MINERALOGY OF FINE-GRAINED CHONDRULE RIMS IN THE KRYMKA (LL3.1) METEORITE
Keywords:
meteorite, chondrite, chondrule rim, fine-grained material, microchondrule.Abstract
The results of mineralogical and chemical study of fine-grained chondrule rims of the Krymka meteorite (LL3.1) are given. The chondrules mostly have microporphyritic structure and pyroxene-olivine composition. Ca-rich pyroxene grains and plagioclase mesostasis are minor. Ca-low pyroxene Fs1–25 and oli-vine Fa1–54 usually are depleted in FeO but ones on the chondrule periphery are slightly enriched in it. Fine-grained rims consist of cryptocrystalline (<< 1 μm) silicate material, fine (< 5 μm) and coarse (> 5 μm) grains and rare microchondrules. Cryptocrystalline material and fine grains form a kind of groundmass for coarse grains and microchondrules. Fine-grained rims are characterized by the following chemical and mineralogical characteris-tics: 1) the rims are opaque; 2) they are macroscopically black coloured in nonoxidized parts and dark-brown or red-brown in oxidized parts; 3) they have cryptocrystalline and homogeneous structure; 4) they show multi-layering consisting of few concentric dust layers of different oxida-tion degree; 5) chondrule rims are brighter in reflected light than the matrix material because of higher abundance of fine metal and/or sulphide grains or FeO-rich silicates; 6) inner parts of the rims are enriched in metal and/or sulphide grains. Bulk chemical composition of the fine-grained rims varies widely and according to the con-tent of main components corresponds to the normative olivine Fa63–83. In accordance with SiO2/MgO ratio the most rims are characterized by enrichment in SiO2, indicating to their forma-tion in a dusty environment depleted in refractory elements. Some rims are similar to those of the carbonaceous xenoliths K1, K3, and to a dark matrix of the Krymka chondrite, and probably be-long to relicts of primary dust. Chemical composition of fine and coarse silicate grains varies a lot: olivine – Fa9–88 and Fa1–96, Ca-low pyroxene – Fs10–37 and Fs1–36 and Ca-high pyroxene – En29–67Wo13–39Fs4–39 and En18–85Wo11–51Fs2–45. Most of them present as fragments. Availability of silicate fragments, in particular fragments of zonal olivine crystals, and mesostasis remains in fine-grained rims indi-cate to intensive collision and fragmentation of mostly microporphyritic chondrules in proto-planetary nebula during Krymka parent body accretion. Microchondrules are rare high-temperature component of the fine-grained rims. They formed by remalting and recondensation of chondrules surface material and fine-grained dust around the chondrules because of a heating event. Unique structures of microchondrules sticking onto two chondrules surface are the evidence of these processes. The results of the study indicate to active processes of interaction between low- and high-temperature components during accretion of the meteorite parent body within a chemically and mineralogically variable gas-dust environment of protoplanetary nebula.
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