HR: 09:15h
AN: H41J-06    [Abstracts]
TI: Selenium enrichment in weathering zones: Se isotopes at Yutangba, China
AU: Zhu, J
EM: jmzhu@uiuc.edu
AF: Inst. of Geochemistry, Chinese Academy of Sci., 46 Guanshui Rd., Guiyang, 550002, China
AU: Zhu, J
EM: jmzhu@uiuc.edu
AF: Geology Dept., Univ of Illinois, 245 NHB, MC-102, Urbana, IL 61801, United States
AU: * Johnson, T M
EM: tmjohnsn@uiuc.edu
AF: Geology Dept., Univ of Illinois, 245 NHB, MC-102, Urbana, IL 61801, United States
AU: Clark, S K
EM: skclark@uiuc.edu
AF: Geology Dept., Univ of Illinois, 245 NHB, MC-102, Urbana, IL 61801, United States
AB: Selenium-rich carbonaceous chert and carbonaceous shale occur in the Permian Maokou Fm. of Hubei Province, China; humans and livestock have been adversely affected by the Se. In Yutangba, 81km SE of Enshi city, rock and soil can exceed 1000 mg/kg. We used Se isotope ratio data to reveal the mechanism of Se enrichment. Mass-dependent fractionation of Se isotopes is caused mainly by reduction of Se oxyanions and thus serves as an indicator these reactions. ⁸²Se/⁷⁶Se ratios were measured by double spike MC-ICP-MS, with 0.20 per mil precision. Unweathered drill core samples from >40 m depth, averaging 108 ±54 mg/kg Se (n=25), have a narrow range of δ ^82/76Se, from -2.54 to +1.74 per mil (vs. SRM3149). The mean is 0.08 per mil, similar to published values for shales and the bulk earth (i.e., meteorites and mafic igneous rocks). Outcrop samples from a horizontal transect across the nearly vertical, partially weathered beds are extremely Se- rich (2327 ±4484mg/kg Se; n=45), and show very wide Se isotope variation, from -13.19 to +11.37 per mil. This indicates most of the Se has been subjected to at least one cycle of oxidation, transport, re-reduction, and precipitation in these organic-rich rocks. Se isotope values vary sharply on a small scale (e.g., 30 cm) among the interbedded chert and shale layers, with a tendency toward enrichment in heavier isotopes in chert. The expected pattern of isotopic variation in such a system is heavy isotope depletion in accessible zones that Se-rich infiltrating waters encounter early, and heavy isotope enrichment in less accessible zones that receive Se only after it has been strongly reduced. The observed pattern fits this model, though the fracture-dominated system is complex and other factors may be involved. Overall, the data are consistent with a scenario of extreme Se enrichment in a rolling redox front that has existed for a long time as the near-vertical strata have weathered and eroded.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0432 Contaminant and organic biogeochemistry (0792)
DE: 0471 Oxidation/reduction reactions (4851)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2007 Fall Meeting