RELATIONSHIPS AMONG LODRANITES AND ACAPULCOITES: NOBLE GAS ISOTOPIC ABUNDANCES, CHEMICAL COMPOSITION, COSMIC-RAY EXPOSURE AGES, AND SOLAR COSMIC RAY EFFECTS#

Abstract

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, minor, and trace element compositions of six lodranites (EET84302, Gibson, LEW88280, Lodran, MAC88177, Y791491), are reported. Because existing empirical production rate models for cosmic-ray-produced nuclides in achondrites could not account for the effects of bulk chemical composition and for the unique shielding conditions in lodranites and acapulcoites, we modeled the production rates of cosmogenic nuclides in lodranites and acapulcoites by galactic and solar cosmic rays using a purely physical model. All lodranites and acapulcoites are relatively small meteorites having preatmospheric radii =< 200 mm, one-half of them even =<75 mm. Evidence was found for solar-cosmic ray produced nuclides in the acapulcoites ALH77081, ALH81187, ALH81261, and ALH84190. The derived cosmic-ray 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, and 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 pairing between ALH77081 and ALH81261, and ALH81187 and ALH84190.