HR: 17:30h
AN: H34A-05 INVITED     [Abstracts]
TI: Where is the U in Calcite?
AU: * Rasbury, E T
EM: troy.rasbury@sunysb.edu
AF: Stony Brook University, Department of Geosciences, Stony Brook, NY 11794-2100, United States
AU: Cole, J M
EM: jcole@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rt. 9W P.O. Box 1000, Palisades, NY 10964, United States
AB: In order to better understand the potential for U-Pb dating of calcite, we have been working to characterize the distribution of U in calcite. We use phosphor imaging to compare macroscopic calcite structures with U distribution in hand specimens, fission track maps to compare microscopic structures of calcite to U distribution, and synchrotron micro-XRF analyses to map the distribution of U with respect to other elements, and X-ray Adsorption Near Edge Structure (XANES) analyses to determine the oxidation state(s) of U. Based on our work and published work of others on U in calcite we recognize (at least) three independent avenues for U incorporation in calcite, all of which have yielded reliable U-Pb ages based on comparison to more traditional dating tools: 1) reduced U that substitutes for Ca in the crystal lattice; 2) oxidized U associated with organic matter that substitutes for Ca in the crystal lattice; 3) oxidized U in calcite that has been neomorphosed from an aragonite precursor and substitutes into the crystal lattice. While it may be suggested that the presence of U in the crystal lattice ‘proves the U-Pb dating technique,' we suggest that instead by understanding U behavior in various depositional contexts, we can infer something about the fluids involved in calcite precipitation. Importantly, we do not know of an example of calcite with mixed U oxidation states; all published examples are either fully reduced or oxidized. Thus, the kinetics of mineral formation as an explanation of U oxidation state or alteration after formation do not appear to be viable explanations for the U oxidation state in calcite. Instead it appears that a reduced U species may be soluble in some hydrothermal fluids (perhaps CO2 rich brines), and that oxidized U can be removed from solution by coprecipitation in calcite (and possibly by complexation to coprecipitated organic functional groups in the calcite) in the oxidized state rather than being removed from solution by reduction. Although these tenets are partly contrary to paradigms of U behavior they are the most direct explanations for our observations.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Radiogenic isotope geochemistry
DE: 1051 Sedimentary geochemistry
DE: 1065 Major and trace element geochemistry
SC: Hydrology [H]
MN: 2007 Joint Assembly