Physics – Geophysics
Scientific paper
Mar 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33..349m&link_type=abstract
Meteoritics & Planetary Science, vol. 33, no. 2, pages 349-359.
Physics
Geophysics
13
Scientific paper
About three decades ago, a formerly unknown genetic type of natural diamonds was discovered in impact craters. Impact diamonds are currently known from a number of impact structures in Europe, Asia and North America and its likely that the number of finds will increase with time. The Popigai crater, Northern Siberia, where impact diamonds were first found, was specifically investigated in terms of geology, geophysics, petrography and mineralogy, and extensive programs of drilling to ascertain diamond distribution. Large resources of industrial impact diamonds were discovered, and these minerals were studied in detail. Authigenic impact diamonds occur in situ, in shocked graphite-bearing gneisses which are found as inclusions in impact melt rocks - tagamites and suevites. According to the observed transformation of co-existing minerals, the lower estimated pressure of the coherent martensite transition of graphite to diamonds is 35 GPa. Impact diamonds inherit the original shape of graphite crystals and are composed of a polycrystalline structured aggregate of cubic and probably hexagonal carbon microcrystals 1-5 micrometer across. Numerous properties of diamonds depend on the high density of defects in the crystal lattice. Allothigenic impact diamonds occur in rocks produced by the homogenization and solidification of impact melt, which originated from the complete fusion of graphite-bearing precursor gneisses. These diamonds usually reflect the influence of the hot melt and are strongly corroded. Diamond-bearing tagamites and suevites in the Popigai crater interior occur as extended lens-like and sheet-like bodies, but also as irregular and small bodies. Diamond distribution depends on the original abundance of precursor graphite in the target rocks, on the superimposed shock-metamorphic zonation and on the character of the ejection of shocked and melted material along different trajectories and azimuths. This has resulted in radial and concentric inhomogeneities in diamond distribution in the crater interior. On a second order, the distribution depends on the scale of melt contamination by clasts and fragments and by the duration of cooling of certain melt bodies and their constituents. Enrichment in diamonds at the margins of thick tagamite sheets is the result of rapid cooling, which prevents combustion of diamonds. A positive correlation between diamond content and the amount of phosphorus pentoxide in impactites indicates links to carbon and phosphorus probably in organic matter of the primary sedimentary rocks, which subjected to granulitic metamorphism 2.4 Ga ago and melted at the time of impact 35.7 ma ago.
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