Incipient melting in and shock classification of L-group chondrites

Thirty-three of 52 type L4 to L6 chondrites that we have examined in thin section contain closed bodies of crystal-laden glass or devitrified glass (melt pockets) that testify to in-situ melting. A close correlation between the distribution of melt pockets and shock intensity as inferred from the characteristics of olivine and plagioclase indicates that the pockets reflect shock melting. The appearance of pockets coincides with a sharp decrease of 40Ar in L-group chondrites, suggesting that shock melting was responsible for loss of argon and raising the possibility that this process redistributed other volatile elements as well. The use of three criteria for shock intensity — olivine and plagioclase characteristics, and the presence or absence of melt pockets — leads to a refined shock classification for equilibrated chondrites that is based entirely on petrographic observations.