REEVALUATION OF PRECISE LEAD ISOTOPE MEASUREMENTS BY THERMAL IONIZATION MASS SPECTROMETRY: COMPARISON WITH DETERMINATIONS BY PLASMA SOURCE MASS SPECTROMETRY

Abstract

In this study we focus on precise lead isotope analysis by thermal ionization mass spectrometry (TIMS) with silica gel technique. We present extensive measurements of SRM981 and SRM982 standards, of chemically pure Pb and of an equal-atom mixture of the four stable Pb isotopes. Measurements at temperatures lower than 1175°C display reproducible values and are coherent with the usual mass discrimination laws at the level of precision achievable with multicollector mass spectrometers. In contrast, the evolution of mass fractionation at higher temperatures does not follow classical laws of mass discrimination. We observe systematic shifts, the most pronounced of which concerns 207Pb with a total deviation that can reach values up to 400 ppm. This confirms the recent observation by Thirlwall [Chem. Geol. 163 (2000) 299]. Similarly, we do not have at present a physical explanation for this abnormal behaviour.We propose new precise isotopic compositions for the SRM981 and SRM982 standards. Assuming a 208Pb/206Pb ratio equal to the nominal value (208Pb/206Pb=2.1681 and 1.00016, respectively, cf. J. Res. Natl. Bur. Stand. 72A (1968) 261), the values for 207Pb/206Pb and 204Pb/206Pb ratios are 0.914970+/-17 and 0.059019+/-5 (2σ) for SRM981, and 0.467080+/-10 and 0.027212+/-2 (2σ) for SRM982. These compositions are in agreement with the recent measurements achieved with multicollection mass spectrometers with an inductively coupled plasma-source (MC-ICP-MS).