The problem of inherited 40Ar* in dating impact glass by the 40Ar/39Ar method: Evidence from the Tswaing impact crater (South Africa)

The Tswaing meteorite impact crater is a 1.13 km diameter structure located in the 2.05 Ga Nebo granite of the Bushveld Complex. The impact age had previously been determined by fission track dating to 220 ± 104 ka. 40Ar/39Ar step-heating and total fusion experiments performed on single- and multi-grain impact glass aliquots gave apparent ages ranging from 1.0 ± 0.3 Ma to 204 ± 6 Ma. These “ages” indicate that the radiogenic Ar derived from the target rocks has not been completely degassed as a result of the impact process, despite fusion of the target material. Results of step-heating experiments imply that the 40Arinherited∗ trapped within the glass is located in two distinct reservoirs thought to be the glass matrix and fluid/vapor inclusions (or un-melted residual clasts). Calculations assuming an age of 0.2 ± 0.1 Ma for Tswaing (fission track data) reveal that the amount of inherited 40Ar*(40Arinherited∗) relative to the pre-impact concentration varies from 0.015% to 4.15%. The spread defined by 40Arinherited∗ likely reflects the various quench rates experienced by the glass, most certainly due to the pre-impact position of the sample relative to the center of the crater. We compare the influence of 40Arinherited∗ on the apparent 40Ar/39Ar age determination of five impact structures. Our calculations show that the main characteristic controlling the age offset (for a given proportion of 40Arinherited∗) is the age difference between the impact and the target rocks (i.e., the 40Ar* concentration in the target rock). The buffer effect for a given crater structure can be predicted knowing the age of the basement and having a rough estimation of the age of the crater structure itself. The occurrence of 40Arinherited∗ is likely influenced by (1) the degree of polymerization (i.e., silicate structure complexity) of the target rock and presumably related to the diffusivity of Ar in the melt and glass, (2) the Ar partial pressure at the grain boundary, (3) the quantity of energy involved in the impact, and (4) the porosity of the target rocks. For glass that inevitably suffers inherited and/or excess 40Ar*, the use of the inverse isochron technique can be appropriate but should be applied with careful statistical treatment.