Strontium and neodymium isotopic study of Libyan Desert Glass: Inherited Pan‐African age signatures and new evidence for target material

Abstract— Libyan Desert Glass (LDG) is an impact‐related, natural glass of still unknown target material. We have determined Rb‐Sr and Sm‐Nd isotopic ratios from seven LDG samples and five associated sandstones from the LDG strewn field in the Great Sand Sea, western Egypt. Planar deformation features were recently detected in quartz from these sandstones. ⁸⁷Sr/⁸⁶Sr ratios and ?‐Nd values for LDG range between 0.71219 and 0.71344, and between –16.6 and –17.8, respectively, and hence are distinct from the less radiogenic ⁸⁷Sr/⁸⁶Sr ratios of 0.70910–0.71053 and ?‐Nd values from –6.9 to –9.6 for the local sandstones from the LDG strewn field. Previously published isotopic ratios from the Libyan BP and Oasis crater sandstones are generally incompatible with our LDG values. LDG formation undoubtedly occurred at 29 Ma, but neither the Rb‐Sr nor the Sm‐Nd isotopic system were rehomogenised during the impact event, as we can deduce from Pan‐African ages of ?540 Ma determined from the regression lines from a total of 14 LDG samples from this work and the literature. Together with similar Sr and Nd isotopic values for LDG and granitoid rocks from northeast Africa west of the Nile, these findings point to a sandy matrix target material for the LDG derived from a Precambrian crystalline basement, ruling out the Cretaceous sandstones of the former “Nubian Group” as possible precursors for LDG.     

Geochemical variability of the Yucatàn basement: Constraints from crystalline clasts in Chicxulub impactites

Abstract— The 65 Ma old Chicxulub impact structure with a diameter of about 180 km is again in the focus of the geosciences because of the recently commenced drilling of the scientific well Yaxcopoil‐ 1. Chicxulub is buried beneath thick post‐impact sediments, yet samples of basement lithologies in the drill cores provide a unique insight into age and composition of the crust beneath Yucatàn. This study presents major element, Sr, and Nd isotope data for Chicxulub impact melt lithologies and clasts of basement lithologies in impact breccias from the PEMEX drill cores C‐1 and Y‐6, as well as data for ejecta material from the K/T boundaries at La Lajilla, Mexico, and Furlo, Italy. The impact melt lithologies have an andesitic composition with significantly varying contents of Al, Ca, and alkali elements. Their present day ⁸⁷Sr/⁸⁶Sr ratios cluster at about 0.7085, and ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.5123 to 0.5125. Compared to the melt lithologies that stayed inside the crater, data for ejecta material show larger variations. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.7081 for chloritized spherules from La Lajilla to 0.7151 for sanidine spherules from Furlo. The ¹⁴³Nd/¹⁴⁴Nd ratio is 0.5126 for La Lajilla and 0.5120 for the Furlo spherules. In an ε^t_CHUR(Nd)‐ε^t_UR(Sr) diagram, the melt lithologies plot in a field delimited by Cretaceous platform sediments, various felsic lithic clasts and a newly found mafic fragment from a suevite. Granite, gneiss, and amphibolite have been identified among the fragments from crystalline basement gneiss. Their ⁸⁷Sr/⁸⁶Sr ratios range from 0.7084 to 0.7141, and their ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.5121 to 0.5126. The T^Nd_DM model ages vary from 0.7 to 1.4 Ga, pointing to different source terranes for these rocks. This leads us to believe that the geological evolution and the lithological composition of the Yucatàn basement is probably more complex than generally assumed, and Gondwanan as well as Laurentian crust may be present in the Yucatàn basement.     

Chicxulub impactites: Geochemical clues to the precursor rocks

Abstract— The 65 Ma Chicxulub impact structure, Mexico, with a diameter of ～180 km is the focus of geoscientific research because of its link to the mass extinction event at the Cretaceous‐Tertiary (K/T) boundary. Chicxulub, now buried beneath thick post‐impact sediments, is probably one of the best‐preserved terrestrial impact structures known. Because of its inaccessibility, only limited samples on the impact lithologies from a few drill cores are available. We report major element and Sr‐, Nd‐, O‐, and C‐isotopic data for Chicxulub impact‐melt lithologies and basement clasts in impact breccias of drill cores C‐1 and Y‐6, and for melt particles in the Chicxulub ejecta horizon at the K/T boundary in Beloc, Haiti. The melt lithologies with SiO₂ ranging from 58 to ～63 wt% show significant variations in the content of Al, Ca, and the alkalies. In the melt matrix samples, δ¹³C of the calcite is about ?3%o. The δ¹⁸O values for the siliceous melt matrices of Y‐6 samples range from 9.9 to 12.4%o. Melt lithologies and the black Haitian glass have rather uniform ⁸⁷Sr/⁸⁶Sr ratios (0.7079 to 0.7094); only one lithic fragment displays ⁸⁷Sr/⁸⁶Sr of 0.7141. The Sr model ages T^SrUR for most lithologies range from 830 to 1833 Ma; unrealistic negative model ages point to an open Rb‐Sr system with loss of Rb in a hydrothermal process. The ¹⁴³Nd/¹⁴⁴Nd ratios for all samples, except one basement clast with ¹⁴³Nd/¹⁴⁴Nd of 0.5121, cluster at 0.5123 to 0.5124. In an ?_Nd‐?_Sr diagram, impactites plot in a field delimited by ?_Nd of ?2 to ?6, and ?_Sr of 55 to 69. This field is not defined by the basement lithologies described to occur as lithic clasts in impact breccias and Cretaceous sediments. At least one additional intermediate to mafic precursor component is required to explain the data.     

Argon‐40/argon‐39 age of the El'gygytgyn impact event,Chukotka, Russia

Abstract— Lake El'gygytgyn, Chukotka, Russia, lies in a ～18 km crater of presumably impact origin. The crater is sited in Cretaceous volcanic rocks of the Okhotsk‐Chukotka volcanic belt. Laser ⁴⁰Ar/³⁹Ar dating of impact‐melted volcanic rocks from the rim of Lake El'gygytgyn yields a 10‐sample weighted plateau age of 3.58 ± 0.04 Ma. The Ar step‐heating method was critical in this study in identifying inherited Ar in the samples due to incomplete degassing of the Cretaceous volcanic rocks during impact melting. This age is consistent with, but more precise than, previous K‐Ar and fission‐track ages and indicates an “instantaneous” formation of the crater. This tight age control, in conjunction with the presence of impactites, shocked quartz, and other features, is consistent with an impact origin for the structure and seems to discount internal (volcanogenic) origin models.     

Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil‐1 borehole,Chicxulub impact structure,Mexico: Implications for target composition

Abstract— We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil‐1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end‐member requires a mafic component in order to explain the low SiO₂ content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned—a high‐Ba, low‐Ta group and a high‐Fe, high‐Zn, and high‐Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil‐1 drill core likely register a multistage deformation history that spans the period from pre‐ to post‐impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347–1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event.     

Rb-Sr ages of clasts from within Boulder 1, Station 2, Apollo 17

Rb, Sr and⁸⁷Sr/⁸⁶Sr have been determined for fragments of matrix and clasts from three of the hand-specimens of Boulder 1, 72275, 72255, and 72215. Total-rock and certain plagioclase samples from a crushed norite clast (Civet Cat) define an age of 4.17±0.05AE (2σ) for the pre-Serenitatis igneous differentiation of the norite. Pyroxene and other mineral separates were affected by a later event at about 3.9±0.1AE. An unshocked clast of pigeonite basalt has a well-fitted mineral isochron of 4.01±0.04AE. Samples of the competent breccia matrix comparatively rich in small clasts of highly radiogenic microgranite define a mixing line equivalent to 4.03±0.03AE, which denotes the age of the microgranite. Other samples of the matrix dominated by small anorthosite clasts define a 4.4AE mixing-line and demonstrate that Sr isotope equilibration between plagioclase and matrix did not occur during the high-temperature event that indurated the matrix.     

Target rocks,impact glasses,and melt rocks from the Lonar crater,India: Highly siderophile element systematics and Sr‐Nd‐Os isotopic signatures

The Lonar crater is a ~0.57‐Myr‐old impact structure located in the Deccan Traps of the Indian peninsula. It probably represents the best‐preserved impact structure hosted in continental flood basalts, providing unique opportunities to study processes of impact cratering in basaltic targets. Here we present highly siderophile element (HSE) abundances and Sr‐Nd and Os isotope data for target basalts and impactites (impact glasses and impact melt rocks) from the Lonar area. These tools may enable us to better constrain the interplay of a variety of impact‐related processes such as mixing, volatilization, and contamination. Strontium and Nd isotopic compositions of impactites confirm and extend earlier suggestions about the incorporation of ancient basement rocks in Lonar impactites. In the Re‐Os isochron plot, target basalts exhibit considerable scatter around a 65.6 Myr Re‐Os reference isochron, most likely reflecting weathering and/or magma replenishment processes. Most impactites plot at distinctly lower ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios compared to the target rocks and exhibit up to two orders of magnitude higher abundances of Ir, Os, and Ru. Moreover, the impactites show near‐chondritic interelement ratios of HSE. We interpret our results in terms of an addition of up to 0.03% of a chondritc component to most impact glasses and impact melt rocks. The magnitude of the admixture is significantly lower than the earlier reported 12–20 wt% of extraterrestrial component for Lonar impact spherules, reflecting the typical difference in the distribution of projectile component between impact glass spherules and bulk impactites.     

Strontium and neodymium isotope systematics of target rocks and impactites from the El'gygytgyn impact structure: Linking impactites and target rocks

The 3.6 Ma El'gygytgyn structure, located in northeastern Russia on the Chukotka Peninsula, is an 18 km diameter complex impact structure. The bedrock is formed by mostly high‐silica volcanic rocks of the ~87 Ma old Okhotsk‐Chukotka Volcanic Belt (OCVB). Volcanic target rocks and impact glasses collected on the surface, as well as drill core samples of bedrock and impact breccias have been investigated by thermal ionization mass spectrometry (TIMS) to obtain new insights into the relationships between these lithologies in terms of Nd and Sr isotope systematics. Major and trace element data for impact glasses are added to compare with the composition of target rocks and drill core samples. Sr isotope data are useful tracers of alteration processes and Nd isotopes reveal characteristics of the magmatic sources of the target rocks, impact breccias, and impact glasses. There are three types of target rocks mapped on the surface: mafic volcanics, dacitic tuff and lava of the Koekvun’ Formation, and dacitic to rhyolitic ignimbrite of the Pykarvaam Formation. The latter represents the main contributor to the impact rocks. The drill core is divided into a suevite and a bedrock section by the Sr isotope data, for which different postimpact alteration regimes have been detected. Impact glasses from the present‐day surface did not suffer postimpact hydrothermal alteration and their data indicate a coherent alteration trend in terms of Sr isotopes with the target rocks from the surface. Surprisingly, the target rocks do not show isotopic coherence with the Central Chukotka segment of the OCVB or with the Berlozhya magmatic assemblage (BMA), a late Jurassic felsic volcanic suite that crops out in the eastern part of the central Chukotka segment of the OCVB. However, concordance for these rocks exists with the Okhotsk segment of the OCVB. This finding argues for variable source magmas having contributed to the build‐up of the OCVB.     

Evidence from the Rb‐Sr system for 4.4 Ga alteration of chondrules in the Allende (CV3) parent body

Abstract— The timing and processes of alteration in the CV parent body are investigated by the analysis of Sr isotopes, major and trace elements, and petrographic type and distribution of the secondary minerals (nepheline and sodalite) in 22 chondrules from the Allende (CV3) chondrite. The Sr isotopic compositions of the chondrules are scattered around the 4.0 Ga reference line on the ⁸⁷Sr/⁸⁶Sr evolution diagram, indicating that the chondrules have been affected by late thermal alteration event(s) in the parent body. The degree of alteration, determined for individual chondrules based on the distribution of nepheline and sodalite, is unrelated to the disturbance of the Rb‐Sr system, suggesting that the alteration process that produced nepheline and sodalite is different from the thermal process that disturbed the Rb‐Sr system of the chondrules. Considering the geochemical behavior of Rb and Sr, the main host phase of Sr in chondrules is likely to be mesostasis, which could be most susceptible to late thermal alteration. As there is a poor connection between the alteration degree determined from abundances of nepheline and sodalite and the disturbance of Rb‐Sr isotopic system, we consider the mesostasis to provide a constraint on the late parent body alteration process. From this point of view, 23 mesostasis‐rich chondrules, including those from literature data, were selected. The selected chondrules are closely correlated on the ⁸⁷Sr/⁸⁶Sr evolution diagram, with an inferred age of 4.36 ± 0.08 Ga. This correlation would represent an age of the final major Sr isotopic redistribution of the chondrules in the parent body.     

Rb‐Sr and Sm‐Nd isotopic systematics of the lherzolitic shergottite GRV 99027

Abstract– Rb‐Sr and Sm‐Nd isotopic analyses of the lherzolitic shergottite Grove Mountains (GRV) 99027 are reported. GRV 99027 yields a Rb‐Sr mineral isochron age of 177 ± 5 (2σ) Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio (I_Sr) of 0.710364 ± 11 (2σ). Due to larger uncertainties of the Sm‐Nd isotopic data, no Sm‐Nd isochron age was obtained for GRV 99027. The ε¹⁴³Nd value is estimated approximately +12.2, assuming an age of 177 Ma. The I_Sr of GRV 99027 is distinguishable from other lherzolitic shergottites, confirming our previous conclusion that it is not paired with them ( Lin et al. 2005 ). The new data of GRV 99027 support the same age of approximately 180 Ma for most lherzolitic shergottites, and fill the small gap of I_Sr between Allan Hills A77005 and Lewis Cliff 88516 ( Borg et al. 2002 ). All available data are consistent with a single igneous source for the intermediate subgroup of lherzolitic shergottites.     

Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr‐Nd isotopic evidence

Abstract— The Chesapeake Bay impact structure, which is about 35 Ma old, has previously been proposed as the possible source crater of the North American tektites (NAT). Here we report major and trace element data as well as the first Sr‐Nd isotope data for drill core and outcrop samples of target lithologies, crater fill breccias, and post‐impact sediments of the Chesapeake Bay impact structure. The unconsolidated sediments, Cretaceous to middle Eocene in age, have ?_Sr^{t = 35.7 Ma} of +54 to +272, and ?_Nd^{t = 35.7 Ma} ranging from ?6.5 to ?10.8; one sample from the granitic basement with a T^Nd_CHUR model age of 1.36 Ga yielded an ?_Sr^{t = 35.7 Ma} of +188 and an ?_Nd^{t = 35.7 Ma} of ?5.7. The Exmore breccia (crater fill) can be explained as a mix of the measured target sediments and the granite, plus an as‐yet undetermined component. The post‐impact sediments of the Chickahominy formation have slightly higher T^Nd_CHUR model ages of about 1.55 Ga, indicating a contribution of some older materials. Newly analyzed bediasites have the following isotope parameters: +104 to +119 (?_Sr^{t = 35.7 Ma}), ?5.7 (?_Nd^{t = 35.7 Ma}), 0.47 Ga (T^Sr_UR), and 1.15 Ga (T^Nd_CHUR), which is in excellent agreement with previously published data for samples of the NAT strewn field. Target rocks with highly radiogenic Sr isotopic composition, as required for explaining the isotopic characteristics of Deep Sea Drilling Project (DSDP) site 612 tektites, were not among the analyzed sample suite. Based on the new isotope data, we exclude any relation between the NA tektites and the Popigai impact crater, although they have identical ages within 2s? errors. The Chesapeake Bay structure, however, is now clearly constrained as the source crater for the North American tektites, although the present data set obviously does not include all target lithologies that have contributed to the composition of the tektites.     

87Rb-87Sr DATING OF L CHONDRITES: EFFECTS OF SHOCK AND BRECCIATION

We have analysed whole rock L chondrites and separate minerals from the L3 chondrite Mezö Madaras for ⁸⁷Rb-⁸⁷Sr dating. Contrary to other groups of chondrites, whole rocks do not define a straight line in the (⁸⁷Rb/⁸⁶Sr, ⁸⁷Sr/⁸⁶Sr) diagram. Whereas unshocked meteorites of old K-Ar age plot on the isochron defined by the other chondrites, shocked objects, with younger K-Ar ages, plot above it. Exceptions to this correlation are Bjürbole for the first type and Homestead and Peace River for the second type. Our preferred interpretation is that this displacement in the (⁸⁷Rb/⁸⁶Sr, ⁸⁷Sr/⁸⁶Sr) diagram corresponds to Rb volatilization induced by reheating. Hand-picked and heavy liquid separates from Mezö Madaras scatter in the same diagram. In addition to possible Rb loss from feldspar, evidences are found for migration of Rb and Sr between olivine and feldspar, glass or clinopyroxene. We interpret this as the consequence of a short though important heating at the time of brecciation.     

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.     

Geochemistry of Darwin glass and target rocks from Darwin crater,Tasmania, Australia

Abstract— Darwin glass formed about 800,000 years ago in western Tasmania, Australia. Target rocks at Darwin crater are quartzites and slates (Siluro‐Devonian, Eldon Group). Analyses show 2 groups of glass, Average group 1 is composed of: SiO₂ (85%), Al₂O₃ (7.3%), TiO₂ (0.05%), FeO (2.2%), MgO (0.9%), and K₂O (1.8%). Group 2 has lower average SiO₂ (81.1%) and higher average Al₂O₃ (8.2%). Group 2 is enriched in FeO (+1.5%), MgO (+1.3%) and Ni, Co, and Cr. Average Ni (416 ppm), Co (31 ppm), and Cr (162 ppm) in group 2 are beyond the range of sedimentary rocks. Glass and target rocks have concordant REE patterns (La/Lu = 5.9–10; Eu/Eu* = 0.55–0.65) and overlapping trace element abundances. 87Sr/⁸⁶Sr ratios for the glasses (0.80778–0.81605) fall in the range (0.76481–1.1212) defined by the rock samples. ε‐Nd results range from –13.57 to –15.86. Nd model ages range from 1.2–1.9 Ga (CHUR) and the glasses (1.2–1.5 Ga) fall within the range defined by the target samples. The ⁸⁷Sr/⁸⁶Sr versus 8⁷Rb/⁸6Sr regression age (411 ± 42 Ma) and initial ratio (0.725 ± 0.016), and the initial ⁴³Nd/144Nd ratio (0.51153 ± 000011) and regression age (451 ± 140 Ma) indicate that the glasses have an inherited isotopic signal from the target rocks at Darwin crater. Mixing models using target rock compositions successfully model the glass for all elementsexcept FeO, MgO, Ni, Co, and Cr in group 2. Mixing models using terrestrial ultramafic rocks fail to match the glass compositions and these enrichments may be related to the projectile.     

Petrography,geochemistry, and argon‐40/argon‐39 ages of impact‐melt rocks and breccias from the Ames impact structure,Oklahoma: The Nicor Chestnut 18‐4 drill core

Abstract— The 15 km diameter Ames structure in northwestern Oklahoma is located 2.75 km below surface in Cambro‐Ordovician Arbuckle dolomite, which is overlain by Middle Ordovician Oil Creek Formation shale. The feature is marked by two concentric ring structures, with the inner ring of about 5 km diameter probably representing the collapsed remnant of a structural uplift composed of brecciated Precambrian granite and Arbuckle dolomite. Wells from both the crater rim and the central uplift are oil‐ and gas‐producing, making Ames one of the economically important impact structures. Petrographic, geochemical, and age data were obtained on samples from the Nicor Chestnut 18‐4 drill core, off the northwest flank of the central uplift. These samples represent the largest and best examples of impact‐melt breccia obtained so far from the Ames structure. They contain carbonate rocks, which are derived from the target sequence. The chemical composition of the impact‐melt breccias is similar to that of target granite, with variable carbonate admixture. Some impact‐melt rocks are enriched in siderophile elements indicating the possible presence of a meteoritic component. Based on stratigraphic arguments, the age of the crater was estimated at 470 Ma. Previous ⁴⁰Ar‐³⁹Ar dating attempts of impact‐melt breccias from the Dorothy 1–19 core yielded plateau ages of about 285 Ma, which is in conflict with the stratigraphic age. The new ⁴⁰Ar‐³⁹Ar age data obtained on the melt breccias from the Nicor Chestnut core by ultraviolet (UV) laser spot analysis resulted in a range of ages with maxima around 300 Ma. These data could reflect processes related either the regional Nemaha Uplift or resetting due to hot brines active on a midcontinent‐wide scale, perhaps related to the Alleghenian and Ouachita orogenies. The age data indicate an extended burial phase associated with thermal overprint during Late Pennsylvanian‐Permian.     

Disturbance of isotope systematics during experimental shock and thermal metamorphism of a lunar basalt with implications for Martian meteorite chronology

Abstract– To better determine the effects of impact‐related processes on radiometric chronometers in meteorites, we undertook an isotopic study of experimentally shocked and heated samples of lunar basalt 10017. Shock experiments at 55 GPa were completed on one subsample, and a second subsample was heated in an evacuated quartz tube at 1000 °C for 170 h. A third subsample was maintained as a control. Samarium‐neodymium, Rb‐Sr, ²³⁸U‐²⁰⁶Pb, and ²⁰⁶Pb‐²⁰⁷Pb isotopic analyses were completed on mineral fractions (leached and unleached), leached whole rocks, and complementary acid leachates. Disturbance in the shocked and heated samples was evaluated through comparison of their isochron diagrams with those of the control sample. The Sm‐Nd isotope system was the least disturbed, the Rb‐Sr isotope system was more disturbed, and the ²³⁸U‐²⁰⁶Pb and ²⁰⁶Pb‐²⁰⁷Pb isotope systems were the most disturbed by shock and annealing. Samples that experienced extended heating demonstrated greater isotopic disturbances than shocked samples. In some cases, the true crystallization age was preserved, and in others, age information was degraded or destroyed. In no case did the experiments generate isochrons that maintained linearity while being rotated or completely reset. Although our results show that neither experimental shock nor thermal metamorphism alone can account for the discordant ages represented by different isotope systems in some Martian meteorites, we postulate that shock metamorphism may render a meteorite more susceptible than its unshocked counterpart to subsequent disturbance during extended impact‐related heating or aqueous alteration. The combination of these processes may result in the disparate chronometric information preserved in some meteorites.     

Neodymium,strontium and chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the chronology of the angrite parent body

Abstract— Neodymium, strontium, and chromium isotopic studies of the LEW86010 angrite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar nebula. Pyroxene and phosphate were found to contain ～98% of its Sm and Nd inventory. A conventional ¹⁴⁷Sm-¹⁴³Nd isochron yielded an age of 4.53 ± 0.04 Ga (2 σ) and ?¹⁴³ _Nd = 0.45 ± 1.1. An ¹⁴⁶Sm-¹⁴²Nd isochron gives initial ¹⁴⁶Sm/¹⁴⁴Sm = 0.0076 ± 0.0009 and ?¹⁴³ _Nd = ?2.5 ± 0.4. The Rb-Sr analyses give initial ⁸⁷Sr/⁸⁶Sr (I⁸⁷_Sr) = 0.698972 ± 8 and 0.698970 ± 18 for LEW and ADOR, respectively, relative to ⁸⁷Sr/⁸⁶Sr = 0.71025 for NBS987. The difference, ΔI⁸⁷_Sr, between I⁸⁷_Sr for the angrites and literature values for Allende CAIs, corresponds to ～9 Ma of growth in a solar nebula with a CI chondrite value of ⁸⁷Rb/⁸⁶Sr = 0.91, or ～5 Ma in a nebula with solar photospheric ⁸⁷Rb/⁸⁶Sr = 1.51. Excess ⁵³Cr from extinct ⁵³Mn (t1/2 = 3.7 Ma) in LEW86010 corresponds to initial ⁵³Mn/⁵⁵Mn = 1.44 ± 0.07 × 10^?6 and closure to Cr isotopic homogenization 18.2 ± 1.7 Ma after formation of Allende inclusions, assuming initial ⁵³Mn/⁵⁵Mn = 4.4 ± 1.0 × 10^?5 for the inclusions as previously reported by the Paris group (Birck and Allegre, 1988). The ¹⁴⁶Sm/¹⁴⁴Sm value found for LEW86010 corresponds to solar system initial (¹⁴⁶Sm/¹⁴⁴Sm)_o = 0.0080 ± 0.0009 for crystallization 8 Ma after Allende, the difference between Pb-Pb ages of angrites and Allende, or 0.0086 ± 0.0009 for crystallization 18 Ma after Allende, using the Mn-Cr formation interval. The isotopic data are discussed in the context of a model in which an undifferentiated “chondritic” parent body formed from the solar nebula ～2 Ma after Allende CAIs and subsequently underwent differentiation accompanied by loss of volatiles. Parent bodies with Rb/Sr similar to that of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic systematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, ～2.6 Ma after the CAIs, to satisfy the Sr and Cr isotopic systematics.     

Nd and Sr isotopic evidence for the origin of tektite material from DSDP site 612 off the New Jersey coast

Abstract— Late Eocene tektite material from DSDP site 612 is composed of angular to spherical tektites and microtektites containing abundant vesicles and a few unmelted to partially melted mineral inclusions. The major element compositions of the 612-tektites are generally comparable to those of North American tektites, but the physical features suggest that the DSDP-612 tektites were formed by less severe shock melting. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd compositions of 612-tektites: a) show much wider ranges than the tightly constrained group of North American tektites and microtektites, and b) are significantly different from those of other groups of tektites. The existence of large isotopic variations in tektites from DSDP site 612 requires that they were formed from a chemically and isotopically heterogeneous material in a regime that is distinctive from that of other groups of tektites. T^ND_CHUR and T^Sr_UR model ages of the 612-tektites indicate that they were formed from a crustal source of late Precambrian mean age (800–1000 Ma) which in middle Palaeozoic time (?400 Ma) was further enriched in Rb/Sr during sedimentary processes. These source characteristics suggest that the impact which produced the 612-tektites occurred in rocks of the Appalachian orogeny or sediments derived from this orogenic belt. Potential source materials for both 612-tektites and North American tektites are present on the eastern and southeastern part of the North American continent and its adjacent shelf. The distinct isotopic differences between 612-tektites and North American tektites indicate that the two groups of tektites were either formed by the impact of more than one bolide in the same general area, or by a single impact event that sampled different layers.     

40Ar‐39Ar age determinations of lunar basalt meteorites Asuka 881757, Yamato 793169, Miller Range 05035, La Paz Icefield 02205, Northwest Africa 479, and basaltic breccia Elephant Moraine 96008

Abstract— ⁴⁰Ar‐³⁹Ar data are presented for the unbrecciated lunar basaltic meteorites Asuka (A‐) 881757, Yamato (Y‐) 793169, Miller Range (MIL) 05035, LaPaz Icefield (LAP) 02205, Northwest Africa (NWA) 479 (paired with NWA 032), and basaltic fragmental breccia Elephant Moraine (EET) 96008. Stepped heating ⁴⁰Ar‐³⁹Ar analyses of several bulk fragments of related meteorites A‐881757, Y‐793169 and MIL 05035 give crystallization ages of 3.763 ± 0.046 Ga, 3.811 ± 0.098 Ga and 3.845 ± 0.014 Ga, which are comparable with previous age determinations by Sm‐Nd, U‐Pb Th‐Pb, Pb‐Pb, and Rb‐Sr methods. These three meteorites differ in the degree of secondary ⁴⁰Ar loss with Y‐793169 showing relatively high Ar loss probably during an impact event ?200 Ma ago, lower Ar loss in MIL 05035 and no loss in A‐881757. Bulk and impact melt glass‐bearing samples of LAP 02205 gave similar ages (2.985 ± 0.016 Ga and 2.874 ± 0.056 Ga) and are consistent with ages previously determined using other isotope pairs. The basaltic portion of EET 96008 gives an age of 2.650 ± 0.086 Ga which is considered to be the crystallization age of the basalt in this meteorite. The Ar release for fragmental basaltic breccia EET 96008 shows evidence of an impact event at 631 ± 20 Ma. The crystallization age of 2.721 ± 0.040 Ga determined for NWA 479 is indistinguishable from the weighted mean age obtained from three samples of NWA 032 supporting the proposal that these meteorites are paired. The similarity of ⁴⁰Ar‐³⁹Ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the K‐Ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime.     

Ries crater and suevite revisited—Observations and modeling Part II: Modeling

We present the results of numerical modeling of the formation of the Ries crater utilizing the two hydrocodes SOVA and iSALE. These standard models allow us to reproduce crater shape, size, and morphology, and composition and extension of the continuous ejecta blanket. Some of these results cannot, however, be readily reconciled with observations: the impact plume above the crater consists mainly of molten and vaporized sedimentary rocks, containing very little material in comparison with the ejecta curtain; at the end of the modification stage, the crater floor is covered by a thick layer of impact melt with a total volume of 6–11 km³; the thickness of true fallback material from the plume inside the crater does not exceed a couple of meters; ejecta from all stratigraphic units of the target are transported ballistically; no separation of sedimentary and crystalline rocks—as observed between suevites and Bunte Breccia at Ries—is noted. We also present numerical results quantifying the existing geological hypotheses of Ries ejecta emplacement from an impact plume, by melt flow, or by a pyroclastic density current. The results show that none of these mechanisms is consistent with physical constraints and/or observations. Finally, we suggest a new hypothesis of suevite formation and emplacement by postimpact interaction of hot impact melt with water or volatile‐rich sedimentary rocks.