Chemical variation in Lonar impact glasses and impactites

摘要

The Lonar impact crater, India, is the only well-preserved, simple bowl-shaped crater excavated in continental flood basalts on Earth. The target rocks are 65-Ma basalts of the Deccan Traps. Chemical and petrographic studies were made of a set of 67 samples of Lonar impactites and impact glasses collected from various locations at Lonar. The results show that the composition of the impactites and impact glasses and those of the target basalts are almost identical, except for minor enrichments in K, As, Sb, and Cs in the impactites. There are some differences in the degree of enrichment or depletion of trace elements depending on the location of the samples. Compositional mapping (by back-scattered electron imagery and X-ray analysis) of those impactites and impact glasses show that both compact/dense and vesicular impact glasses/melts can be of homogenous internal composition, or can be composed of an assemblage of melt rock/glass fragments of slightly different composition, reflecting incomplete homogenization during melting and quenching. Some of the glasses/melts contain grains of magnetites and titanomagnetites ranging in diameter from a few to about 150 mu m. These magnetites and titanomagnetites are interpreted to be remnants from the target basalt and imply that these impactites and impact glasses have formed at atemperature of around 1600 degrees C. In agreement with some previous studies no extraterrestrial component was found in the glasses.