NEON-E IN CM-2 CHONDRITE LEW90500 AND COLLISIONAL HISTORY OF CM-2 CHONDRITES, MARALINGA, AND OTHER CK CHONDRITES

We present He, Ne, and Ar data on stepwise heating experiments for the CM-2 chondrite LEW90500 and on noble gas isotopic abundances of the a nomalous CK chondrite Maralinga. In LEW90500 we observe at least 0.20 X 10(-8) cm(3)STP/g Ne-E probably originating from the decay of presol ar Na-22. Planetary He is characterized by He-4/He-3 = 6500. For plane tary-type trapped noble gases we obtain Ne-20(tr), = 13.4 x 10(-8) cm( 3)STP/g and (Ar-36/Ne-20)(tr) = 5.6. The cosmic-ray exposure age, base d on Ne-21(c) is 0.24 Ma. We review the literature data of the other C M-2 chondrites and find a Ne-E component in most of them; the largest concentration is observed in Mighei (0.53 x 10(-8) cm(3)STP/g). The ex posure age distribution of the CM-2 chondrites confirms previous studi es that report generally young ages (<6.5 Ma). A cluster of four meteo rites is observed around 0.28 Ma. Maralinga (anomalous CK-4) contains no solar gases and relatively low amounts of planetary trapped gases. The cosmic-ray exposure age is 6.1 Ma. This is the lowest age of the s ix known CK or CK-like chondrites. Three of them Lie in the range of 3 8-45 Ma. Copyright (C) 1998 Elsevier Science Ltd.