Terrestrial Impact Events Research Articles

Can Archean Impact Structures Be Discovered? A Case Study From Earth's Largest, Most Deeply Eroded Impact Structure

AbstractThe record of terrestrial impact events is incomplete with no Archean impact structures discovered, despite the expected abundance of collisions that must have occurred. Because no Archean impact structures have been identified, the necessary conditions to preserve an impact structure longer than 2 Byr are unknown. One significant effect of shock metamorphism is that the physical properties of the target rocks change, resulting in distinctive geophysical signatures of impact structures. To evaluate the preservation potential of impact structures, we evaluate the deeply eroded Proterozoic Vredefort impact structure to examine the changes in physical properties and the remnant of the geophysical signature and compare the results with the well‐preserved Chicxulub impact structure. The major structural features of Vredefort are similar to the expected profile of the Chicxulub structure at a depth of 8–10 km. The Vredefort target rocks, while shocked, do not preserve measurable changes in their physical properties. The gravity signature of the impact structure is minor and is controlled by the remnant of the collapsed transient crater rim and the uplifted Moho surface. We anticipate that erosion of the Vredefort structure by an additional 1 km would remove evidence of impact, and regardless of initial size, erosion by >10 km would result in the removal of most of the evidence for any impact structure from the geological record. This study demonstrates that the identification of geologically old (i.e., Archean) impact structures is limited by a lack of geophysical signatures associated with deeply eroded craters.

A new U-Pb age for shock-recrystallised zircon from the Lappajärvi impact crater, Finland, and implications for the accurate dating of impact events

Accurate and precise dating of terrestrial impact craters is a critical requirement for correlating impacts with events such as mass extinctions. A number of isotopic systems have been applied to impact chronology but it is important to understand what an age actually represents and, thus, if it accurately represents the ‘true’ impact age and is suitable for use in correlation. Here we report imaging, microstructural characterisation and high spatial resolution ion microprobe U-Pb analysis of shocked zircon from the approximately 23 km-in-diameter Lappajärvi impact structure, Finland, for which a well-established 40Ar/39Ar framework exists. Microstructural analysis identified two distinct styles of shock recrystallisation: (i) granular zircon that displays multiple domains of similarly oriented neoblasts, some of which are interpreted to be the product of reversion from the high-pressure ZrSiO4 polymorph, reidite, and (ii) granular zircon composed entirely of similarly oriented neoblasts. Only the former gave concordant U-Pb data interpreted to record the age of the impact. The U-Pb ‘concordia age’ reported here, 77.85 ± 0.78 Ma (1.0%; MSWD = 0.60; probability = 0.87; n = 8; 2σ; full external uncertainty), is resolvable from the previously published ‘best estimate’ 40Ar/39Ar age for impact melt rock (76.20 ± 0.29 Ma) and 40Ar/39Ar K-feldspar ages as young as 75.11 ± 0.36 Ma, and is therefore interpreted to more accurately reflect the age of the impact event. The resolvable disparity between the zircon U-Pb and the 40Ar/39Ar data indicates that even the oldest statistically robust 40Ar/39Ar ages obtained at medium- and large-sized impact craters may not accurately record the timing of an impact event at a kyr level. The offset between the U-Pb and 40Ar/39Ar data is interpreted to be, at least in part, a result of the zircon data recording a higher isotopic closure temperature, and the younger 40Ar/39Ar ages recording the progressive cooling of different domains of the impact structure. The Lappajärvi impact structure is the first Phanerozoic impact structure dated by U-Pb analysis of shock-recrystallised zircon to better than, or equal to, 1.0% uncertainty. This further demonstrates that well-characterised granular zircon grains are likely to have wide utility in the accurate and precise dating of terrestrial impact events.

Response to comment on “A high-precision 40Ar/39Ar age for the Nördlinger Ries impact crater, Germany, and implications for the accurate dating of terrestrial impact events” by Schmieder et al. (Geochimica et Cosmochimica Acta 220 (2018) 146–157)

Rocholl et al. (2018) claim that the 40Ar/39Ar age of 14.808 ± [0.038] Ma (2σ; full external error) for the Ries impact, recently obtained from step-heating analysis of tektites (Schmieder et al., 2018, Geochim. Cosmochim Acta 220, 146–157), is “geologically impossible” and violates paleomagnetic constraints. We demonstrate that the moldavite age of Schmieder et al. (2018) is in good agreement with recalculated isotopic ages for ash layers and magnetic (sub-)chrons that define the Langhian stage in the Astronomically Tuned Neogene Time Scale, using updated K decay constants and monitor ages in 40Ar/39Ar geochronology. Moreover, we stress that the normally magnetized basal pelitic sediments of the Ries crater lake, which are separated from the reversely magnetized suevite by a thick unit of post-impact conglomerates and sandstones, may not have been deposited ‘immediately’ after the impact. The paleomagnetic constraints proposed by Pohl (1977, 1978) and Rocholl et al. (2018) are, therefore, questionable.

Comment on “A high-precision 40Ar/39Ar age for the Nördlinger Ries impact crater, Germany, and implications for the accurate dating of terrestrial impact events” by Schmieder et al. (Geochimica et Cosmochimica Acta 220 (2018) 146–157)

Schmieder et al. (2018) suggest an impact age for the Ries crater in Southern Germany that is at odds with paleomagnetic systematics and thus geologically impossible, even within its external 2-sigma error. Paleomagnetic systematics allow for only two alternative impact ages that are both tightly constrained by orbital tuning. The relative differences to the Schmieder et al. (2018) age amount to 60 and 200 ka, respectively. Such time intervals are, however, highly significant with respect to the timing of climatic and environmental signals recorded in the Middle Miocene molasse sediments.

A high-precision 40Ar/39Ar age for the Nördlinger Ries impact crater, Germany, and implications for the accurate dating of terrestrial impact events

40Ar/39Ar dating of specimens of moldavite, the formation of which is linked to the Ries impact in southern Germany, with a latest-generation ARGUS VI multi-collector mass spectrometer yielded three fully concordant plateau ages with a weighted mean age of 14.808±0.021 Ma (±0.038 Ma including all external uncertainties; 2σ; MSWD=0.40, P=0.67). This new best-estimate age for the Nördlinger Ries is in general agreement with previous 40Ar/39Ar results for moldavites, but constitutes a significantly improved precision with respect to the formation age of the distal Ries-produced tektites. Separates of impact glass from proximal Ries ejecta (suevite glass from three different surface outcrops) and partially melted feldspar particles from impact melt rock of the SUBO 18 Enkingen drill core failed to produce meaningful ages. These glasses show evidence for excess 40Ar introduction, which may have been incurred during interaction with hydrothermal fluids. Only partially reset 40Ar/39Ar ages could be determined for the feldspathic melt separates from the Enkingen core. The new 40Ar/39Ar results for the Ries impact structure constrain the duration of crater cooling, during the prevailing hydrothermal activity, to locally at least ∼60 kyr. With respect to the dating of terrestrial impact events, this paper briefly discusses a number of potential issues and effects that may be the cause for seemingly precise, but on a kyr-scale inaccurate, impact ages.

Precambrian reidite discovered in shocked zircon from the Stac Fada impactite, Scotland

Terrestrial impact events have had a profound influence on Earth’s geological, geochemical, and biological evolution. However, the record of Precambrian impacts is poorly constrained due to the dynamic nature of plate tectonics, erosion, and deposition of younger rocks that may destroy or cover the evidence. Here we report the first Precambrian occurrence of the rare mineral reidite (ZrSiO4) within grains of shocked zircon in the ca. 1.18 Ga Stac Fada Member (Stoer Group), northwestern Scotland. The reidite, preserved as <2-µm-wide lamellae, is unambiguous evidence of shock pressures in excess of ∼30 GPa and confirms the impact origin for the Stac Fada deposit. The reidite lamellae are locally deformed, and sites of deformation record its decomposition to baddeleyite (ZrO2) and amorphous silica, the first natural example of this transformation. The findings demonstrate that reidite and baddeleyite may form and be transported in high-energy ejecta without physical or chemical breakdown and are stable during sedimentary diagenesis and low-grade metamorphism. Thus, reidite may be preserved over time scales exceeding 1 b.y., establishing the use of reidite within detrital shocked zircon from Precambrian strata as a viable and valuable means of recognizing and characterizing ancient terrestrial impact events.

Dating Terrestrial Impact Structures

Hypervelocity impacts of asteroids and comets have played a key role in the evolution of the Solar System and planet Earth. Geochronology, the science that investigates the ages of rocks, has become a preponderant tool for dating impact events and for assessing whether they are related in time to mass extinctions on Earth. Impact events are instantaneous compared to other geological processes and, in theory, represent easy targets for multitechnique geochronology. Yet, only a few terrestrial impact events are accurately and precisely dated. A dating campaign is urgently needed if we are to fully understand the role of impacts in Earth history.

Optical maturity of ejecta from large rayed lunar craters

Lucey et al. [2000] have developed a methodology for extracting an optical maturity parameter (OMAT) from multispectral Clementine images. The OMAT parameter characterizes the overall maturity of lunar soils and crater ejecta by changes in reflectance spectra. Using these OMAT images, we surveyed large craters (≥20 km diameter) on the Moon that had previously been mapped as possessing or possibly possessing rayed ejecta. We generated average radial profiles of OMAT values for rays of these large craters. From these profiles we classified the craters into three relative age groups: (1) older than Copernicus (inferred age of ∼810 Myr), (2) intermediate, and (3) as young or younger than Tycho (inferred age of ∼109 Myr). We suspect that there is a bias to our classification scheme, such that the OMAT profiles of smaller craters look like that of larger but older craters. Nevertheless, some large craters, such as Eudoxus (67 km) and Aristillus (55 km), are now known from this study to have optically mature ejecta and therefore are suspected to be older than Copernicus (this is consistent with an age of 1.3 Gyr suggested for Aristillus by Ryder et al. [1991]). Such craters were included by McEwen et al. [1997] when estimating the density of craters younger than or contemporaneous with Copernicus. Therefore the case for a modest increase in the cratering rate (in the past 800 Myr versus the previous 2.4 Gyr) indicated from that work has been weakened [Grier and McEwen, 2001]. Given current constraints on dating large and recent lunar craters, we cannot support (or disprove) the hypothesis that there has been a significant increase in the rate of large terrestrial impact events in the past 100–400 Myr.

Dating terrestrial impact events

Abstract— Systematic examination of dating results from various craters indicates that about 90% of the rocks affected by an impact preserve their pre‐shock ages because shock and post‐shock conditions are not sufficient to disturb isotopic dating systems. In the other 10% of target lithologies, various geochronometers show significant shock‐induced effects. Major problems in dating impactites are caused by their non‐equlibrated character. They often display complex textures, where differently shocked and unshocked phases interfinger on the sub‐mm scale. Due to this, dating on whole rock samples or insufficiently pure mineral fractions often yielded ambiguous results that set broad age limits but are not sufficient to answer reliably questions such as a possible periodicity in cratering on Earth, or correlation of impact events with mass extinctions. Dating results from shock recovery experiments indicate that post‐shock annealing plays the most important role in resetting isotopic clocks. Therefore, the major criterion for sample selection in and around craters is the post‐shock thermal regime. Based on their different thermal evolution, the following geological impact formations can be distinguished: (1) the coherent impact melt layer, (2) allochthonous breccia deposits, (3) the crater basement, and (4) distant ejecta deposits.Samples of the coherent impact melt layer are the most suitable candidates for dating. Excellent ages of high precision can be obtained by internal Rb‐Sr, and Sm‐Nd isochrons, U‐Pb analyses on newly crystallized accessory minerals, and K‐Ar (39Ar‐40Ar) dating of clast‐free melt rocks. Fission track counting on glassy material has yielded correct ages, and paleomagnetic measurements have been successfully applied to post‐Triassic craters. In the ideal case of a fast‐cooling impact melt layer, all these different techniques should give identical ages.Allochthonous breccias contain shocked, unshocked, and/or glassy components in various proportions; and, hence, each of these ejecta deposits has its own individual thermal history, making sample evaluation difficult Glassy melt particles in suevitic breccias are well suited for fission track and Ar‐Ar dating. Weakly shocked material may yield reliable Ar‐Ar and fission track ages, if formation temperatures were high, and cooling rates moderate. In contrast, highly shocked but rapidly cooled lithologies show only disturbed and not reset isotopic systems. For ejecta deposits and the crater wall of young craters, dating with cosmogenic nuclides is a new and powerful technique.Crater basement lithologies have a high potential in impact dating, although it has not been exploited so far. A prerequisite for resetting of isotopic clocks in these lithologies is the presence of an overlaying impact melt layer, which causes thermal metamorphism. Fission track and K‐Ar techniques are most promising, because both systems are easily reset at low temperatures. Good candidates for impact dating are long‐term annealed rocks, even if shock metamorphic overprint is very weak. In addition, Ar‐Ar dating dating of pseudotachylites appears promising. In large impact structures, where high temperatures persist for long times, polymict “footwall” breccias beneath the melt sheet are also appropriate for dating, using the isochron approach and U‐Pb on accessory minerals.Distant ejecta material have undergone very fast cooling, and the ejecta deposits have ambient formation temperatures. Among this material, tektites and impact melt glass are ideal objects for Ar‐Ar and fission track impact dating. Dating on other material from distant ejecta deposits, such as U‐Pb analyses on zircons, offers new possibilities. Efforts to correlate distant ejecta with distinct craters critically depend on proper error assignment to a specific age. This aspect is illustrated on the K/T boundary example.

Stishovite at the Cretaceous-Tertiary Boundary, Raton, New Mexico

Stishovite, a dense phase of silica, has become widely accepted as an indicator of terrestrial impact events. Stishovite occurs at several impact structures but has not been found at volcanic sites. Solid-state silicon-29 magic-angle spinning nuclear magnetic resonance (silicon-29 MAS NMR) and X-ray diffraction of samples from the Cretaceous-Tertiary boundary layer at Raton, New Mexico, indicate that stishovite occurs in crystalline mineral grains. Stishovite was indicated by a single, sharp resonance with a chemical shift value of -191.3 ppm, characteristic of silicon in octahedral coordination, that disappeared after heating the sample at 850 degrees Celsius for 30 minutes. An X-ray diffraction pattern of HF residuals from the unheated sample displayed more than 120 peaks, most of which correspond to quartz, zircon, rutile, and anatase. Eight unambiguous weak to moderate reflections could be ascribed to d-spacings characteristic of stishovite.