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Cosmogenic and fissiogenic noble gases and 81 Kr‐Kr exposure age clusters of eucrites
Author(s) -
Shukolyukov A.,
Begemann F.
Publication year - 1996
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1996.tb02054.x
Subject(s) - radiogenic nuclide , krypton , meteorite , meteoroid , caldera , cosmic ray , parent body , noble gas , geology , isotope , xenon , nuclear physics , atomic physics , physics , geochemistry , astrobiology , chondrite , volcano
— 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 81 Kr‐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 78 Kr/ 83 Kr, 80 Kr/ 83 Kr, 82 Kr/ 83 Kr, and 124 Xe/ 131 Xe 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 78 Kr and 126 Xe production rates was found to be small, with a few exceptions the variations aren <10%–15%. Concentrations of spallogenic 3 He 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 4 He shows additional substantial losses that occurred at the time of, or prior to, the separation of the meteoroids from their parent body. Typical 40 Ar retention in eucrites is 50%–60% which corresponds to a 40 Ar‐K retention age of 3.4–3.6 Ga. In all analyzed unbrecciated eucrites, the retention is distinctly larger (70%–100%). The 244 Pu fission ratio ( 86 Kr/ 136 Xe) Pu , was evaluated from the data on Pomozdino samples to be 0.039 ± 0.014.