Glasses Tech

March 8, 2010

Evaporative differentiation of impact-produced melts: Laser-simulation experiments and comparison with impact glasses from the Logoisk crater

Filed under: Glasses 2005 — admin @ 9:51 am

Eyeglasses

Evaporative modification of silicates during impacts by large meteorites is the result of high-temperature processing of colliding silicate materials. There are still unresolved questions about the influence of vaporization on the chemical compositions of impact glasses. Here we present the results of experimental modeling of impact-related high-temperature pulse heating of silicates with compositions representative of Earth’s crust. Trends involving differential volatilization of the main rock-forming elements were determined experimentally and compared with those predicted by thermodynamic equilibrium calculations. Our results indicate that impact-induced vaporization is significant for melts at temperatures of 3000–5000 K. As vaporization begins, melts lose Si, Fe, and alkalis and are enriched in Mg, Ca, Al, and Ti. With increased mass loss, melts start to lose Mg while Si and even Na can still be present. Still greater mass loss produces melts enriched in Ca, Al, and Ti. We then attempted to use the experimentally determined trends to look for signs of evaporative differentiation in impact glasses from the Logoisk structure (Byelorussia). Fresh impact glasses from suevites in the Logoisk crater have compositions that are in good agreement with the differentiation trends obtained both experimentally and via thermodynamic calculations in the range of 3000–5000 K. The most useful methods for recognizing evaporative differentiation in the natural glasses are the conservation of the initial Ca/Al ratio and the anti-correlation of Si and Al.

M.V. Gerasimov15, O.I. Yakovlev2, Yu.P. Dikov3 and F. Wlotzka4
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