NOBLE-GASES, KR-81-KR EXPOSURE AGES AND PU-244-XE AGES OF 6 EUCRITES,BEREBA, BINDA, CAMEL-DONGA, JUVINAS, MILLBILLILLIE, AND STANNERN

Noble gases including radioactive Kr-81 (t(1/2) = 2.1 X 10(5) yr) have been measured for the cumulate eucrite Binda and five noncumulate euc rites, Bereba, Camel Donga, Juvinas, Millbillillie, and Stannern, some of which have been repeatedly analyzed. Concentrations of Kr-81, whic h range from 4.2 X 10(-14) to 3.2 X 10(-13) cm(3) STP/g, roughly corre late with abundances of the main target elements, Sr, Y, and Zr. Kr-81 -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 Bereba, Binda, Camel Donga, Juvinas, Millbillillie, and Stannern, respectively. All the measured eucrites have Pu-244-derived fission Xe. On the basis of fission Xe-136 concentrations, Pu-245 abun dances are calculated to range from 0.40 ppb for Binda to 1.08 ppb for Juvinas. Following the method proposed by Shukolyukov and Begemann (1 996a), Pu-244-Xe ages relative to the absolute crystallization age of 4.5578 Ga for Angra dos Reis are determined as -46 +/- 18 Ma (Bereba), -29 +/- 34 Ma (Binda), -51 +/- 16 Ma (Camel Donga), -10 +/- 23 Ma (Ju vinas), +8 +/- 24 Ma (fine-grained portion of Millbillillie), -51 +/- 21 Ma (coarse-grained portion of Millbillillie), and -124 +/- 13 Ma (S tannern), which correspond to absolute ages ranging from 4.434 Ga (Sta nnern) to 4.566 Ga (fine-grained portion of Millbillillie). Millbillil lie is a mixture of materials with different Pu-244-Xe ages. Stannern' s unusually young Pu-244-Xe age might reflect a secondary shock distur bance, but otherwise, it tends to confirm a distinctive mode of igneou s petrogenesis in comparison to most other eucrites. The Pu-244-Xe age s of the cumulate eucrites obtained so far (Binda, this work; Moore Co unty, Shukolyukov and Begemann, 1996a) do not differ systematically fr om those of noncumulate eucrites. The four eucrites with the older Pu- 244-Xe ages, Bereba, Binda, Juvinas, and the fine-grained portion of M illbillillie, show significant amounts of radiogenic Xe-129, (0.6-2.3) x 10(-12) cm(3) STP/g (after correction for fission Xe-129), which im ply earlier retention of radiogenic Xe-129 from extinct I-129 than oth er eucrites with young Pu-244-Xe ages. Copyright (C) 1998 Elsevier Sci ence Ltd.