Experimentelle und Analytische Planetologie
Krymka: mysteries and established facts
The mineralogy, chemistry, and the carbon and oxygen isotopes of two exotic Krymka carbonaceous
xenoliths
were studied. 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 non-porphyritic 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.
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