Carbonaceous xenoliths in the Krymka LL3.1 chondrite: Mysteries and established facts

Abstract

The results of a detailed study on mineralogy, chemistry, and the carbon and oxygen isotopes of two exotic Krymka carbonaceous xenoliths are presented in this article. The investigated xenoliths are metamorphosed and shocked and have the following characteristics, which distinguish them from the Krymka host: 1. resemblance of their SiO 2/MgO ratio to that of carbonaceous chondrites; 2. higher Fe content and FeO/(FeO + MgO) ratio; 3. lower concentration of Si, Ca, Al and an enrichment of S and probably of Ag; 4. smaller sizes and lower content (10 vol%) of chondrules and their clasts, and correspondingly higher content of matrix; 5. dominance of porphyritic chondrules and lack of nonporphyritic chondrules; 6. occurrence of an amoeboid olivine grain with 16O-rich composition; 7. existence of carbon in three different forms: graphite, carbon-rich material, and organic compounds. The bulk chemistry of the xenoliths is similar, but not identical, to that of carbonaceous chondrites, suggesting that they represent a chondrite parent body that has not been previously sampled. Among any known type of meteoritic material the mineralogy of the xenoliths corresponds only to that of other Krymka graphite-containing xenoliths. It differs, however, from the latter by having a lower grade of metamorphism. We infer that metamorphism of the primary carbonaceous body of the xenoliths and/or shock of the Krymka parent body are responsible for the major metamorphic alteration of the xenoliths, including the crystallization of graphite from primary organic compounds. A comparison of the features of the Krymka xenoliths with the inferred characteristics of cometary meteorites attests that their genetic relationship to cometary material remains highly inconclusive.