Water-susceptible primordial noble gas components in less-altered CR chondrites: A possible link to cometary materials

Abstract

We report detailed characterizations of the petrology, mineralogy, and noble gas signatures of Renazzo-type (CR) chondrites EET 92048, MIL 090657, NWA 801, and hydrothermally treated MIL 090657 samples to investigate primordial noble gases hosted by water-susceptible materials in aqueously less-altered CR chondrites. The noble gases were extracted by a stepwise heating method. The hydrothermal treatment on MIL 090657 resulted in the alteration of amorphous silicates in the matrix and the removal of large amounts of He and Ne that were released at low temperature from the untreated sample. The 36Ar/132Xe ratios in EET 92048 and MIL 090657 are ~5 times higher than that of the Q noble gas component, although both meteorites do not show any evidence for solar wind noble gases. The analyses revealed that aqueously less-altered CR chondrites contain two distinct primordial noble gas components hosted by water-susceptible materials: an isotopically Q-like Ne-rich component and an Ar-rich component. The characteristics of the Q-like Ne-rich component are consistent with those of Q-like Ne in cometary materials. We propose that the CR chondrites formed at a greater heliocentric distance and share similar materials hosting the Q-like Ne-rich component with comets. The presence of the Ar-rich component in the CR chondrites, as well as in ordinary chondrites and other anhydrous carbonaceous chondrites, provides evidence of the global distribution of the Ar-rich carriers in the ordinary and carbonaceous chondrite forming regions.