On the pairing of Antarctic ureilites with refernce to their parent body

Abstract

In spite of their apparent igneous textures, ureilites preserve significant records of primitive nature including oxygen isotope anomaly and high planetary-type noble gas contents in the carbonaceous matrices. Mineralogical studies of new Antarctic ureilites revealed that pairing of the small ureilite specimens may be only minor (two out of twenty-two) in comparison with the polymict eucrites. However, we noticed two groupings, which are almost paired but are distinct in pyroxene assemblages and reduction textures. The magnesian and calcic groups have been recognized only in the Antarctic collections. The individual specimens within the group may have evolved in the vicinity within a parent body. A model in conformity with this evolution has been proposed in the context of the ureilite-carbonaceous chondrite connection. However, the carbon-containing chondrite-like materials do not imply the actual chemistry of the known chondrites, because the preferential sedimentation of mafic silicates in the solar nebula may produce source materials much closer to ureilites