40Ar/39Ar thermochronology of the fossil LL6- chondrite from Morokweng Crater, South Africa

Abstract

Studies of meteorites are based mostly on samples that fell to Earth in the recent past (i.e., a few million years at most). The Morokweng LL-chondrite meteorite is a particularly interesting specimen as its fall is much older (ca. 145 Ma) than most other meteorites and because it is the only macro-meteorite clast (width intersected in drill core: 25 cm) found in a melt sheet of a large impact structure. When applied to the Morokweng meteorite, 40Ar/39Ar thermochronology provides an opportunity to study (1) effects associated with pre-impact and post-impact processes and (2) collision events within a potentially distinct and as yet unsampled asteroid population.A single multi-grain aliquot yielded an inverse isochron age of 625 ± 163 Ma. This suggests a major in-space collisional event at this time. We have modeled the diffusion of 40Ar* within the meteorite and plagioclase during and after the 145 Ma impact on Earth to tentatively explain why pre-terrestrial impact 40Ar* has been preserved within the plagioclase grains. The 145 Ma terrestrial impact age is recorded in the low-retentivity sites of the meteorite plagioclase grains that yielded a composite inverse isochron age at 141 ± 15 Ma and thus, confirms that age information about major (terrestrial or extraterrestrial) impacts can be recorded in the K-rich mineral phases of a meteorite and measured by the 40Ar/39Ar technique. More studies on fossil meteorites need to be carried out to understand if the rough 0.6 Ga age proposed here corresponds to major LL-chondrite asteroid population destructions or, rather, to an isolated collision event.