HR: 0800h
AN: B31B-0997    [Abstracts]
TI: Middle Archean island arc volcano-hydrothermal sequence: 3.2-Ga Dixon Island Formation, coastal Pilbara terrane, Australia
AU: * Kiyokawa, S
EM: kiyokawa@geo.kyushu-u.ac.jp
AF: Kyushu University Earth and Planetary, 6-10-1 Hakozaki Higashiku Fukuoka Japan, Fukuoka, 812-8581 Japan
AU: Katagami, A
EM: honey-candies.0704@docomo.ne.jp
AF: Faculty of Education, Ibaraki UNIV, 2-1-1 Bunkyo Mito-city Ibaraki Japan, Ibaraki, 310-0056 Japan
AU: Ito, T
EM: tito@mx.ibaraki.ac.jp
AF: Faculty of Education, Ibaraki UNIV, 2-1-1 Bunkyo Mito-city Ibaraki Japan, Ibaraki, 310-0056 Japan
AU: Ikehara, M
EM: ikehara@cc.kochi-u.ac.jp
AF: Marine Core Research Center, Kochi U, 200 monobe Nankoku-shi Kochi Japan, KOCHI, 783-8502 Japan
AU: Kitajima, F
EM: kitajima@geo.kyushu-u.ac.jp
AF: Kyushu University Earth and Planetary, 6-10-1 Hakozaki Higashiku Fukuoka Japan, Fukuoka, 812-8581 Japan
AB: The 3.2-Ga Dixon Island Formation in the Cleaverville Group of the coastal Pilbara terrane, Australia, is one of the most complete and best-preserved examples of middle Archean oceanic stratigraphy. Field observations and geochemical evidence suggest that this formation contains a low-temperature hydrothermal-vent system with a biogenic microbial colony from the Archean ocean. The Dixon Island Formation is approximately 350 m thick and consists of the Rhyolite Tuff, Black Chert and Varicolored Chert members, in ascending order. The Rhyolite Tuff Member contains many vein swarms, such as fine quartz vein and two black-chert veins with in highly altered rhyolite tuff layers. This vein rich and highly altered vein zones are identified as an underground bypass zone for circulating hydrothermal fluid. The Black Chert Member, which is 10 - 15 m thick, is composed of massive black chert, laminated black chert, dark greenish siliceous shale, stromatolite-like biomat bed and tuffaceous laminated chert. The absence of detrital sediment of continental origin and the many vein injections imply that this sedimentary facies represents a pelagic hydrothermal environment at about 500 - 2000 m in paleodepth, and may have been on the slope of an immature island arc. More then 500 samples of detail chemical anarysis from black chert veins and black chert bed suggest that the total organic carbon (TOC) value of massive black chert in the lower part of the Black Chert Member is higher (TOC=0.15-0.45%) than that of the overlying laminated chert section (TOC=0.02-0.15%) and the black chert vein (TOC=0.1-0.13). The carbon isotope (delta13C) values of this lithology (-33 - -27 per mil) are also lighter than for the black-chert veins (-29--26 per mill) and the laminated black chert in the upper part of the Black Chert Member and the Vari-colored Chert Member (-27 - -13 per mil). Especially, -40 per mill carbon isotope identified near the biomat beds. These evidences suggest that the carbonaceous grains bearing massive black chert in the lower part of the Black Chert Member is identified as directory from the black chert vein. On the other hand, biogenic materials, biomat bed and very low carbon isotope suggest the biogenic activity formed above a low-temperature hydrothermal vent. The microbial colony may have been rapidly fossilized by silicification related to hydrothermal activity. Laminated black chert in the upper part of the Black Chert and the Varicolored Chert members may have formed by cyanobacterial sedimentation from the ocean surface.
DE: 0444 Evolutionary geobiology
DE: 5225 Early environment of Earth
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
DE: 9330 Australia
DE: 9623 Archean
SC: Biogeosciences [B]
MN: Fall Meeting 2005