Noble gases in enstatite chondrites II: The trapped component

Abstract

The trapped noble gas record of 57 enstatite chondrites (E chondrites) has been investigated. Basically, two different gas patterns have been identified dependent on the petrologic type. All E chondrites of type 4 to 6 show a mixture of trapped common chondritic rare gases (Q) and a subsolar component (range of elemental ratios for E4-6 chondrites: 36Ar/132Xe = 582 +/- 270 and 36Ar/84Kr = 242 +/- 88). E3 chondrites usually contain Q gases, but also a composition with lower 36Ar/132Xe and 36Ar/84Kr ratios, which we call sub-Q (36Ar/132Xe = 37.0 +/- 18.0 and 36Ar/84Kr = 41.7 +/- 18.1). The presence of either the subsolar or the sub-Q signature in particular petrologic types cannot be readily explained by parent body metamorphism as postulated for ordinary chondrites. We therefore present a different model that can explain the bimodal distribution and composition of trapped heavy noble gases in E chondrites. Trapped solar noble gases have been observed only in some E3 chondrites. About 30% of each group, EH3 and EL3 chondrites, amounting to 9% of all analyzed E chondrites show the solar signature. Notably, only one of those meteorites has been explicitly described as a regolith breccia.