Mineralogy and petrology of an unusual Belgica-7904 carbonaceouschondrite: Genetic relationships among the components

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Belgica-7904 (B-7904) carbonaceous chondrite consists of abundant chondrules, various clasts, isolated minerals, and a dehydrated phyllosilicate matrix. Chondrules are divided into magnesian and ferroan types. Magnesian type has forsterite phenocryst, often with pyroxene, plagioclase, and unusual Cr-rich ovoid, which set in a dehydrated phyllosilicate groundmass. Kamacite is abundantly included in the forsterite phenocrysts, and is rarely associated with schreibersite. Ferroan type consists mainly of ferroan olivine phenocryst and dehydrated phyllosilicate groundmass with phosphate, instead of schreibersite in magnesian type. Clasts in B-7904 are classified into seven types. These include matrix-like clasts, phyllosilicate-rich clasts, hortonolite-rich clasts (aggregates of ferroan olivine and opaque minerals), Mn-rich olivine-bearing clasts containing schreibersite, Mn-rich forsterite, and Mn-rich chromite, forsterite-spinel clasts like amoeboid olivine inclusions (AOI's) in C3 chondrites, kamacite-rich clasts, and sulfide-rich clasts consistsing of troilite, pentlandite, taenite, and Co-rich metal. Mineralogical study suggests that magnesian chondrules, phyllosilicate-rich clasts, and Mn-rich olivine-bearing clasts formed under reducing conditions, whereas ferroan chondrules, hortonolite-rich clasts, and the matrix originated under oxidizing conditions. Thus, the components of B-7904 originated in different reservoirs with contrasting fO_2 and fS_2 in the solar nebula, and were later mixed. Glass inclusion in chondrules, which occur in olivine phenocrysts and dehydrated phyllosilicate groundmasses, contains CaO, whereas phyllosilicates in the groundmass are nearly free from CaO. Thus, CaO was mainly lost from these chondrules during hydrous alteration, although some CaO remains as abundant phosphate in ferroan chondrules. All of the CaO lost from the chondrules and clasts appears to have been concentrated in the matrix as Ca-carbonates. The oxidation reaction to form magnetite and magnesiowustite from kamacite also took place in a magnesian chondrule at about 660K and 10^<-36> bars oxygen partial pressure, which is much higher than an oxygen partial pressure (10^<-43> bars) of the canonical solar gas with a total gas pressure of 10^<-4> bars at 600K. This suggests that magnesian chondrules were oxidized by a nebular gas of non-solar composition, probably after mixing with ferroan chondrules. All phyllosilicates in B-7904 were dehydrated by intense heating, and the unequilibrated mineral assemblages and the magmatic temperatures retained in some of the chondrules suggest that the heating took place for a short duration, probably by shock heating.
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Mineralogy and petrology of an unusual Belgica-7904 carbonaceouschondrite: Genetic relationships among the components

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