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

We studied the cosmic-ray records of the H6 chondrite ALH76008 in a bulk sa mple, in a chondrule fragment, and in matrix material. The matrix was separ ated into olivine/pyroxene, plagioclase, and metal concentrates, For all sa mples He, Ne, and Ar isotopic measurements were performed. The cosmic-ray e xposure age of the bulk chondrite is 1.72 +/- 0.11 Ma, whereas the He-3, Ne -21, and Ar-38 ages of the chondrule exceed those of bulk material by 31%, 67%, and 55%, respectively. The radiation environment in the early solar sy stem is ill defined, but for current production rates this translates into a precompaction exposure time of 0.90 Ma. Furthermore, the plagioclase (mai nly anorthite) concentrate, representing a few percent of the total meteori te, contains solar-type noble gases that reveal differential irradiation of ALH76008 components. We observe a trapped ratio Ne-20/Ne-22 = 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 condit ions between a chondrule and the bull; chondrite; the short exposure age ma y favor the resolution of differential exposure histories. Copyright (C) 19 99 Elsevier Science Ltd.