HR: 1340h
AN: B53B-0990    [Abstracts]
TI: A Possibility For Biological Origin Of Iron Oxides In The Marble Bar Chert, Pilbara Craton, Western Australia
AU: * Hoashi, M
EM: hoashi@chan.co.jp
AF: Kagoshima University, 1-21-30 Korimoto, Kagoshima, 890-0065 Japan
AU: Nedachi, N
EM: nedachi@sci.kagoshima-u.ac.jp
AF: Kagoshima University, 1-21-30 Korimoto, Kagoshima, 890-0065 Japan
AU: Suganuma, Y
EM: suga@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai Nakano, Tokyo, 164-8639 Japan
AU: Naraoka, H
EM: naraoka@cc.okayama-u.ac.jp
AF: Department of Earth Science, Faculty of Science, Okayama University, 1-1, Tsushima-Naka 3chome, Okayama, 700-8530 Japan
AU: Kato, Y
EM: ykato@geosys.t.u-tokyo.ac.jp
AF: Department of Geosystem Engineering, Graduate School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-8656 Japan
AU: Tazaki, K
EM: kazuet@kenroku.kanazawa-u.ac.jp
AF: Department of Earth Sciences, Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192 Japan
AU: Okuno, M
EM: okuno@kenroku.kanazawa-u.ac.jp
AF: Department of Earth Sciences, Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192 Japan
AU: Ohmoto, H
EM: ohmoto@geosc.psu.edu
AF: PSARC, The Pennsylvania State University, 435 Deike Bldg., Penn State University, University Park, PA 16802 United States
AB: The Marble Bar Chert contains abundant hematite-rich bands, and has been interpreted that the hematite was produced by modern weathering. On the other hand, other geologists have emphasized the oxidation of aqueous ferrous ions by iron bacteria. To understand the precipitation of the iron minerals in Marble Bar Chert, we have examined the mineral assemblages, textures and chemical compositions in fresh core which obtains by Archean Biosphere Drilling Project (ABDP). The ABDP core of the Marble Bar Chert is composed of organic-rich black chert, silicified white chert, and rhythmically banded chert consisted of white, red and black band, in order of sedimentation. Only the rhythmically banded chert has high magnetic susceptibility, which is associated with the existence of numerous hematite and magnetite grains. The colour from red to black is correlated with the proportion of hematite and magnetite. Euhedral grains of pyrite of 1 to 50 micrometer in diameter are observed to be layered in bands as well as to be precipitated in veins. This texture suggests that the pyrite might be of syngenetic. Red bands in the rhythmically banded chert contain numerous hematite grains, which are accumulated as spherical grains of about 100 nm in diameter to euhedral grains up to 500 nm in quartz with the size of 10-30 micrometer in diameter. The quartz with hematite grains shows botryoidal texture. Black bands also contain numerous euhedral magnetite grains of about 1 micrometer in diameter, and the occurrence is similar to that of red bands. These botryoidal textures resemble that of modern bacterial mat of benthic coccoid cyanobacteria. Both bands contain euhedral grains of siderite of micron size, in the botryoidal quartz. The carbon isotopic ratios of siderite are from _E to +2 per mill, suggesting siderite was produced by the reaction between dissolved ferrous ions and bicarbonate ions in seawater. SEM images show bacteria-like cells connecting with and enveloping hematite grains in the botryoidal quartz. The intensity of cathodoluminescence from the botryoidal quartz is heterogeneous, and its two-dimensional image shows bacteria-like texture. The carbon X-ray photoelectron spectra acquired from banded cherts contain a peak at about 285 eV, suggesting C-C and C-H bonding. Furthermore, the Raman spectrum from the black band chert exhibits the peaks at about 1300cm$^{-1}$ and 1600cm$^{-1}$, derived from organic carbon. The scanning image of Raman spectrum reveals the heterogeneous distribution of organic carbon. The occurrences of mineral and organic carbon suggest that the rhythmically banded cherts have not been affected by hydrothermal alteration, metamorphism or modern weathering, but suggest that the original texture has remained unchanged. The occurrences suggest further that the precipitation of iron oxides was related with biological activity.
DE: 9330 Australia
DE: 9619 Precambrian
DE: 8125 Evolution of the Earth
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting