Extraterrestrial bombardment flux is speculated to have had major consequences for the development of Earth’s surface environment. However, the terrestrial impact record is fragmentary, principally due to tectonics and erosion, and is progressively erased into the geologic past when, conversely, the bombardment rate was larger than today. The oldest record of impacts on Earth are Archaean to Palaeoproterozoic ejecta deposits found within the Kaapvaal craton of southern Africa and the Pilbara Craton in Western Australia, spanning roughly 3470 to 2460 million years ago; however, no corresponding impact craters have been identified. Currently only two precisely dated Precambrian-age impact structures are known, the 2023-million-year-old, 350 km diameter Vredefort Dome in South Africa, and the 1850 million year old 200 km diameter Sudbury structure in Canada. Other purported Palaeoproterozoic-age impact structures have either poorly constrained ages, or highly contentious impact evidence. A consequence of the incomplete terrestrial impact record is that connections between impact events and punctuated changes to the atmosphere, oceans, lithosphere, and life remain difficult to establish, with the notable exception of the Cretaceous–Paleogene Chicxulub impact. Hitherto, the impact cratering record was absent from 2.5–2.1 billion years ago, when significant changes in the Earth’s hydrosphere and atmosphere occurred.
In a paper published in the journal Nature Communications on 21 January 2020. Timmons Erickson of the Astromaterials Research and Exploration Science Division at NASA's Johnson Space Center, the Institute for Geoscience Research at Curtin University, and the Center for Lunar Science and Exploration of the Universities Space Research Association, Christopher Kirkland, Nicholas Timms, and Aaron Cavosie, also of the Institute for Geoscience Research at Curtin University, and Thomas Davison of the Impacts and Astromaterials Research Centre at Imperial College London, identify the Yarrabubba Impact Structure in Western Australia as the oldest surviving impact structure on Earth, and give their reasons for this assessment and the implications for the history of the Earth's geology and biosphere.
Yarrabubba is a recognised impact structure located within the Murchison Domain of the Archaean granite-greenstone Yilgarn Craton of Western Australia. No circular crater remains at Yarrabubba; however, the structure has an elliptical aeromagnetic anomaly (magnitid anomaly detectable from the air) consisting of an even, low total magnetic intensity domain, measuring approximately 20 km north-south by 11 km east-west. The present day exposure represents a deep erosional level, as neither impact breccias nor topographic expressions of the over-turned rim or central uplift are preserved. Therefore, the 20 km diameter magnetic anomaly has been interpreted to represent the remnant of the deeply buried central uplift of the structure, which is consistent with an original crater diameter of 70 km. Unshocked dolerite dykes formed during either the roughly. 1200 million years ago Muggamurra or roughly 1075 million years ago Warakurna regional volcanism cross-cut the elliptical magnetic anomaly and thus post-date the impact event.
Composite aeromagnetic anomaly map of the Yarrabubba Impact Structure within the Yilgarn Craton, Western Australia, showing the locations of key outcrops and samples used in this study. The image combines the total magnetic intensity (TMI, cool to warm colours) with the second vertical derivative of the total magnetic intensity (2VD, greyscale) data. The demagnetised anomaly centred on the outcrops of the Barlangi granophyre is considered to be the eroded remnant of the central uplift domain, which forms the basis of the crater diameter of 70 km. Prominent, narrow linear anomalies that cross-cut the demagnetised zone with broadly east-west orientations are mafic dykes that post-date the impact structure. Erickson et al (2020).
The main target rocks at the Yarrabubba structure are granitoids collectively known as the Yarrabubba Monzogranite. Identification of shocked quartz and shatter cones in the Yarrabubba Monzogranite confirmed an impact origin for the structure. The structure is centred on a large exposure of granophyre known locally as Barlangi Rock. Barlangi Granophyre is a sodic rhyolite that has been interpreted as an impact-generated melt rock, radiating dykelike apophyses of granophyre outcrop as far as 3 km from the centre of the structure. The Barlangi granophyre has thus been interpreted to have intruded into the Yarrabubba Monzogranite along faults rather than forming a flat-lying, crater-filling melt sheet, similar to metanorite dykes and apophyses interpreted as impact melt that are exposed in the core of the deeply eroded Vredefort impact structure.
The age of the Yarrabubba impact structure was previously constrained only to be younger than the 2650-million-year-old Yarrabubba Monzogranite and older than the 1200-1075-million-year-old cross-cutting dolerite dykes. Zircon crystals from the Barlangi Granophyre have previously yielded a complex age spectrum that span nearly 500 million years, from 2.79 to 2.23 billion years ago. Pseudotachylite veins at Yarrabubba yield a sericite Argon³⁹/Argon⁴⁰ age of about 1.13 billion years, which likely records alteration during younger mafic volcanism.
Argon-Argon dating relies on determining the ratio of radioactive
Argon⁴⁰ to non-radioactive Argon³⁹ within minerals from igneous or
metamorphic rock (in this case volcanic ash) to determine how long ago
the mineral cooled sufficiently to crystallise. The ratio of Argon⁴⁰ to
Argon³⁹ is constant in the atmosphere, and this ratio will be preserved
in a mineral at the time of crystallisation. No further Argon³⁹ will
enter the mineral from this point, but Argon⁴⁰ is produced by the decay
of radioactive Potassium⁴⁰, and increases in the mineral at a steady
rate, providing a clock which can be used to date the mineral.
Erickson et al.'s study utilises targeted in situ Uranium-Lead geochronology by secondary ion mass spectrometry to analyse recrystallised domains (neoblasts) in monazite and zircon, which have been shown to yield precise ages for ancient impact events. They present high-resolution orientation mapping and correlated in situ Uranium-Lead analysis to investigate the microstructure and age of shock features in zircon and monazite in target rock and impact melt from the Yarrabubba structure in Western Australia. These results establish Yarrabubba as the oldest preserved impact structure on Earth.
Zircon and monazite are minerals formed by the crystallisation of cooling igneous melts. Whenthey form they often contain trace amounts of uranium, which decays into (amongst other things) lead at a
known rate. Since lead will not have been present in the original crystal, it is possible to calculate the age of a zircon crystal from the
ratio between these elements.
Within the Yarrabubba Monzogranite, zircon and monazite grains preserve a range of impact-related microstructures. Zircon displays primary igneous growth zoning that is cross-cut by planar and subplanar shock microstructures, including shock twins and planar deformation bands. Monazite preserves a broader range of impact-related textures including domains with low-angle subgrain boundaries and multiple sets of shock twins along three fracture plains and domains of strainfree neoblasts (grains formed more recently than the surrounding matrix).
Examples of shocked zircon and monazite grains from Yarrabubba monzogranite sample 14YB07 and Barlangi Granophyre impact melt sample 14YB03. (a) Cathodoluminescence (CL) and Inverse Pole Figure (IPF) images of a shocked zircon with deformation twins. The zircon contains primary oscillatory zoning that is cross-cut by shock deformation twins and subplanar low-angle grain boundaries. (b) Backscattered electron (BSE) image and electron backscatter diffraction (EBSD) all Euler map of a shocked monazite with systematic shock twin domains that are overprinted by neoblasts. (c) Cathodoluminescence and Inverse Pole Figure images of a polycrystalline shocked zircon grain from the granophyric impact melt. Note that individual crystallites exhibit concentric Cathodoluminescence zonation patterns while the overall Cathodoluminescence pattern reflects the original zonation of the grain with a Cathodoluminescence brighter core to Cathodoluminescence dark rim. The Inverse Pole Figure orientation map of the zircon is dominantly blue to pink and many of the granules have systematic grain boundaries of either 65° or 90°. (d) Backscattered electron image and all Euler map of shock-deformed monazite displaying highly deformed and twinned domains that are overprinted by neoblasts, from the Barlangi Granophyre. Location of Uranium-Lead secondary ion mass spectrometry analytical spots are denoted on each grain with the Lead²⁰⁷/Lead²⁰⁶ age and 2σ errors in millions of years. Erickson et al. (2020).
In The Barlangi Granophyre, zircon textures range from unshocked grains preserving primary igneous growth zoning, to grains containing clear evidence of impact metamorphism, such as planar microstructures, polycrystalline aggregates and grains with Zirconium Oxide inclusions. Neoblasts within polycrystalline zircon aggregates contain systematic misorientation relationships with one another, which can only be caused by recrystallisation after formation of twins and the highpressure polymorph reidite, respectively, and are unambiguous indicators of shock metamorphism. While the Zirconium Oxide inclusions index as baddeleyite (monoclinic-Zirconium Oxide), crystallographic orientation relationships among transformation twins demonstrate they originally formed from tetragonal-Zirconium Oxide parent grains. Thermal dissociation of zircon to tetragonal-Zirconium Oxide only occurs in silica-saturated melts above 1673 °C, unequivocally indicating the Barlangi Granophyre was a super-heated impact melt. Monazite grains in the Barlangi Granophyre preserve a similar range of impact-related features to those from Yarrabubba Monzogranite, including crystal-plastic strain, deformation twins diagnostic of shock conditions, and strain-free neoblastic domains.
Yarrabubba Monzogranite (14YB07), zircon. Erickson et al. (2020).
The Uranium-Lead secondary ion mass spectrometry analyses of zircons from the Yarrabubba Monzogranite produced two clusters of dates; the first centred on 2626 million years ago, and the second centred on 1202 million years ago. The older date is interpreted as the primary magmatic crystallisation age of the target rocks, which has previously been constrained to between 2670 and 2630 million years old. The younder date is attributed to partial resetting associated with post-impact dolerite intrusion in the Mesoproterozoic. Monazite Lead²⁰⁷/Lead²⁰⁶ ages from the Yarrabubba Monzogranite also yield a bimodal distribution. Analytical spots from the high-strain shocked host and/or twin domains are variably discordant and record ²⁰⁷Lead/²⁰⁶Lead ages from 2478 to 2323 million years old. These ages may represent either formation during a post-crystallisation metamorphic event or partial radiogenic Lead-loss during the impact event or a subsequent thermal event. In contrast, spots from low-strain, randomly oriented neoblasts cluster around a Lead²⁰⁷/Lead²⁰⁶ age of 2227 million years old (Lead²⁰⁷/Lead²⁰⁶ dating is a varient on Uranium-Lead dating, typically used on bulk rock samples; it relies on the fact that both Lead²⁰⁷ and Lead²⁰⁶ are produced by the decay of uranium in a known ratio, while non-radiogenic Lead²⁰⁴ is not, so that over time the ratio of these isotopes in a sample of rock that originally contained uranium will change at a predictable rate).
Yarrabubba Monzogranite (14YB07), monazite. Erickson et al. (2020).
Barlangi \Granophyre zircon Lead²⁰⁷/Lead²⁰⁶ ages also show a bimodal age distribution. Erickson et al. interpret the oscillatory-zoned cores with apparent ages of 2781 to 2319 million years old to represent inherited (pre-impact) zircon grains that were incorporated into the Barlangi Granophyre as xenocrysts, consistent with zircon ages determined previously. These results indicate the presence of a significant source component in the Barlangi Granophyre that predates the 2.65-billion-year-old Yarrabubba Monzogranite. Individual analyses from polycrystalline zircon domains are variably discordant and yield Lead²⁰⁷/Lead²⁰⁶ ages from 2259 to 2156-million-years-old. Erickson et al. interpret the data to represent near-recent Lead-loss resulting from exposure to surface fluids. The data from recrystallised zircon domains yields an upper age of 2246 million years. Erickson et al. interpret this date to reflect both new zircon growth and near complete resetting of Uraniaum-Lead systematics in pre-existing domains during shock metamorphism. This date falls within the uncertainty of a single previously reported Uraniaum-Lead zircon analysis from the Barlangi Granophyre, which gave a date range of 2262-2206 million years, and which was inferred to indicate a Palaeoproterozoic impact age.
Barlangi Granophyre (14YB03), zircon. Erickson et al. (2020).
Barlangi Granophyre monazite Lead²⁰⁷/Lead²⁰⁶ ages preserve a bimodal distribution similar to monazite from Yarrabubba Monzogranite. Analyses from the highly strained host and twinned domains display variable normal and reverse discordance and record Lead²⁰⁷/Lead²⁰⁶ ages of between 2457 and 2284 million years. In contrast, analyses from low-strain, randomly oriented neoblasts cluster around a Lead²⁰⁷/Lead²⁰⁶ age of 2231 million years.
Barlangi Granophyre (14YB03), zircon. Erickson et al. (2020).
When combined, all neoblastic monazite domains from both Barlangi Granophyre and Yarrabubba Monzogranite define a cluster on a mean Lead²⁰⁷/Lead²⁰⁶ age of 2229 (±5) million years, which Erickson et al. interpret to record monazite recrystallisation during shock metamorphism and the best estimate of the Yarrabubba impact event. The weighted mean Lead²⁰⁷/Lead²⁰⁶ age for neoblastic zircon of 2246 (±17) million years, overlaps with the Monazite age, but is less precise. The new Yarrabubba impact age of 2229 million years determined by Erickson et al. extends the terrestrial record of impact craters by 200 million years, and demonstrates the potential for discovery of ancient impact structures on Archaean cratons.
Barlangi Granophyre (14YB03), monazite. Erickson et al. (2020).
The age constraints presented by Erickson et al. establish Yarrabubba as the first recognised meteorite impact to have occurred during the Rhyacian Period, a dynamic time in the evolution of Earth following the transition from the Archaean to the Proterozoic eon. At least four glacial diamictite deposits, three of which are found on multiple cratons, are recognised between 2.4 and 2.2 billion years ago. Of these deposits, the over 2.42-billion-year-old Makganyene Diamictite from the Kaapvaal Craton of Southern Africa has been interpreted to represent lowlatitude glaciers that may signify global ice conditions. The youngest Palaeoproterozoic glacial deposit, the Rietfontein Diamictite within the Transvaal Basin of South Africa, has a minimum depositional age of 2225 million years, based on the overlying Hekpoort \Basalt, which is within analytical uncertainty of the Yarrabubba Impact Event. Glacial diamictite deposits do not appear again in the geological record for over 400 million years. What caused the extended absence of glacial conditions after about 2225 million years ago is debatable. The end of the Palaeoproterozoic glaciations at 2225 million years ago occurred within an apparent 50 million year lull in global magmatism from 2266 to 2214 million years ago, making it difficult to appeal to volcanic outgassing as having played a significant role in forcing the glacial termination. Therefore, other mechanisms such as impact cratering need consideration. Radiometric age data presented here demonstrate synchronicity, within uncertainty, between the 2229-million-year-old Yarrabubba Impact Event and the termination of glacial conditions (i.e. the Rietfontein Diamictite) at 2225 million years ago. The geographic extent of the Rietfontein Diamictite is poorly constrained, and it is not yet known if global glacial conditions existed at this time. Nonetheless, Erickson et al. apply numerical simulations to explore the potential effects that a Yarrabubba-sized impact may have had on climactic conditions.
Temporal evolution of the early Palaeoproterozoic Earth. Key features include the Yarrabubba and other impact events, the Great Oxidation Event, 2.06–2.45 billion years ago (Ga) and glaciations, 2.23–2.54 billion years ago. Note the close association of the Yarrabubba Impact Event to the end of the final Palaeoproterozoic glaciation, the Rietfontein, at 2225 million years ago and followed by the large positive carbon isotope excursion known as the Lomagundi Event. Other impacts include the 2.02-billion-years-old Vredefort Dome, and the 2.49-billion-years-old correlated Kuruman Spherule Layer in the Griqualand West basin of South Africa and the Dales Gorge Spherule Layer of the Hamersley Basin in Western Australia. Erickson et al. (2020).
Several factors caused by the Yarrabubba Impact Event could have triggered a change in regional or global climate. Depending on the ambient climate state and palaeogeographic nature of the northern Yilgarn Craton at the time of impact (e.g., ice cover, shallow ocean or carbonate platform overlying silicate basement), which is unknown, significant amounts of carbon dioxide, water vapour or other greenhouse gases could have been released into the relatively oxygen-poor Palaeoproterozoic atmosphere by the impact event. Given that the age of the Yarrabubba impact overlaps with the youngest Paleoproterozoic glacial deposits, Erickson et al. explore scenarios where the Yarrabubba Impact Site could have been covered by a continental ice sheet at the time of impact.
Numerical models using the iSALE shock physics code demonstrate that the formation of a 70-km-diameter impact crater into a granitic target with an overlying ice sheet ranging from 2 to 5 km in thickness results in the almost instantaneous vaporisation of 95–240 km³ of ice and up to 5400 km³ total melting. The vapourised ice corresponds to between 90 and 200 million kilotons of water vapour being jetted into the upper atmosphere within moments of the impact. Impact-generated water vapour in the lower atmosphere would have condensed and rapidly precipitated as rain and snow with no significant long-term climate effects, or could have even triggered widespread glacial conditions via cloud albedo effects during interglacial periods. However, ejection of high-altitude water vapour has potential for greenhouse radiative forcing, depending critically on atmospheric residence time. Uncertainties in the structure and composition of Earth’s Palaeoproterozoic upper atmosphere mean that the precise nature of atmospheric interactions of the collapsing vapour plume is inherently difficult to model. Nevertheless, considering that Earth’s atmosphere at the time of impact contained only a fraction of the current level of oxygen, a possibility remains that the climatic forcing effects of water vapour released instantaneously into the atmosphere through a Yarrabubba-sized impact may have been globally significant. Understanding the residence times of impact-produced water vapour in a cold Palaeoproterozoic atmosphere, and the complex interplay of radiative versus insulative effects of clouds, during glacial conditions requires further investigation. The effects of impact cratering have long been recognised as drivers of climate change. Many studies have described the atmospheric effects of the end-Cretaceous Chicxulub impact structure in Mexico, which resulted in global cooling of oceans and production of widespread acidic rains. While the Yarrabubba Structure, dated at 2229 million years, represents the Earth’s oldest dated impact crater, its coincidence with termination of Palaeoproterozoic glacial conditions prompts further consideration of the ability of meteorite impacts to trigger climate change.
Snapshots of the iSALE model with (a) 2-km-thick ice sheet showing a the initial conditions, (b) the transient crater and (c) the final crater. Superimposed on a is the initial position of tracer particles which were shock-heated to the critical entropy required to begin vaporisation (incipient vaporisation, red) and to completely vapourise ice (complete vaporisation, yellow). The colour scale on the right-hand side of (c) shows the peak shock pressure in the granite basement. (d) The calculated mass of ice shock-heated to the critical entropy for incipient and complete vaporisation, as a function of initial ice thickness. In each impact, the impactor size was 7 km and resulted in a final crater diameter of ~70 km. Erickson et al. (2020).
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