Chromium isotopes identify the extraterrestrial component in impactites from Dhala impact structure,India

The Dhala structure in north-central India is a confirmed complex impact structure of Paleoproterozoic age. The presence of an extraterrestrial component in impactites from the Dhala structure was recognized by geochemical analyses of highly siderophile elements and Os isotopic compositions; however, the impactor type has remained unidentified. This study uses Cr isotope systematics to identify the type of projectile involved in the formation of the Dhala structure. Unlike the composition of siderophile elements (e.g., Ni, Cr, Co, and platinum group elements) and their inter-element ratios that may get compromised due to the extreme energy generated during an impact, Cr isotopes retain the distinct composition of the impactor. The distinct ε⁵⁴Cr value of −0.31 ± 0.09 for a Dhala impact melt breccia sample (D6-57) indicates inheritance from an impactor originating within the non-carbonaceous reservoir, that is, the inner Solar System. Based on the Ni/Cr ratio, Os abundance, and Cr isotopic composition of the samples, the impactor is constrained to be of ureilite type. Binary mixing calculations also indicate contamination of the target rock by 0.1–0.3 wt% of material from a ureilite-like impactor. Together with the previously identified impactors that formed El'gygytgyn, Zhamanshin, and Lonar impact structures, the Cr isotopic compositions of the Dhala impactites argue for a much more diverse source of the objects that collided with the Earth over its geological history than has been supposed previously.     

Documentation of shock features in impactites from the Dhala impact structure,India

The fundamental approach for the confirmation of any terrestrial meteorite impact structure is the identification of diagnostic shock metamorphic features, together with the physical and chemical characterization of impactites and target lithologies. However, for many of the approximately 200 confirmed impact structures known on Earth to date, multiple scale‐independent tell‐tale impact signatures have not been recorded. Especially some of the pre‐Paleozoic impact structures reported so far have yielded limited shock diagnostic evidence. The rocks of the Dhala structure in India, a deeply eroded Paleoproterozoic impact structure, exhibit a range of diagnostic shock features, and there is even evidence for traces of the impactor. This study provides a detailed look at shocked samples from the Dhala structure, and the shock metamorphic evidence recorded within them. It also includes a first report of shatter cones that form in the shock pressure range from ~2 to 30 GPa, data on feather features (FFs), crystallographic indexing of planar deformation features, first‐ever electron backscatter diffraction data for ballen quartz, and further analysis of shocked zircon. The discovery of FFs in quartz from a sample of the MCB‐10 drill core (497.50 m depth) provides a comparatively lower estimate of shock pressure (~7–10 GPa), whereas melting of a basement granitoid infers at least 50–60 GPa shock pressure. Thus, the Dhala impactites register a strongly heterogeneous shock pressure distribution between <2 and >60 GPa. The present comprehensive review of impact effects should lay to rest the nonimpact genesis of the Dhala structure proposed by some earlier workers from India.     

The Dhala structure,Bundelkhand craton,Central India—Eroded remnant of a large Paleoproterozoic impact structure

Abstract— The newly discovered Dhala structure, Madhya Pradesh State, India, is the eroded remnant of an impact structure with an estimated present‐day apparent diameter of about 11 km. It is located in the northwestern part of the Archean Bundelkhand craton. The pre‐impact country rocks are predominantly granitoids of ?2.5 Ga age, with minor 2.0–2.15 Ga mafic intrusive rocks, and they are overlain by post‐impact sediments of the presumably >1.7 Ga Vindhyan Supergroup. Thus, the age for this impact event is currently bracketed by these two sequences. The Dhala structure is asymmetrically disposed with respect to a central elevated area (CEA) of Vindhyan sediments. The CEA is surrounded by two prominent morphological rings comprising pre‐Vindhyan arenaceous‐argillaceous and partially rudaceous metasediments and monomict granitoid breccia, respectively. There are also scattered outcrops of impact melt breccia exposed towards the inner edge of the monomict breccia zone, occurring over a nearly 6 km long trend and with a maximum outcrop width of ?170 m. Many lithic and mineral clasts within the melt breccia exhibit diagnostic shock metamorphic features, including multiple sets of planar deformation features (PDFs) in quartz and feldspar, ballen‐textured quartz, occurrences of coesite, and feldspar with checkerboard texture. In addition, various thermal alteration textures have been found in clasts of initially superheated impact melt. The impact melt breccia also contains numerous fragments composed of partially devitrified impact melt that is mixed with unshocked as well as shock deformed quartz and feldspar clasts. The chemical compositions of the impact melt rock and the regionally occurring granitoids are similar. The Ir contents of various impact melt breccia samples are close to the detection limit (1–1.5 ppb) and do not provide evidence for the presence of a meteoritic component in the melt breccia. The presence of diagnostic shock features in mineral and lithic clasts in impact melt breccia confirm Dhala as an impact structure. At 11 km, Dhala is the largest impact structure currently known in the region between the Mediterranean and southeast Asia.     

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.     

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.     

Identification of a meteoritic component using chromium isotopic composition of impact rocks from the Lonar impact structure,India

The existence of mass‐independent chromium isotope variability of nucleosynthetic origin in meteorites and their components provides a means to investigate potential genetic relationship between meteorites and planetary bodies. Moreover, chromium abundances are depleted in most surficial terrestrial rocks relative to chondrites such that Cr isotopes are a powerful tool to detect the contribution of various types of extra‐terrestrial material in terrestrial impactites. This approach can thus be used to constrain the nature of the bolide resulting in breccia and melt rocks in terrestrial impact structures. Here, we report the Cr isotope composition of impact rocks from the ~0.57 Ma Lonar crater (India), which is the best‐preserved impact structure excavated in basaltic target rocks. Results confirm the presence of a chondritic component in several bulk rock samples of up to 3%. The impactor that created the Lonar crater had a composition that was most likely similar to that of carbonaceous chondrites, possibly a CM‐type chondrite.     

Shocked quartz in polymict impact breccia from the Upper Cretaceous Yallalie impact structure in Western Australia

Yallalie is a ~12 km diameter circular structure located ~200 km north of Perth, Australia. Previous studies have proposed that the buried structure is a complex impact crater based on geophysical data. Allochthonous breccia exposed near the structure has previously been interpreted as proximal impact ejecta; however, no diagnostic indicators of shock metamorphism have been found. Here we report multiple (27) shocked quartz grains containing planar fractures (PFs) and planar deformation features (PDFs) in the breccia. The PFs occur in up to five sets per grain, while the PDFs occur in up to four sets per grain. Universal stage measurements of all 27 shocked quartz grains confirms that the planar microstructures occur in known crystallographic orientations in quartz corresponding to shock compression from 5 to 20 GPa. Proximity to the buried structure (~4 km) and occurrence of shocked quartz indicates that the breccia represents either primary or reworked ejecta. Ejecta distribution simulated using iSALE hydrocode predicts the same distribution of shock levels at the site as those found in the breccia, which supports a primary ejecta interpretation, although local reworking cannot be excluded. The Yallalie impact event is stratigraphically constrained to have occurred in the interval from 89.8 to 83.6 Ma based on the occurrence of Coniacian clasts in the breccia and undisturbed overlying Santonian to Campanian sedimentary rocks. Yallalie is thus the first confirmed Upper Cretaceous impact structure in Australia.     

Mineralogy and petrology of melt rocks from the Popigai impact structure,Siberia

Abstract— The late Eocene Popigai impact structure of Siberia comprises an approximately 0.5–1.5 km thick, ˜100 km diameter sequence of clast-rich and clast-poor andesitic to rhyolitic impact melt rocks and impact breccias, underlain by Archean to Proterozoic crystalline basement and Proterozoic to Phanerozoic sedimentary rocks. The fine-grained to cryptocrystalline texture of the more melt-rich rocks, despite their occurrence in bodies locally in excess of 800 m thick and 28 km long, suggests that the melt crystallized in response to (1) cooling by the clast load, and/or; (2) rapid nucleation on finely brecciated clasts, which have since been assimilated and/or; (3) crystallization enhanced by the relatively low water contents of the melts. Rapid crystallisation of the melt is indicated by the lack of zoning in minerals, the presence of glass, the lack of strain recovery features in clasts and the lack of evidence for fractionation in the major and trace elements, including the rare earth elements. Optical and analytical electron microscopy reveal that the previously reported division of the melt rocks into high- and low-temperature variants based on hand sample appearance, or glass content, is not warranted. Clasts within the melt-rich rocks exhibit a wide range of shock metamorphic features, though they are not distributed in the impact melts in a systematic manner. This indicates that the melt-rich rocks were well mixed during their formation, thus juxtaposing unshocked with shocked material. Injection of mesostasis melt into partially melted checkerboard plagioclase and orthopyroxene clasts also occurred during this mixing stage.     

Geochemical studies of impact breccias and country rocks from the El'gygytgyn impact structure,Russia

The complex impact structure El'gygytgyn (age 3.6 Ma, diameter 18 km) in northeastern Russia was formed in ~88 Ma old volcanic target rocks of the Ochotsk‐Chukotsky Volcanic Belt (OCVB). In 2009, El'gygytgyn was the target of a drilling project of the International Continental Scientific Drilling Program (ICDP), and in summer 2011 it was investigated further by a Russian–German expedition. Drill core material and surface samples, including volcanic target rocks and impactites, have been investigated by various geochemical techniques in order to improve the record of trace element characteristics for these lithologies and to attempt to detect and constrain a possible meteoritic component. The bedrock units of the ICDP drill core reflect the felsic volcanics that are predominant in the crater vicinity. The overlying suevites comprise a mixture of all currently known target lithologies, dominated by felsic rocks but lacking a discernable meteoritic component based on platinum group element abundances. The reworked suevite, directly overlain by lake sediments, is not only comparatively enriched in shocked minerals and impact glass spherules, but also contains the highest concentrations of Os, Ir, Ru, and Rh compared to other El'gygytgyn impactites. This is—to a lesser extent—the result of admixture of a mafic component, but more likely the signature of a chondritic meteoritic component. However, the highly siderophile element contribution from target material akin to the mafic blocks of the ICDP drill core to the impactites remains poorly constrained.     

Target rocks,impact glasses,and melt rocks from the Lonar impact crater,India: Petrography and geochemistry

Abstract— The Lonar crater, India, is the only well‐preserved simple crater on Earth in continental flood basalts; it is excavated in the Deccan trap basalts of Cretaceous‐Tertiary age. A representative set of target basalts, including the basalt flows excavated by the crater, and a variety of impact breccias and impact glasses, were analyzed for their major and trace element compositions. Impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes. Comparable geochemical patterns of immobile elements (e.g., REEs) for glass, melt rock and basalt indicates minimal fractionation between the target rocks and the impactites. We found only little indication of post‐impact hydrothermal alteration in terms of volatile trace element changes. No clear indication of an extraterrestrial component was found in any of our breccias and impact glasses, indicating either a low level of contamination, or a non‐chondritic or otherwise iridium‐poor impactor.     

Impact melt rocks from the Late Paleocene Hiawatha impact structure,northwest Greenland

Impact melt rocks formed during hypervelocity impact events are ideal for studying impact structures. Here, we describe impact melt rock samples collected proximal to the 31 km wide 58 Ma Hiawatha impact structure, northwest Greenland, which is completely covered by the Greenland Ice Sheet. The melt rocks contain diagnostic shock indicators (e.g., planar deformation features [PDF] in quartz and shocked zircon) and form three groups based on melt textures and chemistry: (i) hypocrystalline, (ii) glassy, and (iii) carbonate-based melt rocks. The exposed foreland directly in front of the structure consists of metasedimentary successions and igneous plutons; however, the carbonate-based impactites indicate a mixed target sequence with a significant carbonate-rich component. Well-preserved organic material in some melt rocks indicates that North Greenland at the time of impact was host to abundant organic material, likely a dense high-latitude temperate forest. Geochemical signatures of platinum-group elements in selected samples indicate an extraterrestrial component and support previous identification of a highly fractionated iron impactor in glaciofluvial sand. Our results illustrate the possibility to study impact structures hidden beneath a thick ice sheet based on transported samples and this opens a new avenue for identifying other potential impact craters in Greenland and Antarctica.     

Shock metamorphism investigations of quartz grains in clasts from impact breccia of the Eyreville B drill core,Chesapeake Bay impact structure,USA

Abstract– The Chesapeake Bay impact structure, approximately 85 km in diameter, has been drilled in 2005–2006 at Eyreville (Virginia, USA), to a total depth of 1766 m. In the drill cores, the abundance of shock metamorphosed material is very variable with depth. Shocked mineral and lithic clasts, as well as melt particles, are most abundant in suevitic impact breccia section (1397–1451 m depth). Shocked quartz (i.e., quartz grains with planar fractures and/or planar deformation features) and melt particles, although rare, are also dispersed in the Exmore Formation unit (444–867 m depth). Other lithologies in the Eyreville drill cores show no clear evidence of shock metamorphism. Here, we report on the investigations of 40 samples from the impact breccia section. A total of more than 27,000 quartz grains were examined in about 200 clasts. The abundance of highly shocked clasts tends to decrease with increasing depth. Crystalline clasts derived from the crystalline basement are commonly only slightly shocked (contain generally <10 rel% of shocked quartz grains). The clasts of metamorphosed sediments show a low proportion of shocked quartz grains (mostly <10 rel%). Sedimentary clasts show a wide range of proportions of shocked quartz grains, with several of them being highly shocked clasts (most values between 0 and 40 rel%). Conglomerates show the highest proportion of shocked quartz grains of all types of clasts (up to 83 rel%). Polycrystalline quartz clasts are also commonly highly shocked (contain mostly between 10 and 40 rel% of shocked quartz grains). These hard nonporous clasts are possibly more liable to show evidence of shock. The investigations suggest that the intensity of shock metamorphism is the result of several parameters, such as original position in the target (both horizontal and vertical) and the properties of each lithology (e.g., grain size, porosity, and amount of matrix). According to the universal‐stage investigations, the dominant orientations of planar deformation features in quartz are , , and also .     

Traces of an H chondrite in the impact‐melt rocks from the Lappajärvi impact structure,Finland

Abstract— Here we present the results of a geochemical study of the projectile component in impactmelt rocks from the Lappajärvi impact structure, Finland. Main‐ and trace‐element analyses, including platinum group elements (PGEs), were carried out on twenty impact‐melt rock samples from different locations and on two shocked granite fragments. The results clearly illustrate that all the impact melt rocks are contaminated with an extraterrestrial component. An identification of the projectile type was performed by determining the projectile elemental ratios and comparing the corresponding element ratios in chondrites. The projectile elemental ratios suggest an H chondrite as the most likely projectile type for the Lappajärvi impact structure. The PGE composition of the highly diluted projectile component (?0.05 and 0.7 wt% in the impact‐melt rocks) is similar to the recent meteorite population of H chondrites reaching Earth. The relative abundance of ordinary chondrites, including H, L, and LL chondrites, as projectiles at terrestrial impact structures is most likely related to the position of their parent bodies relative to the main resonance positions. This relative abundance of ordinary chondrites suggests a strong bias of the impactor population toward inner Main Belt objects.     

Shocked quartz grains in the polymict breccia of the Granby structure,Sweden–‐Verification of an impact

Abstract— The Middle Ordovician Granby structure in Sweden is generally considered the result of an asteroidal or cometary collision with Earth, although no hard evidence, i.e., shock metamorphic features or traces of the impactor, have been presented to date. In this study, drill core samples of a sedimentary breccia from the Granby structure have been investigated for microscopic shock metamorphic evidence in an attempt to verify the impact genesis of the structure. The finding of multiple sets of decorated planar deformation features (PDFs) in quartz grains in these samples provides unambiguous evidence that the structure is impact derived. Furthermore, the orientation of the PDFs, e.g., ω {101 }, π {101 } and r, z {101 }, is characteristic for impact deformation. The fact that a majority of the PDFs are decorated implies a water‐bearing target. The shocked quartz grains can be divided into two groups; rounded grains found in the breccia matrix likely originated from mature sandstone, and angular grains in fragments from crystalline target rocks. The absence of melt particles provides an estimated maximum shock pressure for the sedimentary derived quartz of 15–20 GPa and the frequency distribution of PDF orientations in the bedrock quartz implies pressures of the order of 10 GPa.     

New impact‐melt rock from the Roter Kamm impact structure,Namibia: Further constraints on impact age,melt rock chemistry,and projectile composition

Abstract— A new locality of in situ massive impact‐melt rock was discovered on the south‐southwestern rim of the Roter Kamm impact structure. While the sub‐samples from this new locality are relatively homogeneous at the hand specimen scale, and despite being from a nearby location, they do not have the same composition of the only previously analyzed impact‐melt rock sample from Roter Kamm. Both Roter Kamm impact‐melt rock samples analyzed to date, as well as several suevite samples, exhibit a granitic‐granodioritic precursor composition. Micro‐chemical analyses of glassy matrix and Al‐rich orthopyroxene microphenocrysts demonstrate rapid cooling and chemical disequilibrium at small scales. Platinum‐group element abundances and ratios indicate an ordinary chondritic composition for the Roter Kamm impactor. Laser argon dating of two sub‐samples did not reproduce the previously obtained age of 3.7 ± 0.3 (1s?) for this impact event, based on ⁴⁰Ar/³⁹Ar dating of a single vesicular impact‐melt rock. Instead, we obtained ages between 3.9 and 6.3 Ma, with an inverse isochron age of 4.7 ± 0.3 Ma for one analyzed sub‐sample and 5.1 ± 0.4 Ma for the other. Clearly a post‐5 Ma impact at Roter Kamm remains indicated, but further analytical work is required to better constrain the currently best estimate of 4–5 Ma. Both impactor and age constraints are clearly obstructed by the inherent microscopic heterogeneity and disequilibrium melting and cooling processes demonstrated in the present study.     

Accessory and opaque minerals in impact melt rocks of the Boltysh structure,Ukraine

Electron microprobe analyses of accessory and opaque minerals from the impact melt rocks of the Boltysh structure, in the central part of the Ukrainian Shield, are presented in this report. Our study establishes a variety of minerals represented by native metals, alloys, oxides, sulfides, phosphates, and silicates, formed during several stages of cooling and solidification of the thick impact melt sheet. Baddeleyite was determined to be the earliest high‐temperature mineral to occur in the impact melt rocks. Iron and titanium oxides crystallized earlier or simultaneously with the microliths of orthopyroxene and feldspars. High concentrations of TiO₂, Al₂O₃, and Cr₂O₃ were identified in some hematite varieties. Cu‐ and Ni‐bearing pyrrhotites occur in impact melt rocks with a glassy matrix. Native metals—copper, platinum, and silver—were likely formed due to the hydrothermal alteration of the upper unit of the impact melt sheet. Zircon is the only accessory mineral found in impact melt rocks that is preserved from the basement granites of the Boltysh structure.     

Confirmation of a meteoritic component in impact‐melt rocks of the Chesapeake Bay impact structure,Virginia, USA—Evidence from osmium isotopic and PGE systematics

Abstract— The osmium isotope ratios and platinum‐group element (PGE) concentrations of impact‐melt rocks in the Chesapeake Bay impact structure were determined. The impact‐melt rocks come from the cored part of a lower‐crater section of suevitic crystalline‐clast breccia in an 823 m scientific test hole over the central uplift at Cape Charles, Virginia. The ¹⁸⁷Os/¹⁸⁸Os ratios of impact‐melt rocks range from 0.151 to 0.518. The rhenium and platinum‐group element (PGE) concentrations of these rocks are 30–270x higher than concentrations in basement gneiss, and together with the osmium isotopes indicate a substantial meteoritic component in some impact‐melt rocks. Because the PGE abundances in the impact‐melt rocks are dominated by the target materials, interelemental ratios of the impact‐melt rocks are highly variable and nonchondritic. The chemical nature of the projectile for the Chesapeake Bay impact structure cannot be constrained at this time. Model mixing calculations between chondritic and crustal components suggest that most impact‐melt rocks include a bulk meteoritic component of 0.01–0.1% by mass. Several impact‐melt rocks with lowest initial ¹⁸⁷Os/¹⁸⁸Os ratios and the highest osmium concentrations could have been produced by additions of 0.1%–0.2% of a meteoritic component. In these samples, as much as 70% of the total Os may be of meteoritic origin. At the calculated proportions of a meteoritic component (0.01–0.1% by mass), no mixtures of the investigated target rocks and sediments can reproduce the observed PGE abundances of the impact‐melt rocks, suggesting that other PGE enrichment processes operated along with the meteoritic contamination. Possible explanations are 1) participation of unsampled target materials with high PGE abundances in the impact‐melt rocks, and 2) variable fractionations of PGE during syn‐ to post‐impact events.     

Incipient melt formation and devitrification at the Wanapitei impact structure,Ontario, Canada

Abstract— The Wanapitei impact structure is ～8 km in diameter and lies within Wanapitei Lake, ～34 km northeast of the city of Sudbury. Rocks related to the 37 Ma impact event are found only in Pleistocene glacial deposits south of the lake. Most of the target rocks are metasedimentary rocks of the Proterozoic Huronian Supergroup. An almost completely vitrified, inclusion-bearing sample investigated here represents either an impact melt or a strongly shock metamorphosed, pebbly wacke. In the second, preferred interpretation, a number of partially melted and devitrified clasts are enclosed in an equally highly shock metamorphosed arkosic wacke matrix (i.e., the sample is a shocked pebbly wacke), which records the onset of shock melting. This interpretation is based on the glass composition, mineral relicts in the glass, relict rock textures, and the similar degree of shock metamorphism and incipient melting of all sample components. Boulder matrix and clasts are largely vitrified and preserve various degrees of fluidization, vesiculation, and devitrification. Peak shock pressure of ～50–60 GPa and stress experienced by the sample were somewhat below those required for complete melting and development of a homogeneous melt. The rapid cooling and devitrification history of the analyzed sample is comparable to that reported recently from glasses in the suevite of the Ries impact structure in Germany and may indicate that the analyzed sample experienced an annealing temperature after deposition of somewhere between 650 °C and 800 °C.     

Osmium isotope constraints on the proportion of bolide component in Chicxulub impact melt rocks

Abstract— The spatial distribution and amount of material transferred from the bolide involved in the Cretaceous/Tertiary (K/T) event to the target rocks at Chicxulub is still poorly constrained. In this study, Re‐Os isotopic analyses of impact melt breccias and lithic clasts from the Yaxcopoil‐1 (Yax‐1) borehole were used to determine the distribution and proportion of the bolide component in the target rocks. Because of the much greater concentration of Os in chondritic meteorites compared to the target rocks, little addition of the bolide component would be necessary to greatly perturb the Os concentration and isotopic composition of target rocks. Hence, this is a very sensitive means of examining bolide contributions to the target rocks. For the examined suite of samples, the initial ¹⁸⁷Os/¹⁸⁸Os ratios vary from 0.19 to 2.3. Conservative mixing calculations suggest that the bolide component comprised as much as approximately 0.1%, by mass, of some samples. Most samples, however, have negligible contributions from the bolide. No samples have Os that is dominated by the bolide component, so for this suite of samples, it is impossible to fingerprint the chemical nature of the bolide using relative abundances of siderophile elements. These results suggest that the bolide did not contribute a significant amount of material to the target rocks. This may, in turn, indicate that most of the bolide was vaporized upon impact or otherwise ejected without mixing with the melt from the target.     

Annealing of radiation damage in zircons from Apollo 14 impact breccia 14311: Implications for the thermal history of the breccia

Impact breccia 14311, was collected from the Apollo 14 landing site as a potential sample of the underlying Fra Mauro Formation. Published zircon U‐Pb ages of >4000 Ma date the source material of the breccia and the apatite U‐Pb age of ~3940 Ma is interpreted as dating thermal resetting of the apatite U‐Pb systems. In this contribution we present new age information on the late stage thermal history of the breccia based on the annealing of radiation damage in the zircons. From Raman spectroscopic determination of the radiation damage within SIMS analytical spots on the zircons and the U and Th concentrations determined on these spots, we demonstrate that the radiation damage in the zircons has been annealed and we estimate the age of annealing at 3410 ± 80 Ma. This age is interpreted as a cooling age following heating of the breccia to above the annealing temperature of ~230 °C for stage 1 radiation damage in zircon, but below the temperature needed to reset the U‐Pb system of apatite (~500 °C). It is proposed that this thermal event was associated with the prolonged period of Mare volcanism, from 3150 to 3750 Ma, that generated massive basalt flows in the vicinity of the sample location.