Cosmogenic effects in Mbale, L5/6 chondrite

Abstract

Abstract— Measurements of particle tracks, cosmogenic radionuclides, and rare gas isotopes 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 production rates and 60Co and 26Al activities, the meteoroid is estimated to be a sphere with a radius of ∼36 cm. The activities of several cosmogenic radionuclides (i.e., 57Co, 54Mn, 22Na, 44Ti, and 26Al) in two fragments having different shielding, as estimated by their track density and 60Co activity, provide the depth variation in their production rates. Cobalt‐57, 54Mn and 22Na activities agree with the production that is expected around the maximum of the solar cycle 22 as calculated from the Sunspot numbers. The U, Th‐4He and K‐40Ar ages are measured to be 0.54 Ga indicating a late thermal event which is in agreement with the thermal history of some other L group chondrites. The trapped N has δ15N of −57 ± 4%o, which is much lighter than the average L‐group chondrite value; this indicates the presence of an isotopically anomalous light N component.