HR: 0830h
AN: B21D-0742    [PDF]
TI: An example of volcano-hydrothermal sequence in Middle Archean: 3.2-Ga Dixon Island Formation, coastal Pilbara terrane, Australia.
AU: * Kiyokawa, S
EM: kiyokawa@geo.kyushu-u.ac.jp
AF: Shoichi kiyokawa, Department of Earth and Planetary Science, Kyushu University, 6-10-1 Hakozaki Higashiku, Fukuoka, 812-8581 Japan
AU: Ito, T
EM: tito@mx.ibaraki.ac.jp
AF: Takashi Ito, Faculty of Education, Ibaraki University, 2-1-1 Bunkyo, mito, 310-8512 Japan
AU: Ikehara, M
EM: ikehara@cc.kochi-u.ac.jp
AF: Minoru Ikeahara, Center for Advanced Marine Core Research, Kochi University, B200 Monobe?Nankoku, Kochi, 783-8502
AU: kitajima, t
EM: kitajima@geo.kyushu-u.ac.jp
AF: Shoichi kiyokawa, Department of Earth and Planetary Science, Kyushu University, 6-10-1 Hakozaki Higashiku, Fukuoka, 812-8581 Japan
AB: The 3.2-Ga Dixon Island Formation in the coastal Pilbara terrane, Australia, is preserves Middle Archean hydrothermal stratigraphy and contains microbial material. Many black-chert vein swarms imply intensive low-temperature hydrothermal activity during deposition of black chert. Absence of detrital sediments of continental origin in this formation implies that this sedimentary facies represents a hydrothermal environment at about 500~2000 m in paleodepth in an immature island arc setting. Microbial material has been preserved well in the Black Chert Member, which is composed of massive black chert, laminated black chert, dark greenish siliceous shale and tuffaceous laminated chert. The massive black chert has carbonaceous peloids (0.3 mm~2 mm in diameter) similar to those in the black chert veins. The massive black chert contains wriggle-, rod- and dendrite-shaped bacterial material. Total organic carbon (TOC) in the black chert and black chert veins varies within 0.05 ~ 0.16% (average 0.1%) and the carbon isotope (delta 13C) values of these rocks are -32~ -27 per mil (average 30 per mill). This evidence suggests that the carbonaceous grains and bacteria-shaped material in the black cherts are biogenic and formed close to a hydrothermal vent system. Field observations and geochemical evidence suggest that the Dixon Island Formation is one of the best examples of a biogenic microbial colony near hydrothermal vents in the Archean ocean. The microbial colony might have been quickly fossilized by silicification related to hydrothermal activity.
DE: 8102 Continental contractional orogenic belts
DE: 8125 Evolution of the Earth
DE: 8130 Heat generation and transport
DE: 9330 Australia
DE: 9619 Precambrian
SC: Biogeosciences [B]
MN: 2003 Fall Meeting