HR: 10:40h
AN: B32B-02    [Abstracts]
TI: Origins of Carbonaceous Matter, Hematite, and Pyrite in the 3.46Ga Marble Bar Chert/Jasper/Basalt Formation, Pilbara, Western Australia
AU: * Ohmoto, H
EM: ohmoto@geosc.psu.edu
AF: Astrobiology Research Center, Penn State University, 435 Deike Bldg., University Park, PA 16802 United States
AU: Bevacqua, D C
EM: dbevacqu@geosc.psu.edu
AF: Astrobiology Research Center, Penn State University, 435 Deike Bldg., University Park, PA 16802 United States
AU: Watanabe, Y
EM: yumiko@geosc.psu.edu
AF: Astrobiology Research Center, Penn State University, 435 Deike Bldg., University Park, PA 16802 United States
AU: Otake, T
EM: otake@geosc.psu.edu
AF: Astrobiology Research Center, Penn State University, 435 Deike Bldg., University Park, PA 16802 United States
AB: The 3.46 Ga Marble Bar Chert/Jasper unit in the Pilbara district, W.A. was probably deposited in a deep ($>$500 m) ocean during the accumulation of a thick ($>$5 km) submarine basalt. Debate focuses on whether organic matter in pre-2.0 Ga cherts is a remnant of marine organisms or the product of abiotic synthesis in hydrothermal systems, whether the hematite crystals in jaspers were primary or products of modern oxidation of siderite and pyrite, and whether the pyrite crystals formed by sulfate-reducing bacteria, hydrothermal fluids, or atmospheric sulfur. At the drilling site, the Marble bar Chert/Jasper is over turned diping about 80 degrees. A continuous 264 m- long core, drilled at 50 degrees, was recovered. The major chert/jasper unit, comprising alternating beds (0.1 - 5 cm thick) of white/green/gray/black/red chert, is 105 m thick. The abundance of red jasper beds increases down hole, indicating that the hematite crystals were not produced by subaerial oxidation of ferrous minerals. Petrological, mineralogical, and geochemical investigations of the core samples, especially using an X-ray chemical microscope, have revealed that the dominant Fe-bearing minerals are siderite, magnetite, and hematite, in the green, gray-black, and red cherts, respectively. These Fe-bearing minerals and disseminated pyrite crystals (ubiquitous in all cherts) are typically very fine grained (less than 10 microns). The basalts (pillows and tuffs), which occur below, above, and interbedded with the chert/jasper unit, are in places heavily hematitized with various characteristics of submarine hydrothermal alteration, such as chloritization, silicification, pyritization, large variations in the contents of Fe, Mg, Ba, and depletions in Ca, Sr, and Na. Veinles containing quartz and pyrite are abundant in the chert/jaster beds and also in the heavily hematitized basalts. These data suggest the hematite, magnetie, siderite, pyrite and silica in the chert/jasper unit, basalt tuffs, and pillow margins formed by rapid mixing of Fe-, H2S- and silica rich hydrothermal fluids with O2- and carbonate-rich deep ocean water. The presence of distinct negative Ce anomalies in the altered basalts also indicates reaction with oxygenated seawater. Evaluation of the relationship between the atmospheric and ocean water chemistry suggests an oxygen-rich ($>$0.5 PAL) and CO2-rich ($>$100 PAL) contemporaneous atmosphere. Carbonaceous matter in the chert/jasper unit typically occurs in thin layers ($<$1 to 5 mm in thickness) parallel to the bedding plane, commonly in clay-rich layers. Significant enrichment of uranium is observed in some organic C-rich and clay-rich layers, suggesting the reduction of U6+ ions in seawater by the carbonaceous matter during quiescent periods of submarine hydrothermal activity. These features suggest the carbonaceous matter was not generated by abiotic hydrothermal reactions, but is a remnant of microbial mats and the modern-style uranium geochemical cycle was already established 3.46 Ga ago.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 0315 Biosphere/atmosphere interactions
DE: 0325 Evolution of the atmosphere
DE: 0400 Biogeosciences
DE: 1010 Chemical evolution
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting