NEON-E IN CM-2 CHONDRITE LEW90500 AND COLLISIONAL HISTORY OF CM-2 CHONDRITES, MARALINGA, AND OTHER CK CHONDRITES - SOLAR NOBLE GASES IN AN ENSTATITE CHONDRITE

Abstract

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 anomalous CK chondrite Maralinga. In LEW90500 we observe at least 0.20 x 10-8 cm3STP/g Ne-E probably originating from the decay of presolar 22Na. Planetary He is characterized by 4He/3He = 6500. For planetary-type trapped noble gases we obtain 20Netr = 13.4 x 10-8 cm3STP/g and (36Ar/20Ne)tr = 5.6. The cosmic-ray exposure age, based on 21Nec is 0.24 Ma. We review the literature data of the other CM-2 chondrites and find a Ne-E component in most of them; the largest concentration is observed in Mighei (0.53 x 10-8 cm3STP/g). The exposure age distribution of the CM-2 chondrites confirms previous studies that report generally young ages (<6.5 Ma). A cluster of four meteorites 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 six known CK or CK-like chondrites. Three of them lie in the range of 38-45 Ma.