Ries impact crater, Germany: Thermal history of the suevite breccia

Details Ries impact crater, Germany: Thermal history of the suevite breccia Ries impact crater, Germany: Thermal history of the suevite breccia

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.463e&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 463-464

Physics

1

Breccia, Geochronology, Germany, Impact Melts, Meteorite Craters, Thermal Analysis, Ejecta, Glass, Nucleation, Remanence, Temperature Effects

Scientific paper

Suevite breccias deposited in a multitude of small patches around the transient Ries crater on top of earlier ejecta of the impact, consist of a chaotic mixture of crystalline rock clasts in all stages of shock metamorphism, fragments and aerodynamically shaped bombs of glass from shock-melted basement gneiss, and a few sedimentary clasts in a fine-grained montmorillonitic groundmass. All suevites show a reversed remanent magnetization, due to magnetite, corresponding to the direction of the terrestrial magnetic field at the time of the Ries impact, 14.8 Ma. It follows that suevite breccias were deposited at a temperature above 580 C, the Curie point of magnetite. The temperature at which the suevites were deposited and their subsequent cooling were explored. Information on this process is stored in the bombs and fragments of vitreous and devitrified glass. We have, therefore, studied the suevite glasses in outcrops, their physical properties, and the nature of devitrification products, and tried to simulate the observed devitrification phases by crystallization and cooling experiments under controlled conditions. It was concluded that the suevite breccia with bombs and lumps of undercooled impact melt was deposited at a temperature of about 700 C, close to the temperature range in which the undercooled melt transforms into rigid glass. Due to heat conduction and convection the temperature dropped within a 1-m-thick basal zone and a probably thicker top layer below about 650 C in times too short for nucleation and formation of visible crystals in the glasses. At larger distances from base and top, devitrification and oxidation took place as if the undercooled melt lumps were under ambient water pressures of about 20 bar -- a consequence of the unequilibrated state of these products of adiabatic release of an impact melt, formed at about 80 Gpa and more than 2000 C.

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