Relationships among lodranites and acapulcoites: Noble gas isotopic abundances, chemical composition, cosmic-ray exposure ages, and solar cosmic ray effects

Noble gas isotopic abundances of ten lodranites (EET84302, FRO90011, Gibson , LEW86220, LEW88280, Lodran, MAC88177, QUE93148, Y74357, Y791491) and four acapulcoites (Acapulco, ALH81187, ALH81261, ALH84190), as well as major, m inor, and trace element compositions of six lodranites (EET84302, Gibson, L EW88280, Lodran, MAC88177, Y791491), are reported. Because existing empiric al production rate models for cosmic-ray-produced nuclides in achondrites c ould not account for the effects of bulk chemical composition and for the u nique shielding conditions in lodranites and acapulcoites, we modeled the p roduction rates of cosmogenic nuclides in lodranites and acapulcoites by ga lactic and solar cosmic rays using a purely physical model. All lodranites and acapulcoites are relatively small meteorites having preatmospheric radi i less than or equal to 200 mm, one-half of them even less than or equal to 75 mm. Evidence was found for solar-cosmic ray produced nuclides in the ac apulcoites ALH77081, ALH81187, ALH81261, and ALH84190. The derived cosmic-r ay exposure ages of all lodranites (with the exception of QUE93148 with 15 Ma) and all acapulcoites cluster around 6 Ma, suggesting, supported by the similar abundances of cosmogenic nuclides, similar shielding conditions, an d similar chemical compositions, that they all originate from one ejection event from the same parent body. Within error limits identical abundances of cosmogenic nuclides, identical shielding conditions, and identical cosmic-ray exposure ages support pairin g between ALH77081 and ALH81261, and ALH81187 and ALH84190. Copyright (C) 1 999 Elsevier Science Ltd.