New impact-melt rock from the Roter Kamm impact structure, Namibia: Further constraints on impact age, melt rock chemistry, and projectile composition

摘要

A new locality of in situ massive impact-melt rock was discovered on the south-southwestern rim of the Roter Kamm impact structure. While the sub-samples fi-orn this new locality are relatively homogeneous at the hand specimen scale, and despite being from a nearby location, they do not have the same composition of the only previously analyzed impact-melt rock sample from Roter Kamm. Both Roter Kamm impact-melt rock samples analyzed to date, as well as several suevite samples, exhibit a granitic-granodioritic precursor composition. Micro-chemical analyses of glassy matrix and Al-rich orthopyroxene microphenocrysts demonstrate rapid cooling and chemical disequilibrium at small scales. Platinum-group element abundances and ratios indicate an ordinary chondritic composition for the Roter Kamm impactor. Laser argon dating of two sub-samples did not reproduce the previously obtained age of 3.7 +/- 0.3 (1 sigma) for this impact event, based on Ar-40/Ar-39 dating of a single vesicular impact-melt rock. Instead, we obtained ages between 3.9 and 6.3 Ma, with an inverse isochron age of 4.7 +/- 0.3 Ma for one analyzed sub-sample and 5.1 +/- 0.4 Ma for the other. Clearly a post-5 Ma impact at Roter Kamm remains indicated, but further analytical work is required to better constrain the currently best estimate of 4-5 Ma. Both impactor and age constraints are clearly obstructed by the inherent microscopic heterogeneity and disequilibrium melting and cooling processes demonstrated in the present study.