The Dixon Island Formation of the Pilbara terrane, Western Australia, extends from Cleaverville Beach to the Dixon Island coast, and is the only example worldwide of a coastal outcrop of a 3.2-3.1Ga low-grade greenstone belt. The Dixon Island Formation was situated in an immature island arc setting and comprises siliceous, carbonaceous deep-water sediments that contain evidence for hydrothermal and microbial activity. The extensive outcrop along the coastline makes it possible to examine in detail the characteristics of Mesoarchean sedimentation in a hydrothermal environment. This study focuses on a continuously exposed carbonaceous, black chert succession in the central part of the northern coastline outcrop on Dixon Island. At this site, a 20-m-thick, carbonaceous, black chert sequence conformably overlies basement rocks of highly altered komatiite-rhyolite tuffs. The black chert sequence formed well-bedded black chert with carbonaceous peloidal matter and fragmented grains, and the sequence is homogeneous and finely laminated. In this sequence, evidence of low-temperature hydrothermal fluid, sediments and alteration stractures are well preserved in the lowermost section, indicating that high levels of hydrothermal activity occurred on the ocean floor during deposition. In particular, swarms of black chert veins provide evidence of post-volcanic hydrothermal activity that released organic matter and silica to the ocean. The carbonaceous peloidal textures and δ13Corg values of sediments located just above the basement, which hosts the vein swarms, suggest that the veins were the conduits for hydrothermal fluid which contained organic-rich silica material and that flowed onto the seafloor to form the homogeneous carbonaceous cherts along with hydrothermal-related sediment. The δ13Corg values of organic matter in the black cherts range from -42‰ to -22‰ (average=-31.9‰ n=313). Lighter δ13C values (-35‰ to -42‰) characterize carbonaceous laminated black chert located ~5m above the basement, where biogenic structures (e.g. biomat bed, microfossil structures) are found. The lighter δ13C values might be related to methanotrophic micro-organic activity within the sediments during hydrothermal activity. In summary, we reconstructed the sedimentary environment upon a Mesoarchean hydrothermal ocean floor that was a site of microbial activity and local methanogenesis.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology