Reevaluation of precise lead isotope measurements by thermal ionization mass spectrometry: comparison with determinations by plasma source mass spectrometry

摘要

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 degreesC 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 Pb-207 with a total deviation that can discrimination. We observe systematic shifts, the most pronounced of which concerns 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 Pb-208/Pb-206 ratio equal to the nominal value (Pb-208/Pb-206 = 2.1681 and 1.00016, respectively, cf. J. Res. Natl. Bur. Stand. 72A (1968) 261), the values for (207)pb/(206)pb and Pb-204/Pb-206 ratios are 0.914970 +/- 17 and 0.059019 +/- 5 (2 sigma) for SRM981, and 0.467080 +/- 10 and 0.027212 +/- 2 (2 sigma) 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). (C) 2001 Elsevier Science B.V. All rights reserved. [References: 16]