Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27s.254m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 254
Physics
Scientific paper
Unshocked chondrites show large apparent Pb/Pb age differences up to 80 x 10^6 y. All of them appear to contain more radiogenic lead than could have been derived from U- and Th decay since 4.56 aeons, assuming the Pb isotopic composition of Canon Diablo as primordial (Tatsumoto et al., 1973). Three general interpretations have been proposed to explain this apparent excess of radiogenic Pb and apparent age differences: 1) complex U-Pb evolutionary histories, 2) variable primordial Pb isotopic compositions and 3) terrestrial Pb contamination in excess of that accounted for by analytical blanks. The terrestrial Pb contamination hypothesis has been first rejected by Gale et al. (1972). Until now no extraterrestrial mechanism has been proposed to explain this pervasive recent U-Pb mobility in chondrites. We consider this apparent excess of radiogenic Pb as real; we interpret it as an addition of mobile Pb produced during the recent impact that has extracted the meteorite from its parent body. The isotopic composition of this "exotic Pb" is deduced from published and new U-Pb data from chondrites: it defines a Pb/Pb model age of ~4.45 aeons. This isotopic signature identifies the nature of the projectile: volatile-rich chondritic material. A consequence of this interpretation concerns the distribution of the highly volatile elements in equilibrated chondrites: it does not reflect a primitive fractionation but results from mobilization caused by the recent impact that has excavated the meteorite. References: Gale N.H., Arden J., and Hutchison R. (1972) Nat. Phys. Sci. 240, 56-57. Tatsumoto M., Knight R.J., and Allegre C.J. (1973) Science 180, 1278-1283
Allègre Claude J.
Gopel Ch.
Manhes Gerard
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