Basalt analyses from different DSDP Holes in the Mozambique Basin and Ridge and from the Southwest Indian Ridge

Basalts from DSDP Sites 248, 249, 250 and 251 in the southwestern Indian Ocean formed in a complex tectonic region affected by the separation of Africa and South America. The different ages and variable geochemical features of these DSDP basalts probably reflect this tectonic complexity. For example, Site 251 on the flanks of the Southwest Indian Ridge is represented by normal MORB which probably originated at the Southwest Indian Ridge. Site 250 in the Mozambique Basin includes an older incompatible- element enriched unit which may represent basalt associated with the Prince Edward Fracture Zone; the upper unit is normal MORB. Basalts at Site 248 also in the Mozambique Basin are geochemically very unlike MORB and have strong continental affinities; they are also comparable in age to some of the continental Karroo basalts. They appear to be related to a subcontinental mantle source or to contamination by continental basement associated with the tectonic elevation of the Mozambique Ridge. Basalts from Site 249 on the Mozambique Ridge are relatively weathered but appear to be normal MORB. Their age, location, and composition are consistent with their origin at an early Cretaceous rift which has been postulated to have separated the Falkland Plateau from the Mozambique Ridge.

Petrology and geochemistry at DSDP Site 89-462

The 16 samples of Deep Sea Drilling Project (DSDP) Leg 89 basalts that we analyzed for whole rock major and trace elements and for mineralogic compositions are identical to some of the basalts recovered during Leg 61. Leg 89 samples are mostly olivine-plagioclase-clinopyroxene sparsely phyric basalts and exhibit a wide variety of textures. These basalts have lower TiO2 at a given Mg/(Mg+Fe2+)*100 than MORB (midocean ridge basalt). We recognize three major chemical types of basalts in the Nauru Basin. We believe that different degrees of partial melting, modified by fractional crystallization and possibly by magma mixing at shallow depths, can explain the chemical differences among the three groups. This petrogenetic model is consistent with the observed downhole chemical-chronostratigraphic relations of the samples. New 87Sr/86Sr and U3Nd/144Nd analyses of basalt samples from DSDP Site 462 indicate that the Nauru Basin igneous complex is within the Sr-Nd isotopic range of ocean island basalt. Thus the Nauru Basin igneous complex resembles MORB in many aspects of its chemistry, morphology, and secondary alteration patterns (Larson, Schlanger, et al., 1981), but not in its isotopic characteristics. If it were not for the unambiguous evidence that the Nauru Basin complex was erupted off-ridge, the complex could easily be interpreted as normal oceanic layer 2. For this reason, we speculate that the Nauru Basin igneous complex was produced in an oceanic riftlike environment when multiple, fast-propagating rifts were formed during the fast seafloor spreading episode in the Cretaceous.

Geochemistry and stratigraphy on sediment cores from Leg198 and Leg 208

The first complete cyclic sedimentary successions for the early Paleogene from drilling multiple holes have been retrieved during two ODP expeditions: Leg 198 (Shatsky Rise, NW Pacific Ocean) and Leg 208 (Walvis Ridge, SE Atlantic Ocean). These new records allow us to construct a comprehensive astronomically calibrated stratigraphic framework with an unprecedented accuracy for both the Atlantic and the Pacific Oceans covering the entire Paleocene epoch based on the identification of the stable long-eccentricity cycle (405-kyr). High resolution X-ray fluorescence (XRF) core scanner and non-destructive core logging data from Sites 1209 through1211 (Leg 198) and Sites 1262, 1267 (Leg 208) are the basis for such a robust chronostratigraphy. Former investigated marine (ODP Sites 1001 and 1051) and land-based (e.g., Zumaia) sections have been integrated as well. The high-fidelity chronology is the prerequisite for deciphering mechanisms in relation to prominent transient climatic events as well as completely new insights into Greenhouse climate variability in the early Paleogene. We demonstrate that the Paleocene epoch covers 24 long eccentricity cycles. We also show that no definite absolute age datums for the K/Pg boundary or the Paleocene - Eocene Thermal Maximum (PETM) can be provided by now, because of still existing uncertainties in orbital solutions and radiometric dating. However, we provide two options for tuning of the Paleocene which are only offset by 405-kyr. Our orbitally calibrated integrated Leg 208 magnetostratigraphy is used to revise the Geomagnetic Polarity Time Scale (GPTS) for Chron C29 to C25. We established a high-resolution calcareous nannofossil biostratigraphy for the South Atlantic which allows a much more detailed relative scaling of stages with biozones. The re-evaluation of the South Atlantic spreading rate model features higher frequent oscillations in spreading rates for magnetochron C28r, C27n, and C26n.

Geochemistry and petrology of ODP Leg 121 basalts

Basement lavas from Sites 756, 757, and 758 on Ninetyeast Ridge are tholeiitic basalts. Lavas from Sites 756 and 757 appear to be subaerial eruptives, but the lowermost flows from Hole 758A are pillow lavas. In contrast to the compositional variation during the waning stages of Hawaiian volcanism, no alkalic lavas have been recovered from Ninetyeast Ridge and highly evolved lavas were recovered from only one of seven drill sites (DSDP Site 214). All lavas from Site 758 have relatively high MgO contents (8-10 wt%), and they are less evolved than lavas from Sites 756 and 757. Although abundances of alkali metals in these Ninetyeast Ridge basalts were significantly modified by postmagmatic alteration, abundances of other elements reflect magmatic processes. At Site 757 most of the lavas are Plagioclase cumulates, but lava compositions require two compositionally distinct, AhCb-rich parental magmas, perhaps segregated at relatively low mantle pressures. In addition, at both Sites 756 and 758 more than one compositionally distinct parental magma is required. The compositions of these Ninetyeast Ridge lavas, especially those from Site 758, require a source component with a depleted composition; specifically, the abundance ratios Th/Ta, Th/La, Ba/Nb, Ba/La, and La/Ce in these lavas are generally less than the ratios inferred for primitive mantle. Lavas from Ninetyeast Ridge and the Kerguelen Archipelago have very different chondrite-normalized REE patterns, with lower light REE/heavy REE (LREE/HREE) ratios in lavas from Ninetyeast Ridge. However, lavas from Sites 757 and 758 have Pb isotope ratios that overlap with the field defined by lavas from the Kerguelen Archipelago (Weis and Frey, this volume). Therefore, these Ninetyeast Ridge lavas contain more of a component that is relatively depleted in LREE and other highly incompatible elements, but have similar amounts of the component that controls radiogenic Pb isotopes. A model involving mixing between components related to a depleted source and an enriched plume source has been proposed for the oldest Kerguelen Archipelago basalts and Ninetyeast Ridge lavas. Although the incompatible element characteristics of the Ninetyeast Ridge lavas are intermediate between depleted MORB and Kerguelen Archipelago basalts, these data are not consistent with a simple two-component mixing process. A more complex model is required.

Geochemistry and paleontology of sediment core GeoB3359-3

A geochemical and paleontological reconstruction of paleoproductivity, upwelling intensity and sea surface temperature (SST) off central Chile at 35°S (GeoB3359-3) reveals marked changes from the Last Glacial Maximum (LGM) through the Early Holocene. Surface-water productivity was determined by the interaction between the atmospheric (the Southern Westerlies) and oceanographic (the Antarctic Circumpolar Current, ACC) systems from the LGM through early Termination I (TI). The northward shift of the climate zones during the LGM brought the ACC, as the main macronutrient source, closer to the GeoB3359-3, SST lowered, and surface water productivity and accumulation rates of biogenic components enhanced. With the poleward return of the Southern Westerlies and the ACC, the subtropical high-pressure system became the dominant atmospheric component southward till 35°S during the late TI and Early Holocene and caused surface water productivity to increase through enhanced upwelling.

Petrology and geochemistry of sedimentary Unit III of ODP Hole 135-840B

The sedimentary sequence recovered at Site 840, on the Tonga frontal-arc platform, is 597.3 m thick and is subdivided into three lithostratigraphic units. The lowermost, late Miocene Unit III is 336.8 m thick and consists of a sequence of volcaniclastic mass-flow deposits (predominantly turbidites) interbedded with pelagic/hemipelagic deposits. Unit III was deposited in the forearc basin of the Lau volcanic arc, probably on a slope dominated by mass flows that built eastward from the ridge front and across the forearc. Upward through the unit a thinning and fining of individual turbidites takes place, interpreted to reflect a reduced sediment supply and a change from large to smaller flows. Decreasing volcanic activity with time is inferred from a decrease in coarse-grained volcaniclastic content in the upper part of the unit. The majority of the turbidites show the typical Bouma-type divisions, although both high- and low-density turbidity currents are inferred. High-density turbidity currents were especially common in the lower part of the unit. Geochemical analyses of detrital glass lie mainly in the low-K tholeiite field with a compositional range from basalt to rhyolite. A coherent igneous trend indicates derivation from a single volcanic source. This source was probably situated on the rifted part of the Lau-Tonga Ridge, within the present Lau backarc basin. The initial opening of the Lau Basin may have been around 6.0 m.y. ago. The onset of more extensive rifting, approximately 5.6 m.y. ago, is reflected in an increase in the silica content of volcanic glass. At the boundary toward Unit II, at approximately 5.25 Ma, an influx of thicker bedded and coarser grained volcaniclastic material is interpreted to reflect increasing volcanism and tectonism during the final breakup of the Lau-Tonga Ridge.