Evidence for a precompaction exposure to cosmic rays in a chondrule from the H6 chondrite ALH76008

Abstract

We studied the cosmic-ray records of the H6 chondrite ALH76008 in a bulk sample, in a chondrule fragment, and in matrix material. The matrix was separated into olivine/pyroxene, plagioclase, and metal concentrates. For all samples He, Ne, and Ar isotopic measurements were performed. The cosmic-ray exposure age of the bulk chondrite is 1.72 ± 0.11 Ma, whereas the 3He, 21Ne, and 38Ar ages of the chondrule exceed those of bulk material by 31%, 67%, and 55%, respectively. The radiation environment in the early solar system is ill defined, but for current production rates this translates into a precompaction exposure time of 0.90 Ma. Furthermore, the plagioclase (mainly anorthite) concentrate, representing a few percent of the total meteorite, contains solar-type noble gases that reveal differential irradiation of ALH76008 components. We observe a trapped ratio 20Ne/ 22Ne = 12.3 ± 0.3, a value that is between those of solar wind and solar energetic particles. This is the first clear identification of differential irradiation conditions between a chondrule and the bulk chondrite; the short exposure age may favor the resolution of differential exposure histories.