HR: 0830h
AN: V51C-0309    [PDF]
TI: Uranyl Incorporation in Organic-Bearing Soil Calcite
AU: * Rasbury, E
EM: erasbury@notes.cc.sunysb.edu
AF: Department of Geosciences, SUNY Stony Brook, Stony Brook, NY 11794-2100 United States
AU: Tait, C
AF: Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AU: Donohoe, R J
AF: Bioscience Division, Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AB: Phosphor imaging and fission track maps reveal a close correlation between uranium and organic-rich calcite that lines roots in an ancient caliche soil. Concordant U-Pb ages show that this uranium has not been mobilized since the soil calcite formed 298 ñ 1 Ma ago. Luminescence spectroscopic analysis of the organic-rich calcite demonstrates that uranium is present in the oxidized state. More or less comparable luminescence spectra were collected using a CW lamp at room temperature and at liquid nitrogen (LN2) temperature. Spectra for inorganic U species are generally greatly enhanced at liquid nitrogen temperatures over room temperature and thus the similarity may suggest that the U is bound to organic matter. UV-excitation Raman spectroscopy of the calcite samples reveals a vibrational signature that is, in addition to the calcite peaks, essentially identical to that of sodium humate, acquired under similar conditions and consistent with those reported previously for humate substances. Since these spectra are dominated by the carbon backbone structure, the persistence of a carbon backbone structure is noted, but persistence of the ligating functional groups cannot be proven directly. We suggest that this sample records a snapshot of the process of uranium complexation with organic acids in natural systems. In our model organic acids with active functional groups that complex uranyl are sorbed onto the calcite surface and then incorporated as humate into the mineral with further calcite precipitation. Further, the uranyl complex is preserved through near-surface reducing conditions and temperatures to the conditions associated with 2700 meters burial depths. These combined observations suggest that the encapsulating calcite not only prevents reduction of uranium in the surface environment, but also inhibits the breakdown of the humate with burial. Due to the long time scale of immobilization of uranium, evidenced by the age of the calcite, this study has profound implications for understanding sampling for U-series and U-Pb dating of carbonates as well as for developing strategies for nuclear waste management. This study also has implications for any other deposits where uranium is associated with organic matter.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1055 Organic geochemistry
DE: 4807 Chemical speciation and complexation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting