval (678.83 to 727.50 mbsf). Strontium isotopic ages of samples increase with increasing depth in the carbonate sequence, except near the bottom of the sequence, where several samples exhibit a consistent reversed age vs. depth trend. Such age reversals are most likely the product of post-depositional rock-water interaction. Preliminary stable isotope data are consistent with diagenetic alteration in the marine and meteoric environments. Several abrupt decreases in d87Sr, and hence age, of sediments are recognized in the carbonate cap at Bougainville Guyot. These disconformities are most likely the product of subaerial exposure in response to relative sea-level fall. Indeed, Sr-isotope ages indicate that 2 to 9 m.y. of sediment deposition is missing across these d87Sr disconformities.