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

The study addresses the absence of precise age constraints for very ancient terrestrial impact structures and evaluates the potential link between impacts and major environmental transitions in early Earth history. The terrestrial impact record is fragmentary due to tectonics and erosion, and is progressively erased deeper into geologic time when impact rates were higher. The oldest impact evidence on Earth comes from Archean–Palaeoproterozoic ejecta layers (ca. 3.47 to 2.46 Ga), but corresponding craters are unidentified. Prior to this work, only two Precambrian impact structures had precise ages: Vredefort (2023 ± 4 Ma) and Sudbury (1850 ± 1 Ma). The period 2.5–2.1 Ga, encompassing significant changes in Earth’s hydrosphere and atmosphere, lacked dated impact craters. Yarrabubba, a deeply eroded structure in the Yilgarn Craton with a ~20 km demagnetized central uplift anomaly implying a ~70 km original crater, had only broad age bounds (younger than 2650 ± 10 Ma host monzogranite and older than ca. 1200–1075 Ma dolerite dykes). The research aims to obtain a precise impact age using shock-recrystallised minerals and to assess implications for Palaeoproterozoic climate, particularly the termination of glacial conditions.

Background work has established a sparse and contentious record of early Earth impacts. Archean to Palaeoproterozoic spherule layers (e.g., in the Kaapvaal and Pilbara cratons) date between ca. 3.47 and 2.46 Ga, but associated craters are unknown. Precisely dated Precambrian structures include Vredefort (2023 ± 4 Ma) and Sudbury (1850 ± 1 Ma); other proposed Palaeoproterozoic structures have poor age constraints or debated impact evidence. The Yarrabubba structure has been recognized as an impact site based on shatter cones and shocked quartz, with a granophyric body (Barlangi Rock) interpreted as impact melt. Previous zircon ages from Barlangi granophyre showed complex spectra (2.79–2.23 Ga), and pseudotachylite sericite 39Ar/40Ar ages (~1.13 Ga) likely reflect later alteration. The broader geologic context includes multiple Palaeoproterozoic glaciations (2.4–2.2 Ga), the Great Oxidation Event (2.45–2.06 Ga), and a lull in global magmatism (2266–2214 Ma), complicating causal links between volcanism and glacial termination. Prior work has suggested that large impacts can perturb climate, but quantitative estimates for water vapour release during Palaeoproterozoic impacts were limited.

- Field sampling and materials: Two samples from the Yarrabubba structure were analyzed: shocked Yarrabubba monzogranite (14YB07) and Barlangi granophyre impact melt (14YB03). The monzogranite sample was collected ~1.2 km WSW of Barlangi Rock; the granophyre sample was from an apophyse ~2.7 km NNW of Barlangi Rock intruding along a shallowly dipping fault or fracture. - Sample preparation: ~1 kg splits were disaggregated using a Selfrag electric pulse disaggregator. Heavy minerals were concentrated with methylene iodide, then separated with a Frantz magnetic separator. Zircon and monazite grains were handpicked and mounted in epoxy, mechanically polished to 1 µm and finished with chemical–mechanical polish (5 nm silica in NaOH). - Microstructural characterization: High-resolution orientation mapping (electron backscatter diffraction, EBSD), cathodoluminescence (CL), backscattered electron (BSE) imaging, and inverse pole figure (IPF) mapping were used to identify shock microstructures in zircon and monazite. Diagnostic features included {112} twins and {100} planar deformation bands in zircon; deformation twins on (001), (100), (101), subgrain boundaries, and strain-free neoblasts in monazite; polycrystalline zircon aggregates with systematic misorientations (65°/<110> indicative of recrystallisation after {112} twins; 90°/<110> indicative of reidite transformation) and ZrO2 inclusions indexing as baddeleyite with evidence for origin as tetragonal ZrO2, implying super-heated, silica-saturated impact melt (>1673 °C). - Geochronology: In situ U–Pb analyses by secondary ion mass spectrometry (SIMS/SHRIMP) targeted recrystallised (neoblastic) domains and shock-modified zones in zircon and monazite. Data reduction employed concordia/discordia regression (Tera-Wasserburg), weighted mean 207Pb/206Pb ages for near-concordant neoblasts, and interpretation of upper/lower intercepts. Ages and uncertainties are reported at 2σ; MSWD values assess data coherence. - Numerical impact modelling: The iSALE shock physics code simulated formation of a ~70 km-diameter crater in granite overlain by ice sheets 2–5 km thick. Simulations used a 7 km diameter impactor to reproduce final crater size. Tracer particles quantified volumes of ice shock-heated to incipient and complete vaporisation, yielding estimates of water vapour mass injected into the atmosphere and total ice melt volumes. Peak shock pressures in granite basement and temporal crater evolution were analyzed.

- Precise impact age from monazite neoblasts: Combined low-strain, randomly oriented monazite neoblasts from both Yarrabubba monzogranite and Barlangi granophyre yield a weighted mean 207Pb/206Pb age of 2229 ± 5 Ma (n = 26, MSWD = 1.4), interpreted as the Yarrabubba impact age. - Consistent ages within individual lithologies: Monazite neoblasts in monzogranite: 2227 ± 5 Ma (n = 12, MSWD = 0.89). Monazite neoblasts in granophyre: 2231 ± 8 Ma (n = 14, MSWD = 1.9). Shock-recrystallised zircon in granophyre defines an upper intercept age of 2246 ± 17 Ma (n = 13, MSWD = 1.2), overlapping the monazite age but less precise. - Host rock zircon systematics: Zircon from Yarrabubba monzogranite yields upper and lower concordia intercepts at 2626 ± 36 Ma (magmatic crystallisation, consistent with prior 2650 ± 20 Ma) and 1202 ± 210 Ma (partial resetting during Mesoproterozoic dolerite intrusion), respectively (MSWD = 1.8). - Evidence for super-heated impact melt: Presence of zircon with ZrO2 inclusions (originally tetragonal), polycrystalline zircon aggregates with specific misorientation relationships (65°/<110>, 90°/<110>) tied to {112} twins and reidite transformation, and baddeleyite intergrowths demonstrate shock metamorphism and thermal dissociation of zircon in superheated, silica-saturated melt (>1673 °C). - Climatic implications and modelling: The new age extends the terrestrial record of dated impact craters by >200 Myr and coincides, within uncertainty, with the youngest Palaeoproterozoic glacial deposit (Rietfontein diamictite; 2225 ± 3 Ma). iSALE simulations for a 70 km crater impacting granite overlain by a 2–5 km-thick ice sheet predict vaporisation of ~95–240 km3 of ice and up to ~5400 km3 total melting, corresponding to ~9 × 10^13 to 2 × 10^14 kg of H2O vapour jetted into the upper atmosphere within moments of impact. The abstract further estimates potential H2O release between 8.7 × 10^13 and 5.0 × 10^15 kg for a continental glacier scenario.

The study resolves the long-standing uncertainty regarding the age of the Yarrabubba structure by directly dating shock-recrystallised monazite and zircon domains, establishing an impact age of 2229 ± 5 Ma. This precise age situates Yarrabubba as Earth’s oldest recognised impact crater and fills a gap in the 2.5–2.1 Ga impact record. The temporal overlap with the termination of Palaeoproterozoic glaciations (Rietfontein diamictite at 2225 ± 3 Ma) and a contemporaneous lull in global magmatism (2266–2214 Ma) suggests that impact processes could have contributed to climatic transitions independent of volcanic outgassing. Numerical models show that a Yarrabubba-sized impact into a continental ice sheet could inject substantial water vapour into the upper atmosphere nearly instantaneously, providing a potential greenhouse forcing mechanism, contingent on atmospheric residence time and Palaeoproterozoic atmospheric composition. Although lower-atmosphere vapour would rapidly condense, high-altitude vapour may have had more persistent radiative effects. The findings underscore the potential for large impacts to perturb climate, analogous to recognized climatic impacts from the Chicxulub event, while highlighting that the Palaeoproterozoic context (low oxygen atmosphere, possible ice cover) could have led to different outcomes. The microstructural and geochronological concordance across minerals and lithologies strengthens the interpretation that the derived ages date the impact event rather than later metamorphism or alteration.

This work establishes a precise age of 2229 ± 5 Ma for the Yarrabubba impact structure using in situ U–Pb dating of shock-recrystallised monazite, supported by shock-reset zircon, making it the oldest recognised meteorite impact structure on Earth and extending the crater record by over 200 Myr. Microstructural evidence confirms extreme shock and thermal conditions, including super-heated impact melt. The impact age coincides with the end of Palaeoproterozoic glaciations, and numerical models indicate that a Yarrabubba-sized impact into an ice-covered continent could rapidly inject large masses of water vapour into the upper atmosphere, potentially contributing to climatic change. Future research should seek additional ancient impact structures on Archean cratons, refine atmospheric modelling to determine residence times and radiative effects of impact-generated water vapour in a low-oxygen Palaeoproterozoic atmosphere, better constrain the extent and synchronicity of late Palaeoproterozoic glaciations, and integrate geochemical proxies to test links between impacts and climate transitions.

- Paleogeographic and environmental conditions at the time of impact (presence/thickness of ice cover, ocean depth, carbonate platforms) are uncertain. - Composition and structure of the Palaeoproterozoic atmosphere are not well constrained, limiting precise modelling of vapour plume interactions and residence times. - Geographic extent and global significance of the Rietfontein diamictite (youngest glacial deposit) remain poorly constrained, hindering assessment of global climate impacts. - Some U–Pb datasets show discordance likely due to later Pb-loss (e.g., surface fluid exposure) and partial resetting by younger events (Mesoproterozoic dolerite intrusion), introducing interpretive complexities. - Numerical simulations depend on assumptions (e.g., ice thickness, impactor size) and may not capture all coupled climatic feedbacks (e.g., cloud albedo vs greenhouse effects).