Composition, petrology, and chondrule‐matrix complementarity of the recently discovered Jbilet Winselwan CM2 chondrite

Abstract

AbstractJbilet Winselwan (JW) is a recently found CM2 chondrite breccia containing two lithologies. A study of 508 chondrules provides the first statistically reliable size distribution for CM chondrite chondrules, which is about log normal with a mean chondrule size of 149 μm (lithology I) and 141 μm (lithology II). Chondrules are surrounded by fine‐grained rims. Apparent chondrule diameters and their apparent rim thicknesses are positively correlated with slopes of 0.12 (lithology II) and 0.18 (lithology I), the latter typical of CM chondrites. The CM chondrites are generally primitive and parts of JW experienced only mild aqueous alteration. Bulk JW element ratios are solar (=CI chondritic), e.g., Si/Mg (1.12), Fe/Mg (1.80–1.83), Ti/Al (0.053), and about solar for Ca/Al. The 26 chemically studied chondrules have subchondritic Si/Mg (0.88) and Fe/Mg ratios (0.21). Matrix and the fine‐grained chondrule rims on the other hand have superchondritic Si/Mg ratios with means of 1.34 and 1.41, respectively. The Fe/Mg ratios are also superchondritic, with means of 2.41 (matrix) and 2.61 (fine‐grained rims). The refractory element ratios in chondrules are superchondritic (Ti/Al: 0.106; Ca/Al: 1.64), and subchondritic in the JW matrix (Ti/Al: 0.031; Ca/Al: 0.71) and in the fine‐grained rims (Ti/Al: 0.023; Ca/Al: 0.68). These complementary element ratios require formation of chondrules and matrix/rims from the same reservoir in order to obtain a chondritic bulk composition. Most chondrules are mineralogically zoned, with olivine in the core and low‐Ca pyroxene in the rim; hence, CM chondrules were open systems, exchanging material with the surrounding gas.