The Chelyabinsk meteorite: Thermal history and variable shock effects recorded by the 40Ar‐39Ar system

We studied three lithologies (light and dark chondritic and impact melt rock) differing in shock stage from the LL5 chondrite Chelyabinsk. Using the 40Ar‐39Ar dating technique, we identified low‐ and high‐temperature reservoirs within all samples, ascribed to K‐bearing oligoclase feldspar and shock‐induced jadeite–feldspar glass assemblages in melt veins, respectively. Trapped argon components had variable 40Ar/36Ar ratios even within low‐ and high‐temperature reservoirs of individual samples. Correcting for trapped argon revealed a lithology‐specific response of the K‐Ar system to shock metamorphism, thereby defining two distinct impact events affecting the Chelyabinsk parent asteroid (1) an intense impact event ~1.7 ± 0.1 Ga ago formed the light–dark‐structured and impact‐veined Chelyabinsk breccia. Such a one‐stage breccia formation is consistent with petrological observations and was recorded by the strongly shocked lithologies (dark and impact melt) where a significant fraction of oligoclase feldspar was transformed into jadeite and feldspathic glass; and (2) a young reset event ~30 Ma ago particularly affected the light lithology due to its low argon retentivity, while the more retentive shock‐induced phases were more resistant against thermal reset. Trapped argon with 40Ar/36Ar ratios up to 1900 was likely incorporated during impact‐induced events on the parent body, and mixed with terrestrial atmospheric argon contamination. Had it not been identified via isochrons based on high‐resolution argon extraction, several geochronologically meaningless ages would have been deduced.