Loss of radiogenic argon from shocked granitic clasts in suevite deposits from the Ries Crater

Abstract

Five granitic clasts from the Otting and Aumühle quarries in the suevite ejecta deposits beyond the rim of the Ries, Germany, impact crater have been characterized as to modal composition, degree of shock, and loss of radiogenic argon. The shock pressures experienced by this selected suite of samples range from < 15 to > 60 GPa, and the samples have been modestly shocked to heavily shocked, or even melted. Ages for these samples were determined by the 39Ar 40Ar technique. The least shocked specimen (10–15 GPa) shows very little loss of radiogenic Ar relative to the approximately 320 My age of the Variscan basement at the Ries. All other samples (~28 GPa, ~42 GPa, ~52 GPa, and an impact melt) suggest complete to nearly complete loss of radiogenic Ar at the time of the 15 My old impact event. Most of the radiogenic Ar in these granitic samples is contained in feldspar, with a lesser amount in biotite. Diffusive loss of Ar occurs relatively easily from these highly disordered feldspars. The essentially complete loss of radiogenic Ar from 4 of the 5 samples is, in large part, ascribed to thermal activation during the cooling history of the suevite rather than during initial passage of the shock wave. These data are consistent with the conclusion from previous paleomagnetic studies that the suevite was deposited at temperatures above 500°C. The fact that Staudacher et al. (1982) found almost no loss of radiogenic Ar in a wide variety of Ries samples is ascribed to two causes: a) most of their samples were shocked to < 15 GPa and were collected in drill cores from the modestly shocked crater bottom, which was not as hot as the suevite; and b) their observations were primarily made on hornblende and biotite separates that are more resistant to shock effects and to diffusive loss of Ar, compared to shocked feldspars. Loss of argon from our suevite samples likely occurred in a “hot”, post-impact ejecta layer. From data presented here we estimate that < 50% of the material composing the “hot” Ries ejecta should show at least partially reset K-Ar ages; such materials compose no more than 5% of the total displaced crater volume.