Cosmogenic radionuclides in meteorites including recently fallen ones –Constraints on the exposure history of chondrites–

Abstract

Activities of cosmogenic nuclides have been numerously reported for extraterrestrial material, meteorites (especially chondrites). In addition to noble gases, cosmogenic radionuclides (e.g., ²²Na, ²⁶Al, ⁴⁶Sc, ⁵³Mn, ⁵⁴Mn and ⁶⁰Co) have also kept important records of their history during last ten million years. For example, low ⁶⁰Co activity observed in most chondrites (<30 dpm/kg) suggests that their preatmospheric radii are less than 〜30 cm, and activities of ²²Na and ²⁶Al reflect various irradiation conditions by cosmic-ray such as shielding effect, exposure age and cosmic-ray flux with time and space. Terrestrial age (Antarctic meteorites; H group, <0.4×10⁶y; L, LL,<10⁶y estimated from ²⁶Al, ³⁶Cl and ¹⁴C activities) as well as exposure age (H, 3 -40×10⁶y; L, LL, 3 -50×10⁶y) shows different distributions between H- and L-, LL-group chondrites, which have possibly preserved the information about breakage of parent body and so on. The combined data of exposure age and activities of cosmogenic nuclides also give constraints on the complex history of chondrites (e.g., multi-stage irradiation as a result of fragmentation) until the collision with the Earth. Recently, with the progress of nondestructive γ-ray techniques, activities of relatively short-lived nuclides such as ⁴⁶Sc, ²²Na implied the irradiation conditions just before fall to the Earth. In this paper, "the evolution history of chondrites after separation from the parent body" is represented from activities of cosmogenic radionuclides in meteorites including recently fallen ones.