First known EL5 chondrite—evidence for dual genetic sequence for enstatite chondrites

Abstract

The enstatite chondrites form two compositionally distinct groups—the EH and EL chondrites—and four petrologic types (types 3–6), where each type represents increasing degrees of metamorphic alteration^1,2. Prior to the recovery of the Antarctic meteorite Reckling Peak A80259, the subject of the present report, all known EL chondrites were petrologic type 6, whereas all known EH chondrites were types 3–5. It has long been appreciated that the variations in bulk composition preclude a simple conversion of EH4 material into EL6 material^3–7. However, complex models involving simultaneous variations in bulk composition and petrologic type have been discussed^1,5,8,9 and may be implied by other classification schemes in common use; that of Anders-Keil (type I–intermediate–type II) and that of Van Schmus and Wood (E3-E4-E5-E6). We report here the discovery of the first EL5 chondrite. An EL5 breaks the EH3,4-EH5-EL6 sequence, and indicates that the enstatite chondrites constitute two discrete, isochemical metamorphic sequences, EH3–5 and EL5–6.