40Ar-39Ar step-heating of impact glasses from the Nordlinger Ries impact crater-Implications on excess argon in impact melts and tektites

Seven impact melts from various places in the Nordlinger Ries were dated by 40Ar-39Ar step-heating. The aim of these measurements was to increase the age data base for Ries impact glasses directly from the Ries crater, because there is only one Ar-Ar step-heating spectrum available in the literature. Almost all samples display saddle-shaped age spectra, indicating the presence of excess argon in most Ries glass samples, most probably inherited argon from incompletely degassed melt and possibly also excess argon incorporated during cooling from adjacent phases. In contrast, moldavites usually contain no inherited argon, probably due to their different formation process implying solidification during ballistic transport. The plateau age of the only flat spectrum is 14.60 +/- 0.16 (0.20) Ma (2 sigma), while the total age of this sample is 14.86 +/- 0.20 (0.22) Ma (isochron age: 14.72 +/- 0.18 [0.22] Ma [2 sigma]), proofing the chronological relationship of the Ries impact and moldavites. The total ages of the other samples range between 15.77 +/- 0.52 and 20.4 +/- 1.0 Ma (2 sigma), implying approximately 2-40% excess 40Ar (compared to the nominal age of the Ries crater) in respective samples. Thus, the age of 14.60 +/- 0.16 (0.20) (2 sigma) (14.75 +/- 0.16 [0.20 Ma] [2 sigma], calculated using the most recent suggestions for the K decay constants) can be considered as reliable and is within uncertainties indistinguishable from the most recent compilation for the age of the moldavite tektites.