Abstract
An 840-g specimen of the Bruderheim chondrite was subjected to magnetic and heavy-liquid mineral separation procedures, resulting in a number of chemically distinct samples. These samples were analyzed for cosmogenic Al 26 by non-destructive gamma-gamma coincidence counting. The observed Al 26 specific activities were correlated with the chemical composition of potential target elements by a weighted least-squares fitting technique. The calculated Al 26 production rates, in dpm per kilogram of target element, are: Al, 1130 ± 190; Si, 245 ± 31; S, 133 ± 11; Mg, 28 ± 30. Production rates from Ca and Ni + Fe were estimated to be 24 and 2.2 dpm/kg, respectively, from spallation systematics. Most meteorite classes show a distribution of (Al 26) obs (Al 26) clac , primarily between 0·80 and 1·10 (excluding short exposure age effects). The only exception is the eucrites. The five eucrites with the highest relative Al 26 activities have only 0·77 ± 0·03 of their respective calculated activities. Two Apollo 12 samples, from mean depths of 15–20 cm, have approximately 0·78 of the Al 26 activities calculated for their chemical compositions. A depletion in Al 26 in lunar samples shielded from solar radiation is in accord with a reduced cosmic-ray flux near the earth's orbit, relative to that experienced by most meteorites. The Al 26depletion in the eucrites suggests that they spent a greater proportion of their orbital periods near 1 AU than have most meteorites. The similarity in relative Al 26 contents of the lunar samples and the eucrites may not be a coincidence.
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