Precise radiometric age establishes Yarrabubba, Western Australia, as Earth's oldest recognised meteorite impact structure

Extraterrestrial bombardment significantly influenced Earth's surface environment. However, the terrestrial impact record is incomplete due to tectonics and erosion, becoming increasingly fragmented in the geologic past. While Archaean to Palaeoproterozoic ejecta deposits exist in the Kaapvaal and Pilbara cratons (ca. 3470–2460 Ma), corresponding impact craters remain unidentified. Only two Precambrian impact structures are precisely dated: the 2023 ± 4 Ma Vredefort Dome and the 1850 ± 1 Ma Sudbury structure. Other Palaeoproterozoic structures have poorly constrained ages or contentious impact evidence. This incomplete record hinders establishing connections between impact events and Earth's environmental changes. The impact cratering record was notably absent from 2.5–2.1 Ga, a period of significant hydrosphere and atmosphere changes. Yarrabubba, a structure in Western Australia's Yilgarn Craton, is a potential candidate. While lacking a circular crater, its elliptical aeromagnetic anomaly suggests a deeply buried central uplift, indicating an original crater diameter of ~70 km. The structure's age was previously constrained to be younger than 2650 ± 10 Ma and older than ca. 1200–1075 Ma. Zircon crystals from the Barlangi granophyre (interpreted as impact melt) yielded ages ranging from 2.79 to 2.23 Ga, a complex spectrum spanning nearly 500 Myr. This study uses in situ U–Pb geochronology by secondary ion mass spectrometry (SIMS) to analyze recrystallized domains (neoblasts) in monazite and zircon, which provides precise ages for ancient impact events. This approach is used to determine a precise age for the Yarrabubba impact structure.

The existing literature highlights the incomplete nature of the terrestrial impact record, particularly for the Precambrian. While ejecta layers provide evidence of past impacts, the identification and dating of the corresponding craters remain challenging due to geological processes like erosion and tectonic activity. The lack of precisely dated impact structures, especially during periods of significant environmental change like the Paleoproterozoic era (2.5-1.6 billion years ago), hinders the understanding of the relationship between large impacts and global climate shifts. Previous studies on the Yarrabubba structure identified shocked quartz and shatter cones, suggesting an impact origin. However, the lack of a well-preserved crater and the complex zircon age spectrum necessitated further investigation using more precise dating techniques. The existing age constraints for Yarrabubba were based on the age of surrounding rocks, offering only a broad timeframe for the impact event.

This study employed in situ U–Pb geochronology using secondary ion mass spectrometry (SIMS) to analyze shock-recrystallized domains (neoblasts) within monazite and zircon grains. Two samples were collected: one from the shocked Yarrabubba monzogranite and another from the Barlangi granophyre (interpreted as impact melt). Detailed petrographic characterization and identification of shock features were performed on thin sections. Zircon and monazite grains were separated from the samples using standard techniques. The grains were then mounted in epoxy, polished, and analyzed using SIMS. High-resolution orientation mapping was correlated with in situ U–Pb analysis to investigate the microstructure and age of shock features. The U-Pb ages obtained from the neoblasts are interpreted as representing the time of shock metamorphism and therefore the age of the impact event. To investigate potential climatic effects, numerical impact simulations using the iSALE shock physics code were performed to estimate water vapor release from an impact into a continental glacier.

Analysis of shock microstructures in zircon and monazite grains revealed evidence of impact metamorphism, including planar deformation features, shock twins, and recrystallized neoblasts. U-Pb SIMS analysis of zircon from the Yarrabubba monzogranite showed a primary magmatic crystallisation age consistent with previous studies. However, the analysis also indicated partial resetting associated with later dolerite intrusion. Monazite analyses from the monzogranite showed a bimodal age distribution; older ages possibly representing a post-crystallisation event or partial Pb loss and younger ages from low-strain neoblasts clustering around 2227 ± 5 Ma. Analysis of Barlangi granophyre zircon revealed inherited (pre-impact) zircon grains as xenocrysts and a younger population which showed an upper intercept age of 2246 ± 17 Ma indicating near complete resetting of U-Pb systematics during shock metamorphism. Monazite from the Barlangi granophyre showed a similar bimodal age distribution. Combining all neoblastic monazite data from both samples yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2229 ± 5 Ma. This age is consistent with the zircon results and represents the best estimate for the Yarrabubba impact event. Numerical simulations indicated that an impact into a 2-5 km thick ice sheet could vaporize 95-240 cubic kilometers of ice, releasing 9 × 10¹³ to 2 × 10¹⁴ kg of water vapor into the atmosphere.

The precise age of 2229 ± 5 Ma for the Yarrabubba impact structure extends the terrestrial impact record by over 200 million years. The remarkably close proximity of this age to the termination of the youngest Palaeoproterozoic glacial deposits (Rietfontein diamictite, 2225 ± 3 Ma) raises the possibility of a causal link. While the extent of glaciation at that time is not fully understood, the numerical simulations suggest that a significant amount of water vapor could have been released into the atmosphere by the impact. The Paleoproterozoic atmosphere had a lower oxygen concentration compared to today, which means it is possible that the amount of water vapor released could have had significant climatic effects. However, the exact nature of these effects, including the atmospheric residence time of the water vapor and the interplay of radiative and insulative effects of clouds, requires further investigation. The study highlights the potential role of meteorite impacts in influencing the Earth's climate, particularly during periods of significant environmental change. Future research should focus on further refining the age constraints, understanding the extent of the Rietfontein diamictite, and more sophisticated climate modelling to investigate the potential impact of water vapor release on the Paleoproterozoic climate.

This study successfully established the Yarrabubba structure as Earth's oldest known meteorite impact structure, with a precise age of 2229 ± 5 Ma. This significantly extends the known terrestrial cratering record. The temporal coincidence with the termination of the last Palaeoproterozoic glaciation suggests a potential connection between the impact and climatic changes, prompting further investigation into the climatic effects of large impacts on early Earth. Future research should focus on improving the precision of the age data, investigating the atmospheric and climatic consequences of such an event, and searching for similar structures within other ancient cratons.

The study relied on a limited number of samples, though they were strategically selected. The interpretation of the age data assumes that the neoblasts formed solely during the impact event, without influence from subsequent thermal events or alteration. Uncertainty remains regarding the precise extent of glacial conditions at the time of impact and the atmospheric conditions of the early Paleoproterozoic, which are needed to better refine climate models to account for vapor release.