Noble gases, 81Kr-Kr exposure ages and 244Pu-Xe ages of six eucrites, Béréba, Binda, Camel Donga, Juvinas, Millbillillie, and Stannern

Abstract

Noble gases including radioactive 81Kr ( t 1/2 = 2.1 × 10 5 yr) have been measured for the cumulate eucrite Binda and five noncumulate eucrites, Béréba, Camel Donga, Juvinas, Millbillillie, and Stannern, some of which have been repeatedly analyzed. Concentrations of 81Kr, which range from 4.2 × 10 −14 to 3.2 × 10 −13 cm 3 STP/g, roughly correlate with abundances of the main target elements, Sr, Y, and Zr. 81Kr-Kr cosmic-ray exposure ages are obtained as 24.5 ± 0.6 Ma, 21.0 ± 1.6 Ma, 36.6 ± 1.4 Ma, 10.6 ± 0.8 Ma, 20.8 ± 0.5 Ma and 35.1 ± 0.7 Ma, for Béréba, Binda, Camel Donga, Juvinas, Millbillillie, and Stannern, respectively. All the measured eucrites have 244Pu-derived fission Xe. On the basis of fission 136Xe concentrations, 244Pu abundances are calculated to range from 0.40 ppb for Binda to 1.08 ppb for Juvinas. Following the method proposed by Shukolyukov and Begemann (1996a), 244Pu-Xe ages relative to the absolute crystallization age of 4.5578 Ga for Angra dos Reis are determined as −46 ± 18 Ma (Béréba), −29 ± 34 Ma (Binda), −51 ± 16 Ma (Camel Donga), −10 ± 23 Ma (Juvinas), +8 ± 24 Ma (fine-grained portion of Millbillillie), −51 ± 21 Ma (coarse-grained portion of Millbillillie), and −124 ± 13 Ma (Stannern), which correspond to absolute ages ranging from 4.434 Ga (Stannern) to 4.566 Ga (fine-grained portion of Millbillillie). Millbillillie is a mixture of materials with different 244Pu-Xe ages. Stannern’s unusually young 244Pu-Xe age might reflect a secondary shock disturbance, but otherwise, it tends to confirm a distinctive mode of igneous petrogenesis in comparison to most other eucrites. The 244Pu-Xe ages of the cumulate eucrites obtained so far (Binda, this work; Moore County, Shukolyukov and Begemann, 1996a) do not differ systematically from those of noncumulate eucrites. The four eucrites with the older 244Pu-Xe ages, Béréba, Binda, Juvinas, and the fine-grained portion of Millbillillie, show significant amounts of radiogenic 129Xe, (0.6–2.3) × 10 −12 cm 3 STP/g (after correction for fission 129Xe), which imply earlier retention of radiogenic 129Xe from extinct 129I than other eucrites with young 244Pu-Xe ages.