POPIGAI CRATER - ORIGIN AND DISTRIBUTION OF DIAMOND-BEARING IMPACTITES

About three decades ago, a formerly unknown genetic type of natural di amonds was discovered in impact craters. Impact diamonds are currently known from a number of impact structures in Europe, Asia and North Am erica, and it's likely that the number of finds will increase with tim e. The Popigai crater, Northern Siberia, where impact diamonds were fi rst found, was specifically Investigated in terms of geology, geophysi cs, petrography and mineralogy. Large resources of industrial impact d iamonds were discovered, and these minerals were studied in detail. Au thigenic impact diamonds occur in situ in shocked graphite-bearing gne isses that are found as inclusions in impact melt rocks: tagamites and suevites. According to the observed transformation of coexisting mine rals, the lower estimated pressure of the coherent martensite transiti on of graphite to diamonds is 35 GPa. Impact diamonds inherit the orig inal shape of graphite crystals and are composed of a polycrystalline structured aggregate of cubic and probably hexagonal carbon microcryst als 1-5 mu m across. Numerous properties of diamonds depend on the hig h density of defects in the crystal lattice. Allothigenic impact diamo nds occur in rocks produced by the homogenization and solidification o f impact melt, which originated from the complete fusion of graphite-b earing precursor gneisses. These diamonds usually reflect the influenc e of the hot melt and are strongly corroded. Diamond-bearing tagamites and suevites in the Popigai crater interior occur as extended lens an d sheets bodies and also as irregular small bodies. Diamond distributi on depends on the original abundance of precursor graphite in the targ et rocks, on the superimposed shock-metamorphic zonation, and on the c haracter of the ejection of shocked and melted material along differen t trajectories and azimuths. This has resulted in radial and concentri c inhomogeneities in diamond distribution in the crater interior. On a second order, the distribution depends on the scale of melt contamina tion by clasts and fragments and by the duration of cooling of certain melt bodies and their constituents. Enrichment in diamonds at the mar gins of thick tagamite sheets is the result of rapid cooling, which pr events combustion of diamonds. A positive correlation between diamond content and the amount of phosphorus pentoxide in impactites indicates links to C and P probably in organic matter of the primary sedimentar y rocks, which were subjected to granulitic metamorphism 2.4 Ga ago an d melted at the time of impact 35.7 Ma ago.