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 for bulk samples than for some separated clasts or mineral separates. A possible interpretation for this difference in apparent exposure ages is a complex 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 immediately preceding its 4 pi exposure in a smaller body. To test this scenario we measured cosmogenic radionuclides in several clasts from Kapoeta. These measurements are consistent with a simple single-stage 4 pi exposure history during which the entire inventory of cosmogenic radionuclides was produced. Taken together, these data are most consistent with a single-stage 4 pi exposure lasting similar to3 Ma. This scenario is nevertheless consistent with models in which the exposure of some constituents of Kapoeta to energetic particles occurred at an earlier time, as is indicated by Ne-21 measurements. However, from our data we conclude that insubstantial quantities of cosmogenic radionuclides were inherited from this earlier irradiation; this earlier exposure to energetic particles must have predated the recent exposure by at least similar to 10 Ma to allow for the decay of the long half-life cosmogenic radionuclides.