Exposure history of separated phases from the Kapoeta meteorite

The cosmogenic radionuclides, Be-10, Al-26, Cl-36, and Mn-53 were measured in selected clasts and matrix samples from the howardite Kapoeta. Previous measurements of cosmogenic Ne-21 indicate higher cosmic-ray exposure ages f or bulk samples than for some separated clasts or mineral separates. A poss ible interpretation for this difference in apparent exposure ages is a comp lex recent exposure history for Kapoeta. In this scenario some constituents are exposed to cosmic rays in a 2 pi geometry as part of a larger body imm ediately preceding its 4 pi exposure in a smaller body. To test this scenar io we measured cosmogenic radionuclides in several clasts from Kapoeta. The se measurements are consistent with a simple single-stage 4 pi exposure his tory during which the entire inventory of cosmogenic radionuclides was prod uced. Taken together, these data are most consistent with a single-stage 4 pi exposure lasting similar to3 Ma. This scenario is nevertheless consisten t with models in which the exposure of some constituents of Kapoeta to ener getic particles occurred at an earlier time, as is indicated by Ne-21 measu rements. However, from our data we conclude that insubstantial quantities o f cosmogenic radionuclides were inherited from this earlier irradiation; th is earlier exposure to energetic particles must have predated the recent ex posure by at least similar to 10 Ma to allow for the decay of the long half -life cosmogenic radionuclides.