Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.541m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 541
Other
Craters, Impact, Diamonds, Graphite, Melting, Shock Effects
Scientific paper
The impact diamonds are accessory minerals of impactites (impact melt rocks) and usually display presence of hexagonal phase. Popigai impact crater was the first place of their find in indigenous rocks in 1971; later these minerals were revealed in impactites of some other impact structures. Diamond-bearing impactites originate due to impact melting of crystalline and sedimentary target rocks containing accordingly graphite or coal, which transform to high-pressure carbon phases having polycrystalline texture. Apographite impact diamonds inherit the shape of the precursor and are distinct in appearance and properties from apocoal ones. Original carbon-bearing rocks which underwent fusion after pressure release from about 45-55 GPa but remained unmixed due to quick quenching are considered as "protoimpactites" and made up mostly of monomineral fusion glasses with palimpsest initial texture. Authigenic impact diamonds were found in this types of rocks in Popigai and Ries craters. The coexistence of fusion and diaplectic glasses originated after rock-forming minerals gives opportunity to evaluate P-T regimes of formation of apographite or apocoal high-pressure phases. Mixed and homogenized impact melt may form after quenching or crystallization two other types of diamond-bearing impactites - suevites and tagamites. In the first ones diamonds occur in bombs and fragments of glass; in the second ones diamonds are included in hemicrystalline or holocrystalline matrix containing clasts. Both these rocks occur in coherent sheets, covers, irregular bodies, more rare as dykes, just as "protoimpactites" are usually observed in form of inclusions in suevites and tagamites. Definite regularities in distribution of impact diamonds in these rocks are controlled by their formational conditions. Diamond concentrations in tagamites and suevites show the distinct positive correlation in some local areas in crater interiors, moreover individual radial zones of impactites are relatively enriched in diamonds. These heterogeneities may be referred to those of the first order. Heterogeneities of the second order are revealed in diamond distribution in separate bodies of tagamites. Increased concentrations are more typical in the parts that experienced quick chilling. Such tagamites are distinguished by low crystallinity, thin reaction rims surrounding clasts, they often contain significant quantity of rock inclusions. The primary concentric zonality of shock alterations appeared in the target rocks (characterized by irregularity in distribution of original carbon) due to impact transforms during excavation and ejection. The radial heterogeneities in diamond distribution are formed simultaneously. High-pressure phases in the impact melt (in forms of large bodies or small splashes and bombs) may be preserved from graphitization and oxidation in case of rather quick heat loss. The overheated fractions of melt originated in the central part of impact site may appear to be virtually barren of diamonds. Re-washing of diamond-bearing impactites of certain craters or rewashing of diamond-bearing distant ejecta lead to formation of placers. In some cases polycrystalline impact diamonds found in them are taken erroneously for carbonado.
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