UNIQUE COSMIC-RAY HISTORY OF MALAKAL CHONDRITE

Cosmogenic radionuclides, including 22Na, 26Al and 54Mn, were measured in a sample of the recently-fallen Malakal hypersthene chondrite. The high 26Al activity, 79 ± 2 dpm/kg, greatly exceeds the levels expected from elemental production rates, shielding considerations, or comparisons with other ordinary chondrites, and can only be explained by exposure to a uniquely high cosmic-ray flux. Calculations including noble gas, 3H, and 53Mn data from other laboratories require a two-stage irradiation. Malakal's most probable history is: exposure in excess of 4 m.y. to an effective cosmic-ray flux approximately three times that experienced by other chondrites, an orbit change (very possibly caused by a collision), and a final period of about 2 m.y. during which it was exposed to a normal" cosmic-ray flux. © 1974."