Determination of lead in meteorites by alpha activation analysis

Abstract

Alpha activation analysis has been applied to the problem of lead isotopic abundances in meteorites. The procedure involves the bombardment of samples with 30-Mev alpha particles to induce an (α, 2n) reaction on the lead isotopes. This produces radioactive polonium isotopes which may be isolated and counted. Relative abundances of Pb208, Pb206, and Pb204 have been measured on four stone meteorites, a troilite, and a terrestrial basalt. Lead from troilite of the Canyon Diablo iron meteorite had an isotopic composition, Pb208/ Pb204 = 38.0 ± 1.7 and Pb208/pb206 = 2.2 ± 0.07, similar to that reported by Starik in several iron meteorites. This is distinct from the isotopic composition Pb208/Pb204 = 29.5 and Pb208/Pb206 = 3.1 found in other iron meteorites, including Canyon Diablo. The Canyon Diablo result confirms previous reports that these two kinds of lead can exist in different parts of the same meteorite. The results for the stone meteorite samples show a more radiogenic lead than can be accounted for by their lead to uranium and lead to thorium ratios. These measurements do not support the assumption that stone meteorites, when they crystallized, incorporated lead of the same isotopic composition. However, there is evidence of contamination of the meteorite lead with a few tenths of a ppm terrestrial lead, which could explain this discrepancy.