Complete Pb/U analysis of unspiked samples by measuring Pb isotopes only

Abstract

For the analysis of samples with a complex Pb-loss history during their geological evolution, the measurement of single grains without chemical separation and spiking of their U and Pb is desirable. In such cases a 207Pb/ 206Pb age can only be determined if concordant 206Pb/ 238U and 207U/ 235U ages are assumed. Because of the completely different ionisation behavior of Pb and U, the Pb/U ratio cannot be obtained. In radioactive equilibrium, however, 238U and 210Pb are present in a fixed ratio of 210Pb/ 238U= 238λ/ 210λ= 5 · 10 −9 so that, in principle, the Pb/U measurement can be replaced by an isotope abundance ratio 210 Pb Pb. A sample with a (concordant) age of 300 Ma corresponding to a 206Pb/ 238U ratio of 4.76 · 10 −2 would, for example, have a 210Pb/ 206Pb r ratio of 1.05 · 10 −7. Using a Finnigan MAT 262 mass spectrometer with variable multicollector arrangement, masses 204, 206, 207 and 208 were measured simultaneously with Faraday cups, while mass 210 was determined with an electron multiplier (SEM) in ion counting mode (IC). The system was calibrated by means of artificial mixtures, with 210Pb/ 206Pb ratios of 10 −3, 10 −4, 10 −5, 10 −6 and 10 −7. The 208Pb abundance sensitivity to mass 210 is 1.7 · 10 −7. The accuracy of the 210Pb/ 206Pb method was tested by comparison of data obtained by the conventional isotope dilution technique with those obtained by our 210Pb/ 206Pb method for different tobernite samples, and good agreement was recorded.