Cosmogenic effects in Mbale, L5/6 chondrite

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 δ ¹⁵N 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.