Argon isotopic analysis of breccia veins from the Roter Kamm crater, Namibia, and implications for their thermal history

Abstract

The rocks exposed in the rim of the 2.5-km-wide and 3.7-Ma-old Roter Kamm crater in southwest Namibia are cut by breccia veins that macroscopically resemble, and were originally described as, pseudotachylytes. The veins were later shown to be cataclasites with no evidence for melting. 40Ar/39Ar data for vein and host rock samples indicate a low-grade metamorphic event at around 300 Ma, but provide no evidence for an impact age. The samples have suffered 5-7% Ar loss, which we associate with the impact event. All the samples record similar ranges of possible timetemperature conditions and there are no resolvable differences between the results for the vein and the host rock samples, as would be expected if frictional heating played an important role in breccia formation. Modeling the 40Ar/39Ar data, assuming instantaneous impact heating followed by extended cooling, and coupling these results to published data on fluid inclusions, quartz precipitation, shock effects, and crater degradation, suggest that the veins reached maximum temperatures of 230-290 °C during impact and never approached melting temperatures of the precursor rocks.