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Petrographic and trace element analysis of uranium-rich tufa calcite, middle Miocene Barstow Formation, California, USA |
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| Authors: | Jennifer M Cole E Troy Rasbury† Isabel P Montañez‡ Vicki A Pedone§ Antonio Lanzirotti¶ Gilbert N Hanson† |
| Institution: | Interdepartmental Doctoral Program in Anthropological Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794-4364, USA; Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100, USA; Department of Geology, University of California, Davis, CA 95616-8605, USA; Department of Geological Sciences, California State University, Northridge, CA 91330-8266, USA; Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, USA |
| Abstract: | An integrated petrographic and spectroscopic (X‐ray diffraction, phosphor imaging and synchrotron X‐ray absorption spectroscopy) study of tufas from the Miocene Barstow Formation, California, relates sample morphology, mineralogy and geochemical composition. The tufas, composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters, have textures similar to those of microbially mediated terrestrial stromatolites and travertines. The tufas have elevated concentrations of a number of trace elements including Mn, Fe, Sr and U. Synchrotron X‐ray fluorescence analyses show that U concentrations can exceed 500 p.p.m. X‐ray absorption spectroscopy indicates that the U in these samples is incorporated as U(IV). It is suggested here that alkaline lake waters had a high U/Ca ratio and tufa calcite formed where groundwater or (possibly epithermal) springs brought in Ca and trace elements such as Sr. The rapid, and possibly microbially mediated, precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine‐scale inclusions. |
| Keywords: | Carbonate lacustrine synchrotron XRF trace elements tufa |