Geochemistry and 40Ar‐39Ar geochronology of impact‐melt clasts in feldspathic lunar meteorites: Implications for lunar bombardment history

Abstract— We studied 42 impact‐melt clasts from lunar feldspathic regolith breccias MacAlpine Hills (MAC) 88105, Queen Alexandra Range (QUE) 93069, Dar al Gani (DaG) 262, and DaG 400 for texture, chemical composition, and/or chronology. Although the textures are similar to the impactmelt clasts identified in mafic Apollo and Luna samples, the meteorite clasts are chemically distinct from them, having lower Fe, Ti, K, and P, thus representing previously unsampled impacts. The ⁴⁰Ar‐³⁹Ar ages on 31 of the impact melts, the first ages on impact‐melt samples from outside the region of the Apollo and Luna sampling sites, range from ～4 to ～2.5 Ga. We interpret these samples to have been created in at least six, and possibly nine or more, different impact events. One inferred impact event may be consistent with the Apollo impact‐melt rock age cluster at 3.9 Ga, but the meteorite impact‐melt clasts with this age are different in chemistry from the Apollo samples, suggesting that the mechanism responsible for the 3.9 Ga peak in lunar impact‐melt clast ages is a lunar‐wide phenomenon. No meteorite impact melts have ages more than 1s? older than 4.0 Ga. This observation is consistent with, but does not require, a lunar cataclysm.     

Exploring the variability of argon loss in Apollo 17 impact melt rock 77135 using high‐spatial resolution 40Ar/39Ar geochronology

⁴⁰Ar/³⁹Ar incremental heating experiments on whole‐rock lunar samples commonly provide evidence of varying degrees of radiogenic ⁴⁰Ar (⁴⁰Ar*) loss. However, these experiments provide limited information about whether or not ⁴⁰Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP) ⁴⁰Ar/³⁹Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on ⁴⁰Ar/³⁹Ar dates. Potassium‐rich glass and K‐feldspar in the mesostasis are the dominant sources for Ar released during low‐temperature steps of published ⁴⁰Ar/³⁹Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate‐ to high‐temperature steps. Additionally, UVLAMP analysis of a mm‐scale plagioclase clast demonstrates the potential to use stranded ⁴⁰Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or ⁴⁰Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon's evolution.     

39Ar‐40Ar ages of eucrites and thermal history of asteroid 4 Vesta

Abstract— Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new ³⁹Ar‐⁴⁰Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic‐brecciated eucrites. Precise Ar‐Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar‐Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb‐Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4–4.515 Gyr, and ¹⁴⁷Sm‐¹⁴³Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41–4.55 Gyr. Similar Ar‐Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K‐Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar‐Ar ages at ?4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ?4.48 Gyr ago, which quickly cooled the samples and started the K‐Ar chronometer. A large (?460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb‐Pb and Sm‐Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar‐Ar age of 4.48 Gyr, and the few older Pb‐Pb and Sm‐Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar‐Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar‐Ar ages of 3.4–3.7 Gyr; and 2 unbrecciated eucrites give Ar‐Ar ages of ?3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar‐Ar impact‐reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5–4.1 Gyr. Among these, Piplia Kalan, the first eucrite to show evidence for extinct ²⁶Al, was strongly impact heated ?3.5 Gyr ago. When these data are combined with eucrite Ar‐Ar ages in the literature, they confirm that several large impact heating events occurred on Vesta between ?4.1–3.4 Gyr ago. The onset of major impact heating may have occurred at similar times for both Vesta and the moon, but impact heating appears to have persisted for a somewhat later time on Vesta.     

Evidence for multiple 4.0–3.7 Ga impact events within the Apollo 16 collection

In a histogram of lunar impact ages from the Apollo 16 site, there is a spike circa 3.9 Ga that has been interpreted to represent either a large number of nearly synchronous events or an abundance of samples that were affected slightly differently by the event that produced the Imbrium basin. To further scrutinize those age relationships, we extracted six centimeter‐sized clasts of impact melt from ancient regolith breccia 60016 and performed petrological and geochronological (⁴⁰Ar‐³⁹Ar) analyses. Three clasts have similar poikilitic textures, while others have porphyritic, aphanitic, or intergranular textures. Compositions and abundances of relict minerals are different in all six clasts and variously imply Mg‐suite and ferroan anorthosite target sequences. Estimated bulk compositions of four clasts are similar to previously defined group 1 Apollo 16 impact melt rocks, while the other two have higher Al₂O₃ and lower FeO+MgO compositions. All six clasts have similar K₂O and P₂O₅ concentrations, which could have been derived from a KREEP‐bearing component among target sequences. Eighteen ⁴⁰Ar/³⁹Ar analyses of the six clasts produced an age range from 3823 ± 75 to 4000 ± 23 Ma, consistent with estimates for the proposed late heavy bombardment. Four clasts have multiple temperature steps that define plateau ages. These ages are distinct, so they cannot be explained by a single impact event, such as the one that produced the Imbrium impact basin. The conclusion that these represent distinct ages remains after considering the possibility of artifacts in defining plateaus.     

The Ar‐Ar age and petrology of Miller Range 05029: Evidence for a large impact in the very early solar system

Abstract– Miller Range (MIL) 05029 is a slowly cooled melt rock with metal/sulfide depletion and an Ar‐Ar age of 4517 ± 11 Ma. Oxygen isotopes and mineral composition indicate that it is an L chondrite impact melt, and a well‐equilibrated igneous rock texture with a lack of clasts favors a melt pool over a melt dike as its probable depositional setting. A metallographic cooling rate of approximately 14 °C Ma^?1 indicates that the impact occurred at least approximately 20 Ma before the Ar‐Ar closure age of 4517 Ma, possibly even shortly after accretion of its parent body. A metal grain with a Widmanstätten‐like pattern further substantiates slow cooling. The formation age of MIL 05029 is at least as old as the Ar‐Ar age of unshocked L and H chondrites, indicating that endogenous metamorphism on the parent asteroid was still ongoing at the time of impact. Its metallographic cooling rate of approximately 14 °C Ma^?1 is similar to that typical for L6 chondrites, suggesting a collisional event on the L chondrite asteroid that produced impact melt at a minimum depth of 5–12 km. The inferred minimum crater diameter of 25–60 km may have shattered the 100–200 km diameter L chondrite asteroid. Therefore, MIL 05029 could record the timing and petrogenetic setting for the observed lack of correlation of cooling rates with metamorphic grades in many L chondrites.     

Establishing a 14.6 ± 0.2 Ma age for the Nördlinger Ries impact (Germany)—A prime example for concordant isotopic ages from various dating materials

Abstract– ⁴⁰Ar/³⁹Ar dating of recrystallized K‐feldspar melt particles separated from partially molten biotite granite in impact melt rocks from the approximately 24 km Nördlinger Ries crater (southern Germany) yielded a plateau age of 14.37 ± 0.30 (0.32) Ma (2σ). This new age for the Nördlinger Ries is the first age obtained from (1) monomineralic melt (2) separated from an impact‐metamorphosed target rock clast within (3) Ries melt rocks and therewith extends the extensive isotopic age data set for this long time studied impact structure. The new age goes very well with the ⁴⁰Ar/³⁹Ar step‐heating and laser probe dating results achieved from mixed‐glass samples (suevite glass and tektites) and is slightly younger than the previously obtained fission track and K/Ar and ages of about 15 Ma, as well as the K/Ar and ⁴⁰Ar/³⁹Ar age data obtained in the early 1990s. Taking all the ⁴⁰Ar/³⁹Ar age data obtained from Ries impact melt lithologies into account (data from the literature and this study), we suggest an age of 14.59 ± 0.20 Ma (2σ) as best value for the Ries impact event.     

Laser argon‐40‐argon‐39 age studies of Dar al Gani 262 lunar meteorite

Abstract— The laser ⁴⁰Ar‐³⁹Ar dating technique has been applied to the Dar al Gani (DaG) 262 lunar meteorite, a polymict highland regolith breccia, to determine the crystallisation age and timing of shock events experienced by this meteorite. Laser stepped‐heating analyses of three dominantly feldspathic fragments (DaG‐1, DaG‐2, and DaG‐3) revealed the presence of trapped Ar, mostly released at intermediate and high temperatures, with an ⁴⁰Ar/³⁶Ar value of ～2.8. Trapped Ar is most likely released from melt glass present as small veins within the fragments. The ⁴⁰Ar‐³⁹Ar ages determined for the three fragments are ～3.0 Ga for DaG‐1 and DaG‐2 and 2.0 Ga for DaG‐3 and probably relate to major impact events. Laser spot analyses were performed on a feldspathic clast, an impact crystalline melt basalt (ICMB), and the matrix in a polished section of DaG 262. The feldspathic and ICMB clasts have low contents of trapped Ar compared with that in the matrix. The feldspathic clast shows a wide range of ages from 3.0 to 1.7 Ga similar to those obtained by stepped heating. The younger age is interpreted as a minimum age for the last major event that assembled this meteorite. The ICMB shows two age clusters at 3.37 and 3.07 Ga, where the older age may be that of the impact event that formed the impact melt. Several cosmic‐ray exposure (CRE) ages were obtained as expected for a polymict regolith breccia. The CRE ages are 106 and 141 Ma for the feldspathic clast and the ICMB, respectively. One of the feldspathic fragments, DaG‐2, shows a range between 200–400 Ma. These CRE ages, which are similar to those determined for returned samples of the lunar regolith, indicate that the different components of DaG 262 experienced preexposure prior to assemblage of the meteorite.     

The two Suvasvesi impact structures,Finland: Argon isotopic evidence for a “false” impact crater doublet

The two neighboring Suvasvesi North and South impact structures in central‐east Finland have been discussed as a possible impact crater doublet produced by the impact of a binary asteroid. This study presents ⁴⁰Ar/³⁹Ar geochronologic data for impact melt rocks recovered from the drilling into the center of the Suvasvesi North impact structure and melt rock from glacially transported boulders linked to Suvasvesi South. ⁴⁰Ar/³⁹Ar step‐heating analysis yielded two essentially flat age spectra indicating a Late Cretaceous age of ~85 Ma for the Suvasvesi North melt rock, whereas the Suvasvesi South melt sample gave a Neoproterozoic minimum (alteration) age of ~710 Ma. Although the statistical likelihood for two independent meteorite strikes in close proximity to each other is rather low, the remarkable difference in ⁴⁰Ar/³⁹Ar ages of >600 Myr for the two Suvasvesi impact melt samples is interpreted as evidence for two temporally separate, but geographically closely spaced, impacts into the Fennoscandian Shield. The Suvasvesi North and South impact structures are, thus, interpreted as a “false” crater doublet, similar to the larger East and West Clearwater Lake impact structures in Québec, Canada, recently shown to be unrelated. Our findings have implications for the reliable recognition of impact crater doublets and the apparent rate of binary asteroid impacts on Earth and other planetary bodies in the inner solar system.     

Jiddat al Harasis 556: A howardite impact melt breccia with an H chondrite component

Abstract– A petrographic and geochemical study was undertaken to characterize Jiddat al Harasis (JaH) 556, a howardite find from the Sultanate of Oman. JaH 556 is a polymict impact melt breccia containing highly shocked clasts, including mosaicized olivine and recrystallized plagioclase, set in a finely recrystallized vesicular matrix (grain diameter <5–10 μm). Plagioclase (An_76–92) and clinopyroxene (En_48–62Wo_7–15) are associated with orthopyroxene and olivine clasts like in a howardite. JaH 556 oxygen isotope data indicate that it has an anomalous bulk‐rock composition as howardite, resulting from a mixture between HED material and at least one second reservoir characterized by a higher Δ¹⁷O. The bulk meteorite has a composition consistent with howardites, but it is enriched in siderophile elements (Ni = 3940 and Co = 159 ppm) arguing for a chondritic material as second reservoir. This is independently confirmed by the occurrence of chondrule relics composed of olivine (Fo_56–80), orthopyroxene (En₇₉Wo₂), and plagioclase (An_61–66). Based on oxygen isotopic signature, siderophile composition, and chondrule core Mg number (Fo₈₀ and En₇₉Wo₂), it is proposed that JaH 556 is a howardite containing approximately 20% H chondrite material. This percentage is high compared with that observed petrographically, likely because chondritic material dissolved in the impact melt. This conclusion is supported by the observed reaction of orthopyroxene to olivine, which is consistent with a re‐equilibration in a Si‐undersaturated melt. JaH 556’s unique composition enlarges the spectrum of howardite‐analogs to be expected on the surface of 4 Vesta. Our data demonstrate that oxygen isotopic anomalies can be produced by a mixture of indigenous and impactor materials and must be interpreted with extreme caution within the HED group.     

40Ar‐39Ar ages of H‐chondrite impact melt breccias

Abstract— ⁴⁰Ar‐³⁹Ar analyses of a total of 26 samples from eight shock‐darkened impact melt breccias of H‐chondrite affinity (Gao‐Guenie, LAP 02240, LAP 03922, LAP 031125, LAP 031173, LAP 031308, NWA 2058, and Ourique) are reported. These appear to record impacts ranging in time from 303 ± 56 Ma (Gao‐Guenie) to 4360 ± 120 Ma (Ourique) ago. Three record impacts 300–400 Ma ago, while two others record impacts 3900–4000 Ma ago. Combining these with other impact ages from H chondrites in the literature, it appears that H chondrites record impacts in the first 100 Ma of solar system history, during the era of the “lunar cataclysm” and shortly thereafter (3500–4000 Ma ago), one or more impacts ?300 Ma ago, and perhaps an impact ?500 Ma ago (near the time of the L chondrite parent body disruption). Records of impacts on the H chondrite parent body are rare or absent between the era of planetary accretion and the “lunar cataclysm” (4400‐4050 Ma), during the long stretch between heavy bombardment and recent breakup events (3500‐1000 Ma), or at the time of final breakup into meteorite‐sized bodies (<50 Ma).     

High crustal diversity preserved in the lunar meteorite Mount DeWitt 12007 (Victoria Land,Antarctica)

The meteorite Mount DeWitt (DEW) 12007 is a polymict regolith breccia mainly consisting of glassy impact‐melt breccia particles, gabbroic clasts, feldspathic clasts, impact and volcanic glass beads, basaltic clasts, and mingled breccia clasts embedded in a matrix dominated by fine‐grained crystals; vesicular glassy veins and rare agglutinates are also present. Main minerals are plagioclase (typically An_>85) and clinopyroxene (pigeonites and augites, sometimes interspersed). The presence of tranquillityite, coupled with the petrophysical data, the O‐isotope data (Δ¹⁷O = ?0.075), and the FeO_tot/MnO ratios in olivine (91), pyroxene (65), and bulk rock (77) indicate a lunar origin for DEW 12007. Impactites consist of Al‐rich impact‐melt splashes and plagioclase‐rich meta‐melt clasts. The volcanic products belong to the very low titanium (VLT) or low titanium (LT) suites; an unusual subophitic fragment could be cryptomare‐related. Gabbroic clasts could represent part of a shallow intrusion within a volcanic complex with prevailing VLT affinity. DEW 12007 has a mingled bulk composition with relatively high incompatible element abundances and shows a high crustal diversity comprising clasts from the Moon's major terranes and rare lithologies. First‐order petrographic and chemical features suggest that DEW 12007 could be launch‐paired with other meteorites including Y 793274/981031, QUE 94281, EET 87521/96008, and NWA 4884.     

Tenoumer impact crater,Mauritania: Impact melt genesis from a lithologically diverse target

Impact melt rocks from the 1.9 km diameter, simple bowl‐shaped Tenoumer impact crater in Mauritania have been analyzed chemically and petrologically. They are heterogeneous and can be subdivided into three types based on melt matrix color, occurrence of lithic clast components, amount of vesiculation (melt degassing), different proportions of carbonate melt mingled into silicate melt, and bulk rock chemical composition. These heterogeneities have two main causes (1) due to the small size of the impact crater, there was probably no coherent melt pool where a homogeneous mixture of melts, derived from different target lithologies, could be created; and (2) melt rock heterogeneity occurring at the thin section scale is due to fast cooling during and after the dynamic ejection and emplacement process. The overall period of crystal growth from these diverse melts was extremely short, which provides a further indication that complete chemical equilibration of the phases could not be achieved in such short time. Melt mixing processes involved in the generation of impact melts are, thus, recorded in nonequilibrium growth features. Variable mixing processes between chemically different melt phases and the formation of hybrid melts can be observed even at millimeter scales. Due to extreme cooling rates, different mixing and mingling stages are preserved in the varied parageneses of matrix minerals and in the mineral chemistry of microlites. ⁴⁰Ar³⁹Ar step‐heating chronology on specimens from three melt rock samples yielded five concordant inverse isochron ages. The inverse isochron plots show that minute amounts of inherited ⁴⁰Ar* are present in the system. We calculated a weighted mean age of 1.57 ± 0.14 Ma for these new results. This preferred age represents a refinement from the previous range of 21 ka to 2.5 Ma ages based on K/Ar and fission track dating.     

An Early Jurassic age for the Puchezh‐Katunki impact structure (Russia) based on 40Ar/39Ar data and palynology

The Puchezh‐Katunki impact structure, 40–80 km in diameter, located ~400 km northeast of Moscow (Russia), has a poorly constrained age between ~164 and 203 Ma (most commonly quoted as 167 ± 3 Ma). Due to its relatively large size, the Puchezh‐Katunki structure has been a prime candidate for discussions on the link between hypervelocity impacts and extinction events. Here, we present new ⁴⁰Ar/³⁹Ar data from step‐heating analysis of five impact melt rock samples that allow us to significantly improve the age range for the formation of the Puchezh‐Katunki impact structure to 192–196 Ma. Our results also show that there is not necessarily a simple relationship between the observed petrographic features of an impact melt rock sample and the obtained ⁴⁰Ar/³⁹Ar age spectra and inverse isochrons. Furthermore, a new palynological investigation of the postimpact crater lake sediments supports an age significantly older than quoted in the literature, i.e., in the interval late Sinemurian to early Pliensbachian, in accordance with the new radioisotopic age estimate presented here. The new age range of the structure is currently the most reliable age estimate of the Puchezh‐Katunki impact event.     

Impact ages of meteorites: A synthesis

Abstract— Isotopic ages of meteorites that indicate chronometer resetting due to impact heating are summarized. Most of the ages were obtained by the ³⁹Ar-⁴⁰Ar technique, but several Rb-Sr, Pb-Pb, and Sm-Nd ages also suggest some degree of impact resetting. Considerations of experimental data on element diffusion in silicates suggest that various isotopic chronometers ought to differ in their ease of resetting during shock heating in the order K-Ar (easiest), Rb-Sr, Pb-Pb, and Sm-Nd, which is approximately the order observed in meteorites. Partial rather than total chronometer resetting by impacts appears to be the norm; consequently, interpretation of the event age is not always straightforward. Essentially all ³⁹Ar-⁴⁰Ar ages of eucrites and howardites indicate partial to total resetting in the relatively narrow time interval of 3.4–4.1 Ga ago (1 Ga = 10⁹ years). Several disturbed Rb-Sr ages appear consistent with this age distribution. This grouping of ages and the brecciated nature of many eucrites and all howardites argues for a large-scale impact bombardment of the HED parent body during the same time period that the Moon received its cataclysmic bombardment. Other meteorite parent bodies such as those of mesosiderites, some chondrites, and IIE irons also may have experienced this bombardment. These data suggest that the early bombardment was not lunar specific but involved much of the inner Solar System, and may have been caused by breakup of a larger planetismal. Although a few chondrites show evidence of age resetting ～3.5–3.9 Ga ago, most impact ages of chondrites tend to fall below 1.3 Ga in age. A minimum of ～4 impact events, including events at 0.3, 0.5, 1.2, and possibly 0.9 Ga appear to be required to explain the younger ages of H, L, and LL chondrites, although additional events are possible. Most L chondrites show evidence of shock, and the majority of ³⁹Ar-⁴⁰Ar ages of L chondrites fall near 0.5 Ga. The L chondrite parent body apparently experienced a major impact at this time, which may have disrupted it. The observations (1) that lunar highland rocks experienced major impact resetting of various isotopic chronometers ～3.7–4.1 Ga ago; (2) that the HED parent body experienced widespread impact resetting of the K-Ar chronometer but only modest disturbance of other isotopic systems, during a similar time period; (3) that ordinary chondrite parent bodies show much more recent and less extensive impact resetting; and (4) that impacts, which initiated cosmic-ray exposure of most stone meteorites almost never reset isotopic chronometers, may all be a consequence of relative parent body size. Greater degrees of isotopic chronometer resetting occur in larger and warmer impact ejecta deposits that cool slowly. The relatively greater size of bodies like the Moon and Vesta (assumed to be the parent asteroid of HED meteorites) both permit such favorable ejecta deposits to occur more easily compared to smaller parent bodies (generally assumed for chondrites) and also protect parent objects from collisional disruption. Thus, impacts on larger bodies would tend to more easily reset chronometers, consistent with the observed relative ease of resetting of Moon (easiest), HED, chondrites and of K-Ar (easiest), Rb-Sr, other chronometers. In contrast, the more recent impact ages of chondrites are postulated to represent collisional disruption of smaller parent objects whose fragments are more readily removed from the meteorite source reservoirs. Impacts that initiate cosmic-ray exposure are mostly small in scale and produce little heating.     

The age of the Kara impact structure,Russia

Abstract— For the ～65 km sized Kara impact structure close to the polar Ural, we report an age of 70.3 ± 2.2 Ma (2s?), defined by the mean of ⁴⁰Ar-³⁹Ar plateau ages for three glassy or crystalline impact melt rocks cleaned from mineral and rock clasts. The fine structure of the age spectra of these samples can quantitatively be simulated by modeling taking into account ³⁹Ar recoil effects, without assuming the presence of excess Ar. The calculations corroborate our age results by showing that ³⁹Ar recoil does not affect the plateau fractions. Previously, Kara has been proposed as a probable K/T impact site or was related to the Campanian-Maastrichtian boundary at 73 Ma. At the 2s? level, both suggestions are ruled out by the well-constrained age for the Kara impact structure.     

Empirical and theoretical comparisons of the Chicxulub and Sudbury impact structures

Abstract— Chicxulub and Sudbury are 2 of the largest impact structures on Earth. Research at the buried but well‐preserved Chicxulub crater in Mexico has identified 6 concentric structural rings. In an analysis of the preserved structural elements in the eroded and tectonically deformed Sudbury structure in Canada, we identified ring‐like structures corresponding in both radius and nature to 5 out of the 6 rings at Chicxulub. At Sudbury, the inner topographic peak ring is missing, which if it existed, has been eroded. Reconstructions of the transient cavities for each crater produce the same range of possible diameters: 80–110 km. The close correspondence of structural elements between Chicxulub and Sudbury suggests that these 2 impact structures are approximately the same size, both having a main structural basin diameter of ?150 km and outer ring diameters of ?200 km and ?260 km. This similarity in size and structure allows us to combine information from the 2 structures to assess the production of shock melt (melt produced directly upon decompression from high pressure impact) and impact melt (shock melt and melt derived from the digestion of entrained clasts and erosion of the crater wall) in large impacts. Our empirical comparisons suggest that Sudbury has ?70% more impact melt than does Chicxulub (?31,000 versus ?18,000 km³) and 85% more shock melt (27,000 km³ versus 14,500 km³). To examine possible causes for this difference, we develop an empirical method for estimating the amount of shock melt at each crater and then model the formation of shock melt in both comet and asteroid impacts. We use an analytical model that gives energy scaling of shock melt production in close agreement with more computationally intense numerical models. The results demonstrate that the differences in melt volumes can be readily explained if Chicxulub was an asteroid impact and Sudbury was a comet impact. The estimated 70% difference in melt volumes can be explained by crater size differences only if the extremes in the possible range of melt volumes and crater sizes are invoked. Preheating of the target rocks at Sudbury by the Penokean Orogeny cannot explain the excess melt at Sudbury, the majority of which resides in the suevite. The greater amount of suevite at Sudbury compared to Chicxulub may be due to the dispersal of shock melt by cometary volatiles at Sudbury.     

The Lawn Hill annulus: An Ordovician meteorite impact into water‐saturated dolomite

The Lawn Hill Impact Structure (LHIS) is located 250 km N of Mt Isa in NW Queensland, Australia, and is marked by a highly deformed dolomite annulus with an outer diameter of ~18 km, overlying low metamorphic grade siltstone, sandstone, and shale, along the NE margin of the Georgina Basin. This study provides detailed field observations from sections of the Lawn Hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the Lawn Hill impact. ⁴⁰Ar‐³⁹Ar dating of impact‐related melt particles provides a time of impact in the Ordovician (472 ± 8 Ma) when the Georgina Basin was an active depocenter. The timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed Thorntonia Limestone at the time of impact. The exposed annulus is remarkably well preserved, with preservation attributed to postimpact sedimentation. The LHIS has an atypical crater morphology with no central uplift. The heterogeneous target materials at Lawn Hill were probably low‐strength, porous, and water‐saturated, with all three properties affecting the crater morphology. The water‐saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus, and restricted melt production. The impact timing raises the possibility that the Lawn Hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid‐Ordovician (470 ± 6 Ma).     

Shocked rocks and impact glasses from the El'gygytgyn impact structure,Russia

Abstract— The El'gygytgyn impact structure is about 18 km in diameter and is located in the central part of Chukotka, arctic Russia. The crater was formed in volcanic rock strata of Cretaceous age, which include lava and tuffs of rhyolites, dacites, and andesites. A mid‐Pliocene age of the crater was previously determined by fission track (3.45 ± 0.15 Ma) and ⁴⁰Ar/³⁹Ar dating (3.58 ± 0.04 Ma). The ejecta layer around the crater is completely eroded. Shock‐metamorphosed volcanic rocks, impact melt rocks, and bomb‐shaped impact glasses occur in lacustrine terraces but have been redeposited after the impact event. Clasts of volcanic rocks, which range in composition from rhyolite to dacite, represent all stages of shock metamorphism, including selective melting and formation of homogeneous impact melt. Four stages of shocked volcanic rocks were identified: stage I (≤35 GPa; lava and tuff contain weakly to strongly shocked quartz and feldspar clasts with abundant PFs and PDFs; coesite and stishovite occur as well), stage II (35–45 GPa; quartz and feldspar are converted to diaplectic glass; coesite but no stishovite), stage III (45–55 GPa; partly melted volcanic rocks; common diaplectic quartz glass; feldspar is melted), and stage IV (>55 GPa; melt rocks and glasses). Two main types of impact melt rocks occur in the crater: 1) impact melt rocks and impact melt breccias (containing abundant fragments of shocked volcanic rocks) that were probably derived from (now eroded) impact melt flows on the crater walls, and 2) aerodynamically shaped impact melt glass “bombs” composed of homogeneous glass. The composition of the glasses is almost identical to that of rhyolites from the uppermost part of the target. Cobalt, Ni, and Ir abundances in the impact glasses and melt rocks are not or only slightly enriched compared to the volcanic target rocks; only the Cr abundances show a distinct enrichment, which points toward an achondritic projectile. However, the present data do not allow one to unambiguously identify a meteoritic component in the El'gygytgyn impact melt rocks.     

Analyses of the chondritic meteorite Orvinio (H6): Insight into the origins and evolution of shocked H chondrite material

Abstract— We have studied the petrography, reflectance spectra, and Ar‐Ar systematics of the Orivinio meteorite. Orvinio is an H chondrite not an L chondrite as sometimes reported. The material in the meteorite was involved in several impact events. One impact event produced large swaths of impact melt from H chondrite material surrounding relict clasts of chondrule‐bearing material. Not only were portions of a bulk H chondrite planestesimal melted during the impact event, but shock redistribution of metal and sulfide phases in the meteorite dramatically altered its reflectance spectra. Both the melt and relict clasts are darker than unshocked H chondrite material, bearing spectral similarities to some C‐class asteroids. Such shock metamorphism, which lowers the albedo of an object without increasing its spectral slope, may partially explain some of the variation among S‐class asteroids and some of the trends seen on asteroid 433 Eros. Noble gases record the evidence of at least two, and perhaps three, impact events in the meteorite and its predecessor rocks. The most significant evidence is for an event that occurred 600 Ma ago or less, perhaps ?325 Ma ago or less. There is also a signature of 4.2 Ga in the Ar‐Ar systematics, which could either reflect complete degassing of the rock at that time or partial degassing of even the most retentive sites in the more recent event.     

A new high‐precision 40Ar/39Ar age for the Rochechouart impact structure: At least 5 Ma older than the Triassic–Jurassic boundary

The Rochechourt impact structure in south‐central France, with maximum diameter of 40–50 km, has previously been dated to within 1% uncertainty of the Triassic–Jurassic boundary, at which time ~30% of global genera became extinct. To evaluate the temporal relationship between the impact and the Triassic–Jurassic boundary at high precision, we have re‐examined the structure's age using multicollector ARGUS‐V ⁴⁰Ar/³⁹Ar mass spectrometry. Results from four aliquots of impact melt are highly reproducible, and yield an age of 206.92 ± 0.20/0.32 Ma (2σ, full analytical/external uncertainties). Thus, the Rochechouart impact structure predates the Triassic–Jurassic boundary by 5.6 ± 0.4 Ma and so is not temporally linked to the mass extinction. Rochechouart has formerly been proposed to be part of a multiple impact event, but when compared with new ages from the other purported “paired” structures, the results provide no evidence for synchronous impacts in the Late Triassic. The widespread Central Atlantic Magmatic Province flood basalts remain the most likely cause of the Triassic–Jurassic mass extinction.