The Solar System primordial lead

摘要

Knowledge of the primordial isotope composition of Pb in the Solar System is critical to the understanding of the early evolution of Earth and other planetary bodies. Here we present new Pb isotopic data on troilite (FeS) nodules from a number of different iron meteorites: Canyon Diablo, Mundrabilla, Nantan, Seel?sgen, Toluca (IAB-IIICD), Cape York (IIIA), Mt Edith (IIIB), and Seymchan (pallasite). Lead abundances and isotopic compositions typically vary from one troilite inclusion to another, even within the same meteorite. The most primitive Pb was found in three leach fractions of two exceptionally Pb-rich Nantan troilite nodules. Its ~(204)Pb/~(206)Pb is identical to that of Canyon Diablo troilite as measured by Tatsumoto et al. [M. Tatsumoto, R.J. Knight, C.J. Allàgre, Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206, Science 180(1973) 1279-1283]. However, our measurements of ~(207)Pb/~(206)Pb and ~(208)Pb/~(206)Pb are significantly higher than theirs, as well as other older literature data obtained by TIMS, while consistent with the recent data of Connelly et al. [J.N. Connelly, M. Bizzarro, K. Thrane, J.A. Baker, The Pb-Pb age of Angrite SAH99555 revisited, Geochim. Cosmochim. Acta 72(2008) 4813-4824], a result we ascribe to instrumental mass fractionation having biased the older data. Our current best estimate of the Solar System primordial Pb is that of Nantan troilite, which has the following isotopic composition: ~(204)Pb/~(206)Pb=0.107459(16), ~(207)Pb/~(206)Pb=1.10759(10), and ~(208)Pb/~(206)Pb=3.17347(28). This is slightly less radiogenic than the intercept of the bundle of isotopic arrays formed in ~(207)Pb/~(206)Pb-~(204)Pb/~(206)Pb space by our measurements of Canyon Diablo, Nantan, Seel?sgen, Cape York, and Mundrabilla, as well as literature data, which, in spite of rather large uncertainties, suggests a common primordial Pb component for all of these meteorites. The radiogenic Pb present in most of these irons is dominantly asteroidal and indicates evolution in a high-U/Pb environment. The apparent age of the radiogenic Pb component is consistent with the ~(39)Ar-~(40)Ar ages of silicate inclusions found in the same meteorites. We propose that the radiogenic Pb was introduced more recently into troilite, from the surface rubble of the parent asteroid, possibly during the impacts that generated the IAB iron meteorites. The excellent correlation between ~(208)Pb/~(206)Pb and ~(204)Pb/~(206)Pb translates into a Th/U ratio of 3.876±0.016 for the asteroid, which is the most precise estimate for the solar nebula to date.