Cosmogenic effects in Mbale, L5/6 chondrite

Measurements of particle tracks, cosmogenic radionuclides, and rare gas iso topes in Mbale indicate that the meteoroid had a simple, one-stage exposure for 30.2 Ma in interplanetary space. On the basis of the measured track pr oduction rates and Co-60 and Al-26 activities, the meteoroid is estimated t o be a sphere with a radius of similar to 36 cm. The activities of several cosmogenic radionuclides (i.e., Co-57, Mn-54, Na-22, Ti-44, and Al-26) in t wo fragments having different shielding, as estimated by their track densit y and Co-60 activity, provide the depth variation in their production rates . Cobalt-57, Mn-54 and Na-22 activities agree with the production that is e xpected around the maximum of the solar cycle 22 as calculated from the Sun spot numbers. The U, Th-He-4 and K-Ar-40 ages are measured to be 0.54 Ga in dicating a late thermal event which is in agreement with the thermal histor y of some other L group chondrites. The trapped N has delta(15)N of -57 +/- 4 parts per thousand, which is much lighter than the average L-group chond rite value; this indicates the presence of an isotopically anomalous light N component.