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 SiO₂/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.

Introduction

The Krymka meteorite is one of the most primitive ordinary chondrites. It contains many lithic clasts, some of which are uniquely composed of an exotic material and belong to xenoliths (Semenenko and Girich, 2001). Within the Krymka xenoliths, carbonaceous clasts are of high scientific interest due to their unusual chemistry and mineralogy.

Based on the high Tl/Bi ratio in Krymka, Laul et al. (1973) suggested the presence of a volatile-rich, late nebular condensate. An enigmatic petrographic carrier of the volatiles was called “mysterite.” Subsequently, Higuchi et al. (1977) found that some dark inclusions and veins of the chondrites represent the enigmatic carrier of the volatiles. Their chemistry and mineralogy are similar, but not identical, to CI and CM carbonaceous chondrites (Leitch and Grossman, 1977).

Based on petrography, noble gases, and minor element concentrations, a dark “mysterite”-containing inclusion in the Krymka chondrite was classified as a carbonaceous chondrite of unknown chemical group, enriched in Ag, Tl, and Bi up to ∼1.6× CI (Lewis et al., 1979; Grossman et al., 1980). The inclusion is composed of a fine-grained mixture of silicates and troilite and contains coarse grains of pyroxene, olivine, and rare metal particles. It was speculated that the fine-grained material also contains phyllosilicates (Grossman et al., 1980). Lewis et al. (1979) proposed that the carbonaceous inclusion from Krymka represents a late nebular condensate from a high pressure region.

The study of two other carbonaceous inclusions from Krymka with similar mineralogy (Semenenko et al 1991a, Semenenko et al 1991b; Semenenko, 1996) revealed additional distinctive features: 1. presence of silicate-rich rims around one of the inclusions; 2. occurrence of chondrules and their fragments; 3. existence of graphite and organic compounds—aliphatic hydrocarbons, nitrogenized carbonyl compounds and aromatic hydrocarbons; 4. heterogeneous and slightly metamorphosed composition of minerals; 5. locally extremely high concentrations of Ag and enrichment in Zn. The authors suggested that the volatile-rich organic material is composed of organometallic compounds.

The coexistence of organic compounds and of graphite microcrystals within the slightly metamorphosed material led to the proposal that they were genetically related (Semenenko, 1996).

In addition, to understand the formation and history of “mysterite” per se, our study was also motivated by the results of an investigation by Campins and Swindle (1998). These authors had discussed the question that has been asked for a long time (e.g., Anders, 1975): Is cometary material not already on Earth in the form of meteorites from “burned out” cometary nuclei? On the basis of results from the Halley missions (summarized in Jessberger, 1999), Campins and Swindle (1998) described the expected characteristics of cometary material and suggested that among all known types of meteoritic and xenolithic material, the carbonaceous inclusions from Krymka are the candidates that are most likely to be of cometary origin. To test this hypothesis however, a thorough understanding of the petrology and chemistry of the Krymka carbonaceous xenoliths is required. And that is what we set out to attain.

Here we present the results of a combined mineralogical, chemical, and isotopic study of two Krymka carbonaceous xenoliths, one of them has been previously described by Semenenko et al. (Semenenko et al 1991a, Semenenko et al 1991b; Semenenko, 1996), the other one is new. We find that both xenoliths have similar bulk chemistry and mineralogy and may represent a type of carbonaceous chondrite that has not previously been sampled. Their genetic relationship to cometary material remains highly inconclusive.