History of the Pasamonte achondrite: Relative susceptibility of the SmNd, RbSr, and UPb systems to metamorphic events

The RbSr, SmNd, and UPb systematics of the eucrite Pasamonte have been studied in order to investigate the relative susceptibility of the different systems to post-crystallization events and to determine the age and history of the meteorite. The RbSr and²³⁸U-²⁰⁶Pb data of mineral separates do not define an isochron but the SmNd data define an internal isochron which corresponds to the formation age of4.58 ± 0.12b.y. (10⁹ years). The²⁰⁷Pb-²⁰⁶Pb data of mineral separates define an apparent age of4.53 ± 0.03b.y., however we conclude that this age, while in agreement with the SmNd age, is not strictly valid since the UPb data indicate at least three stages of evolution. The UPb data indicate that the parent body of the meteorite experienced brecciation shortly after the formation of the parent body surface (∼4.2–4.45b.y. ago) and a recent disturbance (collision?)6 ± 30m.y. ago. The latter age is within the range of cosmic ray exposure ages for achondrites.