Friday, 14 July 2006

Distribution of Pb Isotopes in Different Soil Phases Using Tessier et al.'s Sequential Extraction Scheme.

Eric T. Tangumonkem and William Manton. Geosciences Department University of Texas at Dallas, Richardson, TX Box 830688

A long standing question regarding soil sequential extraction techniques is whether they redistribute the elements being investigated. If soils derived from geologically old rocks are analysed Pb may provide a means of detecting redistribution, because different soil phases may have different isotope ratios inherited from progenitor minerals with different U/Pb and Th/Pb ratios. Accordingly, three Canadian soil standards TILL1, TILL3, and  SO2, were studied  and the antiquity of their source rocks confirmed by model Sm-Nd ages of the soils which were 1.5 Ga for SO-2, and TILL-1, and 2.5 Ga for TILL-3 (1Ga = 109 years). The standard SRM 2709 from a Californian agricultural soil was also studied. Thermal ionization was used and Pb isotope ratios and concentrations were simultaneously measured with a 205Pb spike on a Finnigan MAT 261 mass spectrometer. In Tessier et al's method the exchangeable fraction is extracted with MgCl2, the carbonate with NaAc, the Mn-Fe oxides with hot hydroxylamine and the organic with H2O2. The Pb contents of all reagents was <50 pg/mL. All extractions were done in triplicate and good agreement in both concentration and isotope ratio obtained. In all four soils, the amount of Pb increased in the order: exchangeable (0.004-0.11 ppm), carbonate (0.49-2.4 ppm), organic (1.0-2.7 ppm), and Mn–Fe oxides (2.5-9.1 ppm). The residual was 10.6-14.2 ppm and amounted to between 40 and 75% of the Pb processed. The agricultural soil SRM 2079 was isotopically homogeneous as might be expected for a soil derived from young rocks, but large variations were found in the Canadian soils. In 208Pb/207Pb vs. 206Pb/207Pb plots, residual silicates, whole soil, Mn-Fe oxides and organic material were collinear with the organic material having the most radiogenic Pb. Exchangeable and carbonate Pb lay off the line, but in no consistent manner. Redistribution implies homogenization and since each phase had its own Pb ratio, Tessier et al's method appears robust. The most interesting result is that if the ratios 206Pb/204Pb vs. 207Pb/204Pb are plotted for the residual, the whole soil, the Mn-Fe Oxides and the organic phases, good isochrons are obtained for SO-2 and TILL-3 with apparent ages of 1.73 and 2.91 Ga which reflect the model Sm-Nd ages. It is difficult to understand how recently formed secondary minerals can reflect the age of the progenitor materials. Possibly the organic phase is ubiquitous and binds the radiogenic Pb, perhaps from grain boundaries, that is the first to be released by weathering. The Mn-Fe phase forms later and takes Pb from the crystal lattices as weathering progresses. In the 208Pb/207Pb vs. 206Pb/207Pb plots the carbonate and exchangeable Pb appears to the most recent and derived from a high U/Th source possibly limestone in the case of SO-2 and from a low U/Pb source, possibly feldspars in the case of TILL-1. It remains to be seen whether these interpretations hold up under other sequential extraction schemes.



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