Limited response of the K–Ar system to the Nordlinger Ries giant meteorite impact

Abstract

FOR the proper interpretation of radiometric rock ages it is important to know which processes are able to reset radioactive clocks. In particular, the resetting by intense shock effects is significant in lunar chronology because most lunar highland rocks are impact breccias produced in the course of multiple large meteorite impacts. For the important K–Ar scheme, we have dated variously shocked samples from ejecta and from a drill core through one of the best documented terrestrial meteorite craters the Nordlinger Ries (Southern Germany)1. From K–Ar and fission track dating of thoroughly melted impact glasses the 24 km diameter Ries crater was found to have formed 14.7±0.7 Myrago by meteorite impact into the variscian bedrock (for summary of ages see ref. 2). The modern version of K–Ar dating (39Ar–40Ar technique) can reveal quantitative information about partial losses of radiogenic argon (fractional resetting) in addition to yielding ages eventually not affected by these gas losses3,4. We have applied this dating technique to mineral separates (hornblende, biotite, chlorite) from ejecta, fall-back breccias and underlying bedrock of the Ries Crater, including drill core samples taken at depths up to 1,201 m(ref. l).The samples exhibit various degrees of mechanical strain (from ∼ 10 kbar up to >400 kbar shock pressure). Apart from the thoroughly molten glasses which reproduced the impact age of ∼ 14.7 Myr, we found that all other rocks involved in the impact irrespective of the degree of shock metamorphism and within the limits of error, yield the same 39Ar–4oAr age of 313±3 Myr, probably the age of the bedrock. Apparently, rocks exposed to the very intense shock effects frequent on the Moon give reliable K–Ar ages when measured as K–Ar plateau ages. Age resetting, if present, probably results from heating associated with such impacts. In the case of the Ries, we found from the diffusion characteristics of argon that the post-shock temperature of the crystalline fragments within the suevite layer has not exceeded 600 °C.