Geochemistry at DSDP Leg 73 Holes

In the South Atlantic, at Sites 519 to 523, the dissolution of calcareous oozes ended in the formation of red clays rich in iron and manganese. The early authigenesis of manganese oxides and clays is described in Miocene marly calcareous oozes. The mineralogical and geochemical influences of basaltic basement weathering are shown by the occurrence of palagonite, authigenic clays, and oxides in the basal sediments. The development of red clay facies can be inhibited by local topographic and paleoceanographic changes, as at Site 520.

Nature, chemistry, and origin of late Cenozoic megascopic tephras in Legs 89 and 90 Cores from the Southwest Pacific

Several thin (1-10 cm) megascopic vitric tephras occur in the late Cenozoic calcareous oozes on Lord Howe Rise in the Tasman Sea and off eastern South Island, New Zealand. Of the 18 tephras analyzed 15 are silicic (75-78% SiO2) with abundant clear glass shards and a biotite ± hypersthene ± green hornblende ferromagnesian mineralogy. The Neogene silicic tephras were derived from the now-extinct Coromandel volcanic area in New Zealand, and the Quaternary ones from the presently active Central Volcanic Region of New Zealand. On the basis of glass chemistry and age, several of the Quaternary tephras are probably correlatives, and at least two can be matched to the major on-land Mt. Curl tephra (-0.25 m.y.). The occurrence of correlative silicic tephras both northwest and southeast of New Zealand may result from particularly violent eruptions, the ash below and above an altitude of -20 km being dispersed in opposite directions toward the Pacific Ocean and Tasman Sea, respectively. Ash drifting eastward into the southeasterly trade wind belt off northeastern New Zealand could also be carried into the central and northern Tasman Sea. Three megascopic tephras consist of altered basic shards and common labradorite crystals. They record Neogene explosive basaltic to andesitic activity from nearby ocean island or ridge sources in the Ontong-Java Plateau and Vanuatu regions. The megascopic tephras are a very incomplete and biased record of late Cenozoic explosive volcanism in the southwest Pacific because the innumerable, thin, green argillaceous layers in the cores (Gardner et al., this volume) probably represent devitrified intermediate to basic tephras derived mainly from oceanic arc volcanism along the Pacific/Australia plate boundary. In contrast to the New Zealand-derived silicic glass shards, the preservation potential of these more basic shards in Leg 90 calcareous sediments was low.

Grain size composition and geochemistry of volcaniclastic sediments in the Lau Basin

Early Pliocene to Pleistocene volcaniclastic sediments recovered during Ocean Drilling Program Leg 135 from Sites 834 to 839 in the Lau Basin show a wide range of chemical and mineralogical compositions extending the spectrum previously known from the Lau Basin, Lau Ridge and Tofua Arc. The following major types of volcaniclastics have been distinguished: (1) primary fallout ashes originating from eruptions on land, (2) epiclastic deposits that resulted from subaerial and submarine eruptions, (3) subaqueous fallout and pyroclastic flow deposits resulting from explosive submarine eruptions, and (4) hyaloclastites resulting from mechanical fragmentation and spalling of chilled margins of submarine pillow tubes and sheet-lava flows. Vitric shards are mostly basaltic andesitic to rhyolitic and broadly follow two major trends in terms of K2O enrichment: a low-K series (LKS) with about 1 wt% K2O at 70 wt% SiO2, and a very low-K series (VLKS) with only about 0.5 wt% K2O at 70 wt% SiO2. Sites 834 and 835 on "old" backarc basin crust, >4.2 and 3.4 m.y. old, comprise LKS rhyolites >3.3 m.y. old. Calc-alkaline basaltic turbidites originating from the Lau Ridge flowed in at 3.3 Ma. In the period from 3.3 to 2.4 Ma basaltic andesitic to rhyolitic, fine-grained LKS and VLKS volcaniclastics were deposited by turbidity currents and subaerial fallout. Three thin, discrete fallout layers (2.4-3.2 m.y. old) with high-K calc-alkaline compositions probably erupted in New Zealand. Volcaniclastics from Site 836, all <0.6 m.y. old, make up 24% of the sediments and comprise local basaltic andesitic to andesitic hyaloclastites with low Ba/Zr ratios of 0.9 to 1.4 and polymict andesitic sediments with Ba/Zr ratios of up to 5.5, containing clasts altered to lower greenschist facies. In Sites 837-839, drilled on young crust (1.8-2.1 m.y. old), volcaniclastics make up 45%-64% of the total sediment. Glass compositions are often bimodal with a mafic and a rhyolitic population. Large-volume rhyolitic, silt- to lapilli-sized volcaniclastics are interpreted as pyroclastic flows from explosive eruptions on a seamount 25-50 km away from the sites. Ba/Zr ratios are 2 to 4, partially overlapping with some Lau Basin basement lavas that show an "arc" signature, and they can reach values >5 in thin volcaniclastic layers <0.6 m.y. old.

Prokaryotic growth and presence of metabolic products and contamination tracers in rocks samples from ODP Leg 187 sites

The microbial population in samples of basalt drilled from the north of the Australian Antarctic Discordance (AAD) during Ocean Drilling Program Leg 187 were studied using deoxyribonucleic acid (DNA)-based methods and culturing techniques. The results showed the presence of a microbial population characteristic for the basalt environment. DNA sequence analysis revealed that microbes grouping within the Actinobacteria, green nonsulfur bacteria, the Cytophaga/Flavobacterium/Bacteroides (CFB) group, the Bacillus/Clostridium group, and the beta and gamma subclasses of the Proteobacteria were present in the basalt samples collected. The most dominant phylogenetic group, both in terms of the number of sequences retrieved and the intensities of the DNA bands obtained with the denaturing gradient gel electrophoresis analysis, was the gamma Proteobacteria. Enrichment cultures showed phylogenetic affiliation with the Actinobacteria, the CFB group, the Bacillus/Clostridium group, and the alpha, beta, gamma, and epsilon subclasses of the Proteobacteria. Comparison of native and enriched samples showed that few of the microbes found in native basalt samples grew in the enrichment cultures. Only seven clusters, two clusters within each of the CFB and Bacillus/Clostridium groups and five clusters within the gamma Proteobacteria, contained sequences from both native and enriched basalt samples with significant similarity. Results from cultivation experiments showed the presence of the physiological groups of iron reducers and methane producers. The presence of the iron/manganese-reducing bacterium Shewanella was confirmed with DNA analysis. The results indicate that iron reducers and lithotrophic methanogenic Archaea are indigenous to the ocean crust basalt and that the methanogenic Archaea may be important primary producers in this basaltic environment.

Low-pressure melting relations of a basalt from ODP Hole 127-797C

One-atmosphere melting experiments, controlled to approximately the fayalite-magnetite-quartz oxygen buffer, performed on a basalt from Hole 797C crystallized olivine and plagioclase nearly simultaneously from about 1235°C and augite from about 1175°C. The liquid compositions indicate systematic trends of increasing FeO and TiO2 and decreasing Al2O3 with decreasing MgO. Experimental olivine compositions vary from Fo90 to Fo78, plagioclase from An79 to An67, and augite from En49 to En46. The KD value for the Fe2+ and Mg distribution between olivine and liquid is 0.31. The KD value for the distribution of Fetotal and Mg between augite and liquid averages 0.24. These KD values suggest experimental equilibrium. The KD values for Na and Ca distribution between plagioclase and liquid range between 0.55 and 0.99 and are dependent on crystallization temperature. Projected on pseudoternary basaltic phase diagrams, the liquid line of descent moves toward increasing quartz normative compositions, revealing a typical tholeiitic crystallization trend with marked Fe and Ti enrichments. Such enrichments are a reflection of the dominance of plagioclase in the crystallizing assemblage. The experimental results can explain the marked Fe- and Ti-enrichment trends observed for the sills of the lower part of Hole 797C, but have no direct bearing on the origin of the relatively evolved high-Al basalts of Hole 794C.