Unusually High Abundance of Silica Phases in Chondrules In a CR Chondrite, NWA852

Carbonaceous chondrites are the most primitive material in the solar system. A study of chondrules should shed light on the formation conditions of the solar system 4.56 Ga ago. In this study, we measured petrographical properties of 47 chondrules in NWA852 (CR2). Their apparent diameters range from 0.19mm to 2.82mm. The mean diameter is 0.75mm. The density of chondrule is 1.65 /mm2, and they occupy 80 vol. % in the thin section studied here. This is much larger than the mean chondrule abundance, 50-60 vol. %, for CR (Scott et al., 1996). Their textures are divided into 5 types. Porphyritic Olivine-Pyroxene(POP) (63.8 %), Porphyritic Olivine (PO) (14.9 %), Porphyritic Pyroxene (PP) (14.9 %), and Granular Olivine-Pyroxene (GOP) (2.1 %). Thus, pyroxene-bearing chondrules are abundant in NWA852. There are also two compound chondrules (4.3 %), consisting of BO and POP, and PO and BO, respectively. The shapes of chondrules are irregular (66.7 %), round (23.3 %), and ellipsoidal (10.0 %). Several CAIs are also found in this sample. Silica phases have been already reported from CR chondrites (Krot et al., 2002; Campbell et al., 2005). We also found silica phases from chondrules in NWA852. However, abundance in NWA852 is unusually high. About 30 % of the chondrules contain silica phases, typically in their peripheral parts. The common occurrence of silica phases in the chondrules might be related to the high amount of pyroxene, and might be a key to understand the nebular fractionation. Keywords: CR chondrite, chondrule, pyroxene, silica M. Tabata: a7sm6030@ganko.tohoku.ac.jp