244Pu-Xe formation and gas retention age, exposure history, and terrestrial age of angrites LEW86010 and LEW87051: Comparison with Angra dos Reis

Abstract

We have studied noble gas isotopic abundances in the two angrites LEW86010 and LEW87051. Many characteristics of LEW86010 are similar to those observed for the basaltic achondrite Angra dos Reis, until recently the only known angrite. Fission Xe in LEW86010 originates almost entirely from 244Pu fission. We calculate an atomic ratio 244Pu 150Nd = 1.36 × 10 −3 that corresponds to a fission Xe retention age 19 ± 40 Ma younger than that of Angra dos Reis. Thus, LEW86010 formed contemporaneously with Angra dos Reis for which previous analyses yielded a formation age of 4550 Ma. The U/ Th- 4He age (3700 Ma) of LEW86010 is higher than the K- 40Ar gas retention age, a feature observed earlier for Angra dos Reis. LEW87051 shows the lowest nominal U/Th- 4He gas retention age (4 Ma) ever observed for a stone meteorite. The cosmic-ray exposure histories are very different for the three angrites: for LEW86010 and LEW87051 we calculate 17.6 ± 1.0 Ma and ≧ 0.2 Ma, respectively, compared to a previously reported exposure age of 55.5 Ma for Angra dos Reis. For the radionuclide 81Kr we measured an activity of 0.012 dpm/kg in LEW86010 and derive a terrestrial age of 420 000 ± 100 000 years.