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• W300067803 abstract "We studied three dark inclusions (DIs) in the reduced CV3 chondrite Efremovka (E81, E82, E90) using optical and scanning electron microscopy and electron probe microanalysis, in addition to E39, E53, and E80 characterized in detail by Krot et al. [1]. These DIs show moderate degree of alteration similar to those experienced by the Efremovka DI E39. They consist of chondrules and CAIs embedded in finegrained, opaque matrices, rich in metal. In contrast to E39, sulfides are absent and Ca-rich rims around DIs are not observed. Low-Ca pyroxene, olivine and mesostasis in chondrules are completely replaced by an Fe-rich phase consisting mainly of fine-grained fayalitic olivine with minor interstitial, poorlycrystalline Si-Al-rich material and chlorite [1]. HighCa pyroxene grains are stable to alteration. Chondrule pseudomorphs contain small grains of Ca-Ti-Fe-rich phase (possibly Ti-andradite), salite-ferrosalite pyroxene, and Fe-rich Cr-spinel. Small CAIs are abundant throughout the DIs; they consist of Aldiopside and fassaitic pyroxene rims surrounding cores composed of the Fe-rich phase. Based on mineralogical similarities between the DIs studied and those characterized by Krot et al. [1], we conclude that E81, E82, and E90 also experienced aqueous alteration and subsequent thermal metamorphism in asteroidal environment. Textural observations indicate that E81 and E82 are breccias containing clasts of various degrees of alteration. We infer that brecciation and aggregation of these DIs occurred after aqueous alteration and possibly postdated thermal metamorphism. Introduction. Dark inclusions provide important information about nebular and/or asteroidal alteration processes affecting their components [2-14] as well as CAIs, chondrules and matrices of the oxidized CV3 chondrites [10, 12, 14]. It was suggested that DIs in Allende, Vigarano and Leoville experienced various degrees of aqueous alteration and were subsequently metamorphosed in asteroidal environment and that these processes resulted in formation of fayalitic olivine and secondary Ca-rich phases [4, 8-12 , 14]. The discovery of relict chlorite coexisting with secondary, fine-grained fayalitic olivine in Efremovka DIs [1] strongly support this model. However, many questions about formation history of CV3 DIs remain to be answered; e.g., precursor materials of the DIs in CV3s and their geological position within CV3 or other parent asteroid(s), temperature and duration of aqueous alteration and thermal metamorphism, changes in bulk chemical compositions and oxygen isotopic compositions during these processes. In this paper, we describe mineralogical and textural observations on three DIs (E81, E82, E90) in the reduced CV3 chondrite Efremovka. Mineralogy and petrography. The Efremovka DIs studied consist of chondrules, CAIs and mineral grains embedded in fine-grained, opaque matrix, rich in metal; magnetite and sulfide grains are not observed. All coarse-grained components are replaced to various degrees by an Fe-rich phase with an olivine composition (Fa36-43, in wt.%, Al2O3, 1.3±0.6; Cr2O3, 0.4±0.4; CaO, 0.4±0.1; Na2O, up to 1.1; SO2, up to 1.0). The Fe-rich phase probably consists of abundant, fine-grained fayalitic olivine with small amounts of interstitial, poorly-crystallized Si-Al-rich material, and chlorite, as was shown for several other DIs in Efremovka [1]. The degree of alteration increases in the order low-Ca pyroxene, plagioclase mesostasis, opaque nodules, olivine, and high-Ca pyroxene. Relict grains of olivine and high-Ca pyroxene are common in many chondrule pseudomorphs. Pseudomorphs after chondrule phenocrysts commonly show layered textures and contain small grains of a Ca-Ti-Fe-rich phase, possibly Ti-andradite. Small (<150 μm) CAIs are abundant throughout the DIs; they consist of Aldiopside and fassaitic pyroxene rims surrounding cores completely replaced by an Fe-rich phase which is compositionally similar to those in chondrules. Metal nodules consist of kamacite (3.6-6.0 wt.% Ni, 1.2-4.0 wt.% Co) and taenite (33.4-47.0 wt.% Ni, up to 2.4 wt.% Co). E81 and E82 show brecciated textures. E81 contains chondrules with unaltered phenocrysts of forsteritic olivine, glassy mesostasis, troilite, metal nodules and unaltered, fine-grained CAIs. The unaltered chondrules and CAIs are observed in the center of the DI and do not show any evidence of interaction with the host DI. E82 contain chondritic clast which is more heavily altered than the host DI; it contains a large (1200x600μm) altered porphyritic chondrule. The chondrule phenocrysts are replaced by the Fe-rich phase and surrounded by euhedral grains of chromite [Cr/(Cr+Al) = 0.4-0.8, Fetot/(Fetot+Mg) = 0.50.7]; mesostasis is replaced by Fe-rich phase and contains skeletal aggregates of euhedral pyroxene Lunar and Planetary Science XXVIII 1441.PDF" @default.
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• W300067803 date "1997-03-01" @default.
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• W300067803 title "Dark Inclusions in the CV3 Carbonaceous Chondrite Efremovka: Textural and Mineralogical Observations" @default.
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