Advancing meteorite impact chronology with in situ mica Rb–Sr dating: insights from the Australian Acraman and Gosses Bluff impact structures

Meteorite impacts are highly energetic processes that drive deformation under extreme pressure and temperature conditions, far exceeding those of typical terrestrial crustal processes. Constraining the timing of impacts through Earth history is important for understanding its role in Earth's biogeodynamics, yet less than ∼20 % of the impacts worldwide are precisely dated. Here, we present novel in situ mica Rb–Sr isotopic measurements from the Australian Acraman and Gosses Bluff impact sites collected via laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS), including single- and multi-collector instruments to expand the chronological toolbox for dating meteorite impacts. Whilst apatite yield older U–Pb ages (1462±82 Ma) compared to the expected Acraman impact age of 588±35 Ma, in situ muscovite Rb–Sr measurements from the Acraman ejecta layer (580±8 Ma) and texturally-related fine-grained zircon (565±19 Ma) are consistent with the proposed impact age. For the Gosses Bluff impact, an apatite U–Pb age of 132±14 Ma and an aliettite (smectite group phyllosilicate) Rb–Sr isochron age of 137±9 Ma are both consistent, within uncertainty, with the established impact age of 133±3 Ma. The comparison between single- and multi-collector ICP-MS/MS has shown that the latter yielded significantly more precise Sr measurements, likely owing to simultaneous isotope collection using Faraday amplifiers equipped with high-resistance amplifiers (10 13 Ω). This configuration minimizes the effects of temporal signal fluctuations and improves signal-to-noise ratios at low ion-beam intensities, enabling greater precision on Sr isotope measurements. Our results demonstrate that in situ Rb–Sr dating of newly-grown micas during impact metamorphism may be a suitable target to determine the age of meteorite impact events. This novel approach effectively addresses the textural complexities of impact-related rocks, often overlooked by bulk isotope-dilution techniques. The effectiveness of in situ mica Rb–Sr geochronology for dating impact events may reflect its ability to target mineral domains that preserve their original isotopic signatures despite subsequent alteration. This contrasts with previous 40Ar/39Ar results from the Acraman impact structure, which yielded younger apparent ages attributed to post-impact hydrothermal overprinting. We propose that in situ Rb–Sr geochronology offers an efficient approach to impact dating, with the potential to substantially expand the number of reliably dated impact structures worldwide while requiring minimal sample preparation.