The Songyuan ordinary chondrite (China) -- rich in alien rock fragments and its reclassification

Abstract

The Songyuan chondrite is composed mostly of minerals and chondrule types common in equilibrated ordinary chondrites. More distinctively, the Songyuan material includes numerous sintered cryptocrystalline rock lumps, possible chondrule precursors, and less frequent fragments of olivine-pyroxene basalt. The moderate porosity (estimated under the polarizing microscope at a few percent by volume) and uncompacted metal grains, suggest that the meteorite came from very shallow horizons of the regolith. The shallow original position of the Songyuan fragment on its parent body is consistent also with the abundance of numerous euhedral crystals of the main meteoritic components present in the interstitial pores. They crystallized most probably from fluids migrating through the superficial horizons from the inner parts of the Songyuan parent body. The structural characteristics and slight differentiation in olivine composition suggests a high degree of homogenization and, together with the median value of the fayalite content (26.2 ą0.24 wt.%), corresponds to the L5 chemical-petrologic group of equilibrated ordinary chondrites. Moreover, the homogenous extinction in the olivine crystals and the absence of multiple planar fractures indicate the lowest level of the shock metamorphism (S1), whereas the lack of any oxide rims around kamacite and troilite grains points to the lowest level of weathering (W0).