HR: 1340h
AN: V43C-1438    [Abstracts]
TI: The Pb Isotope Records of Archean and Proterozoic Shales Limit Early O-Rich Atmosphere
AU: * Krogstad, E J
EM: ejkrogstad@gsu.edu
AF: Geology Dept., Georgia State University, Atlanta, GA 30303 United States
AB: The relative behavior of Th and U under Earth's surface conditions is well known today, with these elements separated due to the oxidation of U (4+) to U (6+). Thorium does not undergo such a transition, and remains in its 4+ state. U and Th have very limited solubility in their 4+ states, but U (6+) is quite soluble. These differences, and their resulting radiogenic Pb isotope signatures, can be used to provide clues about conditions that existed during crustal weathering and sedimentary transport and diagenesis. Early and Middle Archean fine-grained sediments from Greenland (Rosing and Frei 2004) and southern Africa (Krogstad et al. 2004) have Pb isotopic records (Pb6/4 versus Pb8/4) that show that Th and U behaved in a well-correlated manner. These sample suites have integrated long-term Th/U near the Archean upper crust average. Variations in Pb6/4 versus Pb8/4 of Early Proterozoic shales (2.5 to 2.2 Ga) from North America (Krogstad and Walker 1996, McLennan et al. 2000) also behave in such a manner. This suggests that until 2.2 Ga separation of U from Th due to oxidation of U (4+) was not important. By contrast, Phanerozoic shales show fanning patterns in their Pb6/4 versus Pb8/4, indicating that separation of U from Th occurred in weathering of their provenance, as well during their transport and/or diagenesis. This Pb isotopic record confirms conclusions from other studies that the atmosphere and hydrosphere in Paleoproterozoic and earlier Earth were not oxygen-rich. This conclusion differs from recent suggestions from the Pb isotopic record that oxidizing conditions existed by 3.7 Ga.
DE: 1020 Composition of the crust
DE: 1030 Geochemical cycles (0330)
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting