Distribution of lead and thallium in the matrix of the Allende meteorite and the extent of terrestrial lead contamination in chondrites

Abstract

Selective chemical dissolution has been used to study the distribution of Pb and Tl in an ultrafine −20-μm matrix separate of Allende. The matrix was exposed to high-purity reagents ranging from H 2O, then HCl of increasing concentration and finally HF-HCl mixtures. A total of 17 extractions were obtained, each for a minimum period of 10 days. The isotopic compositions of the Pb released during the slow dissolution of the matrix fall into four distinct groups. The first, consisting of four extractions, released a component of terrestrial Pb isotopic composition with a total abundance of about 1 ppb. The next six extractions, which contained the bulk of the indigenous Pb and Tl corresponding to 96% and 94%, respectively, of the total matrix abundance, was of a reasonably homogeneous Pb isotopic composition with mean ratios of 206Pb 204Pb= 10.00and 207Pb 204Pb= 10.74 . In the final seven extractions, the released Pb falls into two higher isotopic groupings and probably results from the dissolution of debris from chondrules and inclusions. The apparent age of the internal matrix isochron is 4562 ± 14 My. The release of Pb and Tl shows a reasonable correlation with the matrix dissolution. This indicates that the Pb and Tl reside predominantly within the matrix phases rather than as a localised phase. The Tl isotopic composition of two matrix fractions and whole meteorite were measured and found to be indistinguishable from the terrestrial 205Tl/ 203Tl ratio. Measurement of a terrestrial reagent standard in the range 1–10 ng Tl gave, for 20 analyses, a mean 205Tl/ 203Tl ratio of 2.38907 ± 0.00102 (2σ). The estimate of terrestrial Pb contamination is considerably lower than the 6–300 ppb assumed in some recent studies in order to explain the phenomenon of apparent excess radiogenic Pb in chondrites. The problem of terrestrial Pb pollution and the evidence which argues against a relatively severe and homogeneous Pb contamination of meteorites, is briefly considered. The apparent initial isotopic composition of the bulk of the indigenous Pb in the Allende matrix was found to be 206Pb 204Pb= 9.57and 207Pb 204Pb= 10.47 . This is of a higher composition than the Pb in the Can˜on Diablo troilite phase and further indicates that the phenomenon of apparent excess radiogenic Pb in chondrites is real.