Impact Melt Composition of the Obolon Crater: Chlorine as a Possible Indicator of the Submarine Crater Formation

Physics

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Earth, Glasses, Impact Craters, Impact Melting

Scientific paper

The Obolon impact crater, 17 km in diameter, is located on the Northern slope of the Ukrainian shield. The crater target is presented by the Precambrian crystalline rocks and sedimentary deposits of the Late Paleozoic and Early Mesozoic age. The Obolon crater formation in the shallow marine basin was supposed [1]. The crater has a complex structure. The presence of the central uplift was detected by the geophysical investigations. The two boreholes penetrated the allochthonous rock complex and the crater sediments in the Western part of the structure. The allochthonous breccia and suevites strata of 73 m thick was stripped by the hole 5302 bored at about 3 km to the West from the crater center. The impact melt rocks of the Obolon crater are represented by the fragments and particles of the devitrified glass in suevites, from 0.5-1.0 mm to 15-20 mm in size. The numerous inclusions of the shocked minerals are spread in the glass. The impact melt glass composition is presented in the table 1. The composition of the crystalline basement was estimated by calculation of the several rocks content in the allochthonous breccia and suevites. The gneisses and granitic gneisses compose about 65% of the crystalline target rocks. The composition of the impact glass is close to the composition of the gneisses of the crystalline basement. The peculiarity of the impact melt glass composition is its enrichment with chlorine and bromine. The chlorine content in the glass is about 5 times higher than in gneisses and about 7 times higher than in granites of the target. The possible source of the galogenides enrichment of the melt was the marine water covering the Obolon crater target at the time of impact. The enrichment with chlorine of the Pacific Ocean strewn field microtectites was described by S.V.Margolis and coauthors [2]. They proposed the marine water as the source of chlorine (and sodium) enrichment of the micrometeorites formed by impact into the ocean. So the enrichment of the impact melt with the galogenides is a possible indicator of the submarine conditions of the craters formation. References: [1] Valter A. A. et al. (1977) in Space Environment and the Earth, 76-80, Naukova, Kiev. [2] Margolis S. V. et al. (1991) Science, 251, 1594-1597.

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