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

Hecht, Lutz; Reimold, W. Uwe; Sherlock, Sarah; Tagle, Roald; Koeberl, Christian and Schmitt, Ralf-Thomas (2008). New impact-melt rock from the Roter Kamm impact structure, Namibia: Further constraints on impact age, melt rock chemistry, and projectile composition. Meteoritics and Planetary Science, 43(7) pp. 1201–1218.

DOI: https://doi.org/10.1111/j.1945-5100.2008.tb01123.x

Abstract

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 from 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) for this impact event, based on 40Ar/39Ar 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.

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