Comparative xenology of two angrites

Abstract

We report the results of Xe analyses in stepwise heating of the angrite meteorite Lewis Cliff 86010 and of a comparative study of Xe component structures in this meteorite and in the first known angrite, Angra dos Reis. Both meteorites are poor in trapped Xe and rich in fission Xe from 244Pu and in spallation Xe produced by cosmic ray irradiation of Ba and the light rare earth elements (REE). Neither meteorite shows evidence for the presence of excess 129Xe derived from 129I, presumably because the volatile element iodine is depleted along with other volatiles in the angrites. Both meteorites show pronounced variations in the composition of spallation Xe attributable to separation of the Ba and REE components. In both meteorites 244Pu-derived fission Xe is correlated with REE spallation, consistent with Pu-REE coherence in igneous partitioning. The Ba spallation Xe composition in Lewis Cliff 86010 differs somewhat from that in Angra dos Reis, primarily in a lower relative yield of the neutron sensitive isotope 131Xe. This suggests a lower neutron flux during cosmic ray exposure for the Lewis Cliff Meteorite, an effect opposite to that suggested by neutron capture effects in Sm. Resolution of these observations appears to require a complex exposure history for at least Angra dos Reis. The 244Pu/REE ratio in Lewis Cliff 86010 is only about half that in Angra dos Reis. This difference probably does not reflect a difference in the time of Xe retention. Unless this difference arose in igneous processes despite the apparent chemical coherence of Pu and Ree in mineral partitioning, it must have characterized the parent magmas and/or precursor materials from which these two meteorites were derived.