Advancing meteorite impact chronology with in situ mica Rb-Sr dating

Abstract. Meteorite impacts are highly energetic processes that drives deformation on Earth under extreme pressure and temperature conditions, far exceeding those of typical crustal processes. Consequently, it may promote environmental perturbations, blossoming/extinction of life and even produce suitable conditions for the formation of mineral resources. Thus, constraining the timing of impacts is pivotal to shedding light on 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 isotopes from the Australian Acraman and Gosses Bluff impact sites collected via LA-ICP-MS/MS, including single- and multi-collector instruments to expand the chronological toolbox for dating meteorite impacts. Whilst monazite (980 ± 28 Ma) and apatite (1448 ± 79 Ma) yield older ages compared to the expected Acraman impact age of 588 ± 35 Ma, in situ Rb–Sr from muscovite-bearing domains from the Acraman ejecta layer (580 ± 8 Ma; multi-collector age) and associated fine-grained zircon (598 ± 16 Ma) are consistent with the expected impact age. Similarly, apatite (132 ± 14 Ma) and mica-bearing domains (137 ± 9 Ma; multi-collector age) are comparable to the proposed impact age of 133 ± 3 Ma, whilst zircon yields mostly discordant data. The Rb–Sr results comparison between single- and multi-collector ICP-MS/MS has shown that the latter yielded significantly more precise Sr measurements, likely due to measurement of St isotopes with high resistor Faraday cups (10¹³ Ω) resulting in improved signal-to-noise ratio, consequently yielding more precise isotopic ratios and isochron ages. Our findings show that in situ Rb–Sr dating of micas formed during impact metamorphism offers the means for determining the timing of meteorite impact events. This approach effectively addresses the textural complexities commonly present in impact-related rocks, which are often overlooked by bulk isotope-dilution techniques, and delivers accuracy sufficient to establish the age of impacts. The effectiveness of in situ Rb–Sr dating of impact-related micas may be attributed to their higher resistance to hydrothermal alteration compared to Ar isotopes, which often yield complex degassing spectra and younger apparent ages such as for the Acraman impact. This study shows that in situ Rb–Sr isotopes of newly grown micas in impact-related rocks refines the chronology of impacts, with potential to increase the number of dated impacts globally with a low-cost, speedy technique with minimum sample preparation. Such task is crucial for understanding the role of meteorite impacts, including their potential influence on environmental crises, mass extinctions, and the emergence of life.