Aerial crystallization of hematite in impact bombs from the Araguainha astrobleme,Mato Grosso,central Brazil

Abstract— Optical and scanning electron microscope (SEM) observations of hematite impact bombs from the Araguainha astrobleme in central Brazil reveal an aerodynamically stable microstructure, which is interpreted to reflect the aerial crystallization of hypervelocity melt globules derived from Fe-rich sedimentary target rocks. The bombs are composed of an aggregate of preferentially oriented conic units with different degrees of packing, giving rise to porous and massive hematite varieties. These varieties generally occupy the front and rear of the bombs, respectively. Quartz fragments with planar deformation features are common in the porous domains and represent ejected solid particles captured by the bombs during cooling. The absence of typical contorted shapes in these bombs reflects the lack of spin during their ballistic trajectory. The bombs are enriched in Rh (4×); Ru, Ir, and Pd (5×); and Pt (>30×) relative to the indigenous level of target sediments. This suggests the presence of a projectile-derived component in their constitution. Single composition oxidic bombs (pure hematite and pure silica) predominate in this occurrence, indicating that a mechanism of segregation has operated in the ejected melt volumes.     

Structural evolution of the 40 km wide Araguainha impact structure,central Brazil

Abstract— The 40 km wide Araguainha structure in central Brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation. The dominant structural features preserved at Araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity. Slumping of the transient cavity walls resulted in inward‐verging inclined folds and a km‐scale anticline in the outer ring of the structure. The folding stage was followed by radial and concentric faulting, with downward displacement of kilometer‐scale blocks around the crater rim. The central uplift records evidence for km‐scale upward movement of crystalline basement rocks from the transient cavity floor, and lateral moment of sedimentary target rocks detached from the cavity walls. Much of the structural grain in the central uplift relates to structural stacking of km‐scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks. Outward‐plunging radial folds indicate tangential oblate shortening of the strata during the imbrication of the thrust sheets. Each individual sheet records an early stage of folding and thickening due to non‐coaxial strains, shortly before sheet imbrication. We attribute this folding and thickening phase to the kilometer‐scale inward movement of the target strata from the transient cavity walls to the central uplift. The outer parts of the central uplift record additional outward movement of the target rocks, possibly related to the collapse of the central uplift. An inner ring structure at 10–12 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks.     

40Ar/39Ar laser probe dating of the Central European tektite‐producing impact event

Abstract— A new ⁴⁰Ar/³⁹Ar data set is presented for tektites from the Central European strewn field (moldavites). This is the only strewn field that is entirely situated in a continental environment and still characterized by scattered ages (14–15.3 Myr). The main objectives of the study were to define more precisely the moldavite formation age and provide a good calibration for a glass standard proposed for fission‐track dating. The laser total fusion ages obtained on chips from 7 individual specimens from the Southern Bohemian and Moravian subfields are restricted to a narrow interval of time, with an average of 14.34 ± 0.08 Myr relative to the 27.95 ± 0.09 Myr of the Fish Canyon Tuff biotite. This result gives a more precise age not only for the tektite field but also for its producing impact. If the genetic link between the moldavites and the Nördlinger Ries impact crater is maintained, then this new age has to be considered a reliable estimate for the Ries crater also. This new value places the formation of Central European tektites within the Lower Serravallian period in the latest geologic timescales. Evidence of their impact products, such as glass spherules or shocked minerals, can, therefore, be sought in sedimentary marine formations in a more precisely defined age interval.     

A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselkä impact structure,Finland

Field investigations in the eroded central uplift of the ≤30 km Keurusselkä impact structure, Finland, revealed a thin, dark melt vein that intersects the autochthonous shatter cone‐bearing target rocks near the homestead of Kirkkoranta, close to the center of the impact structure. The petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding ~8–10 GPa. The mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia. ⁴⁰Ar/³⁹Ar dating of dark and clast‐poor whole‐rock chips yielded five concordant Late Mesoproterozoic miniplateau ages and one plateau age of 1151 ± 10 Ma [± 11 Ma] (2σ; MSWD = 0.11; P = 0.98), considered here as the statistically most robust age for the rock. The new ⁴⁰Ar/³⁹Ar age is incompatible with ~1.88 Ga Svecofennian tectonism and magmatism in south‐central Finland and probably reflects the Keurusselkä impact, followed by impact‐induced hydrothermal chloritization of the crater basement. In keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration, the Late Mesoproterozoic ⁴⁰Ar/³⁹Ar age of ~1150 Ma should be treated as a minimum age for the impact. The new ⁴⁰Ar/³⁹Ar results are consistent with paleomagnetic results that suggested a similar age for Keurusselkä, which is shown to be one of the oldest impact structures currently known in Europe and worldwide.     

40Ar-39Ar dating of the H3 chondrite Sainte Rose

Abstract— A whole rock sample from the H3 chondrite Sainte Rose, which was discovered in 1983, was dated with the ⁴⁰Ar-³⁹Ar technique. From the K/Ca spectrum and from an analysis of the ³⁹Ar recoil effect we conclude that this meteorite has at least two distinguishable K-carriers: a low temperature, high-K/Ca, fine-grained carrier with a typical grain size of about 2 μm and a high temperature, low-K/Ca, more coarse-grained carrier. The K-Ar clocks of both were started 4.40 ± 0.01 Ga ago.     

39Ar‐40Ar dating of the Zagami Martian shergottite and implications for magma origin of excess 40Ar

Abstract— The Zagami shergottite experienced a complex, petrogenetic formation history (McCoy et al. 1992, 1999). Like several shergottites, Zagami contains excess ⁴⁰Ar relative to its formation age. To understand the origin of this excess ⁴⁰Ar, we made ³⁹Ar‐⁴⁰Ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution. Surprisingly, all these separates show similar concentrations of excess ⁴⁰Ar, ?1 × 10^?6 cm³/g. We present arguments against this excess ⁴⁰Ar having been introduced from the Martian atmosphere as impact glass. We also present evidence against excess ⁴⁰Ar being a partially degassed residue from a basalt that actually formed ?4 Gyr ago. We utilize our experimental data on Ar diffusion in Zagami and evidence that it was shock‐heated to only ?70 °C, and we assume this heating occurred during an ejection from Mars ?3 Myr ago. With these constraints, thermal considerations necessitates either that its ejected mass was impossibly large, or that its shock‐heating temperature was an order of magnitude higher than that measured. We suggest that this excess ⁴⁰Ar was inherited from the Zagami magma, and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old, K‐rich crustal material. Similar concentrations of excess ⁴⁰Ar in the analyzed separates imply that this magma maintained a relatively constant ⁴⁰Ar concentration throughout its crystallization. This likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released. These concepts have implications for excess ⁴⁰Ar in other shergottites.     

A Middle‐Late Triassic 40Ar/39Ar age for the Paasselkä impact structure (SE Finland)

Abstract– ⁴⁰Ar/³⁹Ar dating of recrystallized feldspar glass particles separated from clast‐rich impact melt rocks from the approximately 10 km Paasselkä impact structure (SE Finland) yielded a Middle to Late Triassic (Ladinian‐Karnian) pseudo‐plateau age of 228.7 ± 3.0 (3.4) Ma (2σ). This new age makes Paasselkä the first known Triassic impact structure dated by isotopic methods on the Baltic Shield. The new Paasselkä impact age is, within uncertainty, coeval with isotopic ages recently obtained for the Lake Saint Martin impact structure in Canada, indicating a new Middle to Late Triassic impact crater population on Earth. The comparatively small crater size, however, suggests no relationship between the Paasselkä impact and a postulated extinction event at the Middle/Late Triassic boundary.     

A Rhaetian 40Ar/39Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record

Abstract– ⁴⁰Ar/³⁹Ar dating of potassium feldspar (primary spherulitic‐blocky and secondary idiomorphic K‐feldspar) separated from impact‐metamorphosed gneiss found near Videix in the western central part of the Rochechouart impact structure (NW Massif Central, France) yielded a Rhaetian combined age of 201 ± 2 Ma (2σ), indistinguishable within uncertainty from the age of the Triassic/Jurassic boundary. Ballen quartz intergrown with the primary K‐feldspar indicates post‐shock temperatures exceeding approximately 1000 °C that affected the precursor gneiss. Geochemically, both feldspar types represent essentially pure potassium end‐members. Apart from the approximately 15 km diameter impact deposit area, the youngest crystallization age known for basement rocks in this part of the Massif Central is approximately 300 Ma. No endogenic magmatic‐thermal events are known to have occurred later in this region. The K‐feldspar recrystallized from local feldspar melts and superimposed post‐shock hydrothermal crystallization, probably within some thousands of years after the impact. It is, therefore, suggested that the ⁴⁰Ar/³⁹Ar age for the Videix gneiss (as a potassic “impact metasomatite”) dates the Rochechouart impact, in consistence with evidence for K‐metasomatism in the Rochechouart impactites. The new age value is distinctly younger than the previously obtained Karnian–Norian age for Rochechouart and, thus, contradicts the Late Triassic multiple impact theory postulated some years ago. In agreement with the paleogeographic conditions in the western Tethys domain around the Triassic/Jurassic boundary, the near‐coastal to shallow marine Rochechouart impact is compatible with the formation of seismites and tsunami deposits in the latest Triassic of the British Isles and possible related deposits in other parts of Europe.     

A Carnian 40Ar/39Ar age for the Paasselkä impact structure (SE Finland)—An update

A recrystallized band of pale feldspathic impact melt in a gneissic impact breccia from the approximately 10 km Paasselkä impact structure in southeast Finland was dated via ⁴⁰Ar/³⁹Ar step‐heating. The newly obtained plateau age of 228.7 ± 1.8 (2.2) Ma (2σ) (MSWD = 0.32; p = 0.93) is equal to the previously published pseudoplateau age of 228.7 ± 3.0 (3.4) (2σ) for the impact event. According to the current international chronostratigraphic chart and using the most recent published suggestions for the K decay constants, a Carnian (Late Triassic) age for the Paasselkä impact structure of 231.0 ± 1.8 (2.2) Ma (2σ) is calculated and considered the most precise and accurate age for this impact structure. The new plateau age for Paasselkä confirms the previous dating result but is, based on its internal statistics, much more compelling.     

Araguainha impact crater,Brazil. I. The interior part of the uplift

Abstract— The central uplift of the 40-km wide Araguainha impact structure, Brazil, consists of a ring, about 8 km in diameter, of up to 150-m high blocks of Devonian Furnas sandstone, which surround a central depression of elliptical shape (4.5 × 3.0 km). The depression is occupied by a pre-Devonian alkali-feldspar granite, shocked by pressures of 20–25 GPa and permeated by cataclastic shear zones and dikes of shocked granitic material. The granite is flanked and partly covered by several impact breccias: (1) Impact breccia with melt matrix overlies the granite in places and forms hills, bordering the granitic center in the S and SW. It is chemically identical with the granite and consists of thermally altered granitic clasts in a matrix of sanidine, quartz, biotite, muscovite, chlorite and riebeckite. (2) Polymict breccias form hills which border the central depression in the N and NW. Components are unshocked and shocked sediments, shock-melted sandstone, shocked granite and shock melt rocks in irregular masses and individual bodies, embedded in a fine-grained matrix. ⁴⁰Ar/³⁹Ar analyses show that the melt rocks solidified 246 Ma ago, indicating that the impact occurred at near the Permian-Triassic boundary, possibly when the area was covered by a shallow sea. The present chemistry and petrography of the melt rocks suggest that by reacting with seawater granitic impact melt was depleted of K and Rb and enriched in Na, and that later diagenetic processes produced replacement of feldspar by quartz and deposition of hematite. (3) Monomict breccias, consisting of unshocked, shocked and shock-fused quartz sandstones, form hills which surround the central depression in the SE and S. The Araguainha structure is an eroded complex crater, produced by an impact, 246 Ma ago. The depth of excavation was about 2.4 km, comprising Permian, Permo-Carboniferous and Devonian sediments and the granitic basement. The diameter of the transient crater was about 24 km. Erosion and weathering have removed most of the original crater fill and ejecta deposits, with the exception of remnants, preserved in the central uplift.     

Petrographic and geochemical characterization of the granitic rocks of the Araguainha impact crater,Brazil

Petrographic and geochemical data obtained on the Araguainha impact crater (Goiás/Mato Grosso States, Brazil) indicate the existence of several molten products that originated during impact‐induced congruent melting of an alkali‐granite exposed in the inner part of the central uplift of the structure. Although previous studies have described these melts to some extent, there is no detailed discussion on the petrographic and geochemical variability in the granite and its impactogenic derivatives, and therefore, little is known about the geochemical behavior and mobility of trace elements during its fusion in the central part of the Araguainha crater. This paper demonstrates that the preserved granitoid exposed in the core of the structure is a magnesium‐rich granite, similar to postcollisional, A‐type granites, also found in terrains outside the Araguainha crater, in the Brasília orogenic belt. The molten products are texturally distinct and different from the original rock, but have very similar geochemical composition, making it difficult to separate these lithotypes based on concentrations of major and minor elements. This also applies for trace and rare earth elements (REE), thus indicating a high degree of homogenization during impact‐induced congruent melting under high pressure and postshock temperature conditions. Petrographic observations, along with geochemical data, indicate that melting occurs selectively, where some of the elements are transported with the melt. Simultaneously, there is an effective dissolution of the rock (granite), which leads to entrainment of the most resistant solid phases (intact or partially molten minerals) into the melt. Minerals more resistant to melting, such as quartz and oxides, contribute substantially to a chemical balance between the preserved granite and the fusion products generated during the meteoritic impact.     

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.     

Argon-40/argon-39 analyses of samples from the Gardnos impact structure,Norway

Abstract— We have conducted ⁴⁰Ar/³⁹Ar age dating on a sample of impact melt from the Gardnos impact structure in Norway in an attempt to better constrain the formation age of the crater. Current estimates of the age of the Gardnos crater cover a wide range and are as old as 900 Ma (Dons and Naterstad, 1992; French et al., 1997). The age spectra that we obtained from three samples are consistent with a thermal event at 385 ± 5 Ma (1σ). Because this differs greatly from the best stratigraphic age of ～600 Ma, and because the minerals present in the dated sample are a metamorphic assemblage, we believe we have not dated the formation age of the crater. Instead we have probably dated the effect of the early Devonian collapse of the late Caledonian (Scandian) orogeny on these materials (Dons and Naterstad, 1992; French et al., 1997). Although it may be possible, it will be difficult to determine the age of the impact by isotopic means alone because of this widespread metamorphism. Detailed stratigraphic analyses of the crater fill sediments may be the most promising method for constraining the crater age.     

40Ar/39Ar age of material returned from asteroid 25143 Itokawa

The Hayabusa mission to asteroid 25143, Itokawa, brought back 2000 small particles, which most closely resemble material found in LL4‐6 chondrites. We report an ⁴⁰Ar/³⁹Ar age of 1.3 ± 0.3 Ga for a sample of Itokawa consisting of three grains with a total mass of ~2 μg. This age is lower than the >4.0 Ga ages measured for 75% of LL chondrites but close to one for Y‐790964 and its pairs. The flat ⁴⁰Ar/³⁹Ar release spectrum of the sample suggests complete degassing 1.3 Ga ago. Recent solar heating in Itokawa's current orbit does not appear likely to have reset that age. Solar or impact heating 1.3 Ga ago could have done so. If impact heating was responsible, then the 1.3 Ga age sets an upper bound on the time at which the Itokawa rubble pile was assembled and suggests that rubble pile creation was an ongoing process in the inner solar system for at least the first 3 billion years of solar system history.     

Ultraprecise age and formation temperature of the Australasian tektites constrained by 40Ar/39Ar analyses

The Australasian tektites are quench melt glass ejecta particles distributed over the Asian, Australian, and Antarctic regions, the source crater of which is currently elusive. New ⁴⁰Ar/³⁹Ar age data from four tektites: one each from Thailand, China, Vietnam, and Australia measured using three different instruments from two different laboratories and combined with published ⁴⁰Ar/³⁹Ar data yield a weighted mean age of 788.1 ± 2.8 ka (±3.0 ka, including all sources of uncertainties) (P = 0.54). This age is five times more precise compared to previous results thanks, in part, to the multicollection capabilities of the ARGUS VI noble gas mass spectrometer, which allows an improvement of almost fourfold on a single plateau age measurement. Diffusion experiments on tektites combined with synthetic age spectra and Monte Carlo diffusion models suggest that the minimum temperature of formation of the Thai tektite is between 2350 °C and 3950 °C, hence a strict minimum value of 2350 °C.     

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.     

Laser probe argon-40/argon-39 dating of coesite- and stishovite-bearing pseudotachylytes and the age of the Vredefort impact event

Abstract— Age determinations have been made on pseudotachylytic rocks from the controversial Vredefort structure of South Africa using the laser microprobe ⁴⁰Ar/³⁹Ar dating technique. Coesite- and stishovitebearing veins in a quartzite from the Central Rand Group of the collar rocks were dated using a 10-μm diameter focused ultra-violet laser beam. These yielded a weighted mean age of 2027 ± 18 Ma (2σ). Six pseudotachylytes, sampled from four different locations within the Outer Granite Gneiss of the core, were dated using an 50–100-μm diameter focused infrared laser beam. These pseudotachylytes exhibit altered vein margins with apparent ages considerably younger than ages obtained from the fresher centres of veins. The best weighted mean pseudotachylyte matrix age obtained was 2018 ± 14 Ma (2σ). Most of the clasts within the pseudotachylyte matrices retain significantly older (e.g., Archean) ages, indicative of their parent rock history. Our results show that five of the seven dated samples possess matrix ages of ～2000 Ma, similar to the age of the Granophyre (Walraven et al., 1990), a supposed impact melt rock (French and Nielsen, 1990). The dating of coesite- and stishovite-bearing veins equates the shock event with pseudotachylyte formation, generation of the Granophyre and creation of the Vredefort structure. The results affirm that the Vredefort Dome is a meteorite impact structure and show that it formed at 2018 ± 14 Ma (2σ).     

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.     

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).     

40Ar-39Ar Dating of the Haughton Impact Structure

Abstract— ⁴⁰Ar-³⁹Ar dating of a strongly shocked, vesiculated gneiss clast in a breccia from the Haughton impact structure yields an isochron age of 23.4 ± 1.0 Ma. The sample contains excess ⁴⁰Ar.