Origin of radiogenic 129Xe variations in carbonaceous chondrites

Carbonaceous chondrites are pristine witnesses of the formation of the solar system. Among them, the carbon-rich Tarda and Tagish Lake meteorites are thought to have sampled very distant regions of the outer circumsolar disk (Hiroi et al ., 2001). Here, we show that their noble gas isotopic compositions (especially 129 Xe excesses) are similar, implying their formation in comparable environments. Combined with literature data, we show that the radiogenic excesses of 129 Xe relative to solar wind in carbonaceous chondrites define anti-correlations with their respective iodine and carbon contents. These trends do not result from the heterogeneous distribution of 129 I in the disk but rather evidence a xenon dilution effect; the radiogenic 129 Xe excesses being dominated by trapped xenon in the most carbon-rich carbonaceous chondrites. Our data also suggest that both Tarda and Tagish Lake accreted beyond 10 astronomical units, in regions of the disk that were cold enough for CO 2 to condense.