HR: 0800h
AN: V21D-0762    [Abstracts]
TI: The Lateral Change of Archean Hydrothermal System, The Dixon Island Formation in the Coastal Pilbara Terrane, Australia
AU: * Koge, S
EM: kohge@geo.kyushu-u.ac.jp
AF: Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka, 812-8581, Japan
AU: Kiyokawa, S
EM: kiyokawa@geo.kyushu-u.ac.jp
AF: Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka, 812-8581, Japan
AU: Ito, T
EM: tito@mx.ibaraki.ac.jp
AF: Ibaraki University, Bunkyou 2-1-1, Mito, 310-8512, Japan
AU: Ikehara, M
EM: ikehara@cc.kochi-u.ac.jp
AF: Kochi University, Mononobe Otsu 200, Nannkoku, 783-8502, Japan
AU: Kitajima, F
EM: Kitajima@geo.kyushu-u.ac.jp
AF: Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka, 812-8581, Japan
AU: Yamaguchi, K E
EM: kosei@jamstec.go.jp
AF: JAMSTEC, natushima 2-15, Yokosuka, 237-0061, Japan
AB: The 3.2 Ga Dixon Island Formation in the coastal Pilbara terrane, Western Australia, is one of the most complete and best-preserved examples of Mesoarchean submarine hydrothermal system. Its continuous section is well exposed along the northern coastline of the Dixon Island. Based on our detailed geological mapping, the 6km- wide outcrop could be subdivided into six blocks, namely DX-A to DX-F, which were defined by dextral strike-slip faults. The 400m-thick Dixon Island Formation is composed of, in an ascending order, Komatiite-Rhyolite tuff (KR), Black Chert (BC), and Varicolored Chert (VC) Members. To explore lateral variations of potential biological activity in submarine hydrothermal system and depositional environments, we focus on the organic carbon (Corg) bearing BC member and perform petrographic investigation of thin sections under microscope and measured the Corg contents, Corg isotope compositions, and rare earth elements (REEs) abundance. The massive and laminated black cherts in DX-B are thicker than those in DX-F. The amount of massive dark green chert laterally increases from DX-B to DX-F. The shale-normalized REE+Y patterns of both massive and laminated cherts similarly show overall enrichment of heavy REEs and positive Eu anomalies. The Corg contents are generally very low (<0.05 wt.%) throughout the area. Those of massive black chert in the BC Member slightly decrease laterally from west to east (DX-B to DX-F), ranging from 0.08 wt.% to 0.02 wt.%. Their Corg isotopic compositions decrease from the lower -25 to -30‰ to middle part -30 to -35‰, and increase to the upper part -20 to -30‰. Some thin layers in the BC Member have slightly high Corg contents (0.10%) with very low Corg isotopic compositions -45 to -40‰. However, the average Corg isotopic compositions are similar in DX-A through DX-E -33 ‰ for DX-A and DX-B, 31‰ for DX-C through E. DX-F is an exception; it has less negative 26.6‰ values. Based on the field observation and obtained geochemical data set, we suggest that the chert units with similar Corg isotopic compositions have carbonaceous matters of similar origin. We also suggest that the thickest unit DX-B with positive Eu anomaly represents sediments that were most substantially influenced by submarine hydrothermal activity. Such hydrothermal activity most likely influenced biological activity, which resulted in systematically different Corg contents and Corg isotopic compositions both in lateral and vertical extent.
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting