Radiogenic Lead in Iron Meteorites Revisited

Lead in iron meteorites has long been known to be of surprisingly radiogenic isotope composition (e.g. [1-3]). Primordial Pb is only found (if at all) after very extensive leaching of the samples in strong mineral acids. The widely accepted explanation for this observation is that Pb of radiogenic composition represents terrestrial contamination, although the exact source and mode of contamination have eluded the isotope geochemists. In the most recent study (with the lowest laboratory blanks) Göpel et al. [3] found that leachates and residues of progressively leached group IAB iron meteorites (Canyon Diablo, Toluca and Odessa) have, within analytical uncertainty, identical radiogenic and primordial Pb isotope components. The combined data yield an apparent regression age of 4534+/-8 Ma, identical to the Ar-Ar plateau age of the winonaite Pontlifny (4531+/-12 Ma; [4]), which is believed to represent the silicate counterpart to IAB iron meteorites. We will present high precision MC-ICPMS Pb-isotope data for eight progressively leached magmatic and non-magmatic iron meteorites, as well as Pb-isotopes and rare earth element patterns of bulk dissolutions of iron meteorites. All leachates and bulk dissolutions of the different iron meteorites (excluding the non-magmatic group IAB representatives) display excellent colinearity in the common lead isotope diagram. The slope of the regression line corresponds to an apparent age of 4568.5+/-4.6 Ma, within uncertainty equal to the Pb/Pb age of Ca-Al rich inclusions in the carbonaceous chondrite Allende [5]. Excellent colinearity of Pb data is also found in thorogenic Pb space, where the slope of the regression line corresponds to a κ -value of 3.816+/-29, which is equal to that of chondritic meteorites (i.e. 3.806+/-88) but different from average continental crust. Data for individual IAB meteorites also define excellent colinearity but yield somewhat younger apparent ages. For example, the regression date for our Canon Diablo leachates is 4546.5+/-7.6 Ma, within uncertainties identical to that of Göpel et al. [3]. Chondrite normalized rare earth element patterns of unleached bulk samples of iron meteorites prove absence of terrestrial contamination. Probability for selective contamination of iron meteorites with terrestrial Pb was calculated by comparison with different terrestrial data pools. For a single contamination event of all studied samples with average terrestrial lead the probability is 0.69%. Individual contamination for the different iron meteorites yields a probability as low as 0.0048%. Low probabilities of contamination reflect the well-known observation that the accessible Earth plots well to the left of the meteorite isochron in common Pb space (i.e. the first Pb-isotope paradox) and that average continental crust (and sediment) has a higher time-integrated Th/U ratio than chondritic meteorites. In view of the consistency in Pb-isotope composition of the radiogenic component in iron meteorites, the lack of contamination of other trace elements, and low estimated probabilities for terrestrial contamination we propose to reconsider the possibility of in situ grown (though now largely unsupported) radiogenic lead in iron meteorites. Implications for early planetary evolution and core formation will be discussed. [1] Oversby, V.M. (1970), Geochim. Cosmochim. Acta 34, 65. [2] Chen, J.H. and Wasserburg, G.J. (1983), Geochim. Cosmochim. Acta 47, 1725. [3] Göpel et al. (1985), Geochim. Cosmochim. Acta 49, 1681. [4] Benedix et al. (1998), Geochim. Cosmochim. Acta 62, 2535. [5] Manhès et al. (1988), Geochim. Cosmochim. Acta 69-70, 32.