(Table T1) Magnetic properties of MORB from ODP Leg 187 sites

The core samples of mid-ocean-ridge basalts (including Indian and Pacific type) recovered from the Southeast Indian Ridge (SEIR) area near the Australian Antarctic Discordance during Ocean Drilling Program Leg 187 were studied using rock magnetism, mineralogy, and petrography methods. On the basis of thermomagnetic analyses and low-temperature magnetometry, the dominant magnetic carrier in most of the basalt samples (pillow basalts) is characterized as titanomaghemite, which presumably formed by low-temperature oxidation of primary titanomagnetite. Some samples from unaltered massive basalts contain nearly unoxidized titanomagnetite as the main magnetic mineral. A metadiabase sample showing greenschist facies metamorphism contains magnetic minerals dominated by magnetite. The pillow basalts contain titanomaghemite ranging from stable single-domain to pseudosingle-domain (PSD) grains, and the majority are characterized by a single stable component of remanence. The massive basalts show hysteresis features of larger PSD grains and contain a very low coercivity remanence. The values of natural remanent magnetization (NRM) of the samples in this SEIR area are on the same order as those of other oceanic ridge basalts. They show a general decreasing trend of NRM with increasing crust age. However, the values of NRM show no correlation either with the tectonic zonations (Zone A vs. Zone B) or with the mantle provinces (Pacific vs. Indian types).

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.

Bulk geochemistry, mass change and alteration intensity of basalts from ODP Leg 187 sites and survey dredges

Shipboard studies during Ocean Drilling Program Leg 187 (Australian Antarctic Discordance, AAD) suggested that there was no discernible coincidence between the interpreted age of rocks recovered and the intensity of alteration observed. Samples from the oldest sites occupied appeared to exhibit the least overall effects of alteration, and the intensity of alteration varied from site to site. Previous investigations of low-temperature alteration in oceanic basement samples have been restricted by the myopic perspective provided by single drill holes or dredge collections. Combining core samples from Leg 187 and dredge samples from the AAD collection at Oregon State University (USA) offers the unique opportunity to investigate mineral and bulk chemical changes attending alteration of basalt over a range of ages from 0 to 28 Ma. Results of this research indicate that there is a general increase in the intensity of alteration as the basalts age and mosve off axis, but that this relationship is somewhat veiled by the dominating control on alteration intensity dictated by variations in permeability.