Cosmogenic and fissiogenic noble gases and 81Kr-Kr exposure age clusters of eucrites

Abstract— The isotopic composition and concentrations of noble gases were measured in the eucrites Bereba, Cachari, Caldera, Camel Donga, Chervony Kut, Ibitira, Jonzac, Juvinas, Millbillillie, Moore County, Padvarninkai, Pasamonte, Pomozdino, Serra de Magé, Sioux County, and Vetluga. The distribution of ⁸¹Kr-Kr exposure ages shows “clusters” at (7 ± 1) Ma, (10 ± 1) Ma, (14 ± 1) Ma, (22 ± 2) Ma, and (37 ± 1) Ma that agree with those for howardites, eucrites, and diogenites (HED) at (6 ± 1) Ma, (12 ± 2) Ma, (21 ± 4) Ma, and (38 ± 8) Ma. This most likely indicates a common origin of HED meteorites. Correlation equations for the shielding-sensitive cosmogenic ratios ⁷⁸Kr/⁸³Kr, ⁸⁰Kr/⁸³Kr, ⁸²Kr/⁸³Kr, and ¹²⁴Xe/¹³¹Xe were obtained. Comparison with data from simulation experiments suggests that most eucrites were exposed to the cosmic radiation as somewhat large meteoroids with diameters of ～1 m or more. The shielding-dependence of the ⁷⁸Kr and ¹²⁶Xe production rates was found to be small, with a few exceptions the variations aren <10%–15%. Concentrations of spallogenic ³He indicate diffusive losses of up to 70% that can be, in first approximation, described by a model of quasi-continuous losses during the exposure to the cosmic radiation with a loss rate of the order of ～3 × 10^?8 a^?1. Radiogenic ⁴He shows additional substantial losses that occurred at the time of, or prior to, the separation of the meteoroids from their parent body. Typical ⁴⁰Ar retention in eucrites is 50%–60% which corresponds to a ⁴⁰Ar-K retention age of 3.4–3.6 Ga. In all analyzed unbrecciated eucrites, the retention is distinctly larger (70%–100%). The ²⁴⁴Pu fission ratio (⁸⁶Kr/¹³⁶Xe)_Pu, was evaluated from the data on Pomozdino samples to be 0.039 ± 0.014.