^1,2Jörg Fritz,³Roald Tagle,⁴Luisa Ashworth,²Ralf Thomas Schmitt,⁵Axel Hofmann,⁶Béatrice Luais,⁴Phillip D. Harris,^2,7Desirée Hoehnel,⁸Seda Özdemir,^2,9Tanja Mohr-Westheide,^9,10Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12736]
¹Saalbau Weltraum Projekt, Heppenheim, Germany
²Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
³Bruker-Nano GmbH, Berlin, Germany
⁴GeoSpectral Imaging, Johannesburg, South Africa
⁵Department of Geology, University of Johannesburg, Johannesburg, South Africa
⁶Centre de Recherches Pétrographiques et Géochimiques, CRPG UMR 7358 CNRS-UL, France
⁷Institut für Erd und Umweltwissenschaften, Universität Potsdam, Potsdam-Golm, Germany
⁸Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁹Institut für Geologische Wissenschaften, Freie Universität Berlin (FU Berlin), Berlin, Germany
¹⁰Natural History Museum, Vienna, Austria
Published by arrangement with John Wiley & Sons

A Paleoarchean impact spherule-bearing interval of the 763 m long International Continental Scientific Drilling Program (ICDP) drill core BARB5 from the lower Mapepe Formation of the Fig Tree Group, Barberton Mountain Land (South Africa) was investigated using nondestructive analytical techniques. The results of visual observation, infrared (IR) spectroscopic imaging, and micro-X-ray fluorescence (μXRF) of drill cores are presented. Petrographic and sedimentary features, as well as major and trace element compositions of lithologies from the micrometer to kilometer-scale, assisted in the localization and characterization of eight spherule-bearing intervals between 512.6 and 510.5 m depth. The spherule layers occur in a strongly deformed section between 517 and 503 m, and the rocks in the core above and below are clearly less disturbed. The μXRF element maps show that spherule layers have similar petrographic and geochemical characteristics but differences in (1) sorting of two types of spherules and (2) occurrence of primary minerals (Ni-Cr spinel and zircon). We favor a single impact scenario followed by postimpact reworking, and subsequent alteration. The spherule layers are Al2O3-rich and can be distinguished from the Al2O3-poor marine sediments by distinct Al-OH absorption features in the short wave infrared (SWIR) region of the electromagnetic spectrum. Infrared images can cover tens to hundreds of square meters of lithologies and, thus, may be used to search for Al-OH-rich spherule layers in Al2O3-poor sediments, such as Eoarchean metasediments, where the textural characteristics of the spherule layers are obscured by metamorphism.

^1,2Gregory A. Brennecka, ²Lars E. Borg, ³Stephen J. Romaniello, ^3,4Amanda K. Souders, ^1,2Quinn R. Shollenberger, ²Naomi E. Marks, ³Meenakshi Wadhwa
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.003]
¹Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany
²Nuclear & Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore, CA 94550 USA
³School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287-1404 USA
⁴Department of Geosciences, Texas Tech University, Lubbock, TX 79409, USA
Copyright Elsevier

Although there is limited direct evidence for supernova input into the nascent Solar System, many models suggest it formed by the gravitational collapse of a molecular cloud that was triggered by a nearby supernova. Existing lines of evidence, mostly in the form of short-lived radionuclides present in the early Solar System, are potentially consistent with this hypothesis, but still allow for alternative explanations. Since the natural production of ¹²⁶Sn is thought to occur only in supernovae and this isotope has a short half-life (¹²⁶Sn→¹²⁶Te, t_1/2=235 ky), the discovery of extant ¹²⁶Sn would provide unequivocal proof of supernova input to the early Solar System. Previous attempts to quantify the initial abundance of ¹²⁶Sn by examining Sn-Te systematics in early solids have been hampered by difficulties in precisely measuring Te isotope ratios in these materials. Thus, here we describe a novel technique that uses hydride generation to dramatically increase the ionization efficiency of Te—an approximately 30-fold increase over previous work. This introduction system, when coupled to a MC-ICPMS, enables high-precision Te isotopic analyses on samples with 126Sn. This sample set shows no evidence of live ¹²⁶Sn, implying at most minor input of supernova material during the time at which the CAIs formed. However, based on the petrology of this sample set combined with the higher than expected concentrations of Sn and Te, as well as the lack of nucleosynthetic anomalies in other isotopes of Te suggest that the bulk of the Sn and Te recovered from these particular refractory inclusions is not of primary origin and thus does not represent a primary signature of Sn-Te systematics of the protosolar nebula during condensation of CAIs or their precursors. Although no evidence of supernova input was found based on Sn-Te systematics in this sample set, hydride generation represents a powerful tool that can now be used to further explore Te isotope systematics in less altered materials.

^1,2Changkun Park, ¹Kazuhide Nagashima, ¹Alexander N. Krot, ¹Gary R. Huss, ³Andrew M. Davis, ⁴Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.002]
¹Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, South Korea
³Department of the Geophysical Sciences, Enrico Fermi Institute, and Chicago Center for Cosmochemistry, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637-1433, USA
⁴Centre for Star and Planet Formation, Geological Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark
Copyright Elsevier

Calcium-aluminum-rich inclusions with isotopic mass fractionation effects and unidentified nuclear isotopic anomalies (FUN CAIs) have been studied for more than 40 years, but their origins remain enigmatic. Here we report in situ high precision measurements of aluminum-magnesium isotope systematics of FUN CAIs by secondary ion mass spectrometry (SIMS). Individual minerals were analyzed in six FUN CAIs from the oxidized CV3 carbonaceous chondrites Axtell (compact Type A CAI Axtell 2271) and Allende (Type B CAIs C1 and EK1-4-1, and forsterite-bearing Type B CAIs BG82DH8, CG-14, and TE). Most of these CAIs show evidence for excess 26Mg due to the decay of 26Al. The inferred initial 26Al/27Al ratios [(26Al/27Al)0] and the initial magnesium isotopic compositions (δ26Mg0) calculated using an exponential law with an exponent β of 0.5128 are (3.1±1.6)×10−6 and 0.60±0.10‰ (Axtell 2271), (3.7±1.5)×10−6 and −0.20±0.05‰ (BG82DH8), (2.2±1.1)×10−6 and −0.18±0.05‰ (C1), (2.3±2.4)×10−5 and −2.23±0.37‰ (EK1-4-1), (1.5±1.1)×10−5 and −0.42±0.18‰ (CG-14), and (5.3±0.9)×10−5 and −0.05±0.08‰ (TE) with 2σ uncertainties. We infer that FUN CAIs recorded 26Al and magnesium isotopic heterogeneity in the CAI-forming region(s). Comparison of 26Al-26Mg systematics, stable isotope (oxygen, magnesium, calcium, and titanium) and trace element studies of FUN and non-FUN igneous CAIs indicates that there is a continuum among these CAI types. Based on these observations and evaporation experiments on CAI-like melts, we propose a generic scenario for the origin of igneous (FUN and non-FUN) CAIs: (i) condensation of isotopically normal solids in an 16O-rich gas of approximately solar composition; (ii) formation of CAI precursors by aggregation of these solids together with variable abundances of isotopically anomalous grains—possible carriers of unidentified nuclear (UN) effects; and (iii) melt evaporation of these precursors accompanied by crystallization under different temperatures and gas pressures, leading to the observed variations in mass-dependent isotopic fractionation (F) effects.

^1,2Zuzana Rodovská,¹Tomáš Magna,³Karel Žák,³Roman Skála,⁴Tomasz Brachaniec,^5,6Channon Visscher
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12733]
¹Czech Geological Survey, Prague 1, Czech Republic
²Faculty of Science, Charles University in Prague, Prague 2, Czech Republic
³Institute of Geology, The Czech Academy of Sciences, Prague 6, Czech Republic
⁴Department of Geochemistry, Mineralogy, and Petrology, Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland
⁵Chemistry and Planetary Sciences, Dordt College, Sioux Center, Iowa, USA
⁶Space Science Institute, Boulder, Colorado, USA
Published by arrangement with John Wiley & Sons

Lithium abundances and isotope compositions are presented for a suite of sediments from the surroundings of the Ries Impact structure, paralleled by new Li data for central European tektites (moldavites) from several substrewn fields (South Bohemia, Moravia, Cheb Basin, Lusatia), including a specimen from the newly discovered substrewn field in Poland. The data set was supplemented by three clay fractions isolated from sedimentary samples. Moldavites measured in this study show a very narrow range in δ7Li values (−0.6 to 0.3‰ relative to L-SVEC) and Li contents (23.9–48.1 ppm). This contrasts with sediments from the Ries area which show remarkable range in Li isotope compositions (from −6.9 to 13.4‰) and Li contents (0.6–256 ppm). The OSM sediments which, based on chemical similarity, formed the major part of moldavites, show a range in δ7Li values from −2.0 to 7.9‰ and Li contents from 5.8 to 78.9 ppm. Therefore, the formation of moldavites was apparently accompanied by large-scale mixing, paralleled by chemical and isotope homogenization of their parent matter. The proposed Li mixing model indicates that sands, clayey sediments, and low volumes of carbonates are the major components for tektite formation whereas residual paleokarst sediments could have been a minor but important component for a subset of moldavites. Striking homogenization of Li in tektites, combined with limited Li loss during impacts, may suggest that moderately volatile elements are not scavenged and isotopically fractionated during large-scale collisions, which is consistent with recent models. In general, whether homogenization of bodies with distinct Li isotope systematics takes place, or collision of bodies with similar Li systematics operates cannot be resolved at present stage but Li isotope homogeneity of solar system planets and asteroidal bodies tentatively implies the latter.

¹Gerrit Budde, ¹Christoph Burkhardt, ¹Gregory A. Brennecka, ¹Mario Fischer-Gödde, ¹Thomas S. Kruijer, ¹Thorsten Kleine
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.09.020]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
Copyright Elsevier

Nucleosynthetic isotope anomalies are powerful tracers to determine the provenance of meteorites and their components, and to identify genetic links between these materials. Here we show that chondrules and matrix separated from the Allende CV3 chondrite have complementary nucleosynthetic Mo isotope anomalies. These anomalies result from the enrichment of a presolar carrier enriched in s-process Mo into the matrix, and the corresponding depletion of this carrier in the chondrules. This carrier most likely is a metal and so the uneven distribution of presolar material probably results from metal–silicate fractionation during chondrule formation. The Mo isotope anomalies correlate with those reported for W isotopes on the same samples in an earlier study, suggesting that the isotope variations for both Mo and W are caused by the heterogeneous distribution of the same carrier. The isotopic complementary of chondrules and matrix indicates that both components are genetically linked and formed together from one common reservoir of solar nebula dust. As such, the isotopic data require that most chondrules formed in the solar nebula and are not a product of protoplanetary impacts.

Allende chondrules and matrix together with bulk carbonaceous chondrites and some iron meteorites (groups IID, IIIF, and IVB) show uniform excesses in 92Mo, 95Mo, and 97Mo that result from the addition of supernova material to the solar nebula region in which these carbonaceous meteorites formed. Non-carbonaceous meteorites (enstatite and ordinary chondrites as well as most iron meteorites) do not contain this material, demonstrating that two distinct Mo isotope reservoirs co-existed in the early solar nebula that remained spatially separated for several million years. This separation was most likely achieved through the formation of the gas giants, which cleared the disk between the inner and outer solar system regions parental to the non-carbonaceous and carbonaceous meteorites. The Mo isotope dichotomy of meteorites provides a new means to determine the provenance of meteoritic and planetary materials, and to assess genetic links between chondrites and differentiated meteorites.

¹R. Brasser, ^2,3S.J. Mojzsis, ⁴S.C. Werner, ⁵S. Matsumura, ¹S. Ida
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.09.013]
¹Earth Life Science Institute, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan
²Department of Geological Sciences, University of Colorado, UCB 399, 2200 Colorado Avenue, Boulder, CO 80309-0399, USA
³Institute for Geological and Geochemical Research, Research Center for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 45 Budaörsi Street, H-1112 Budapest, Hungary
⁴The Centre for Earth Evolution and Dynamics, University of Oslo, Sem Saelandsvei 24, 0371 Oslo, Norway
⁵School of Science and Engineering, Division of Physics, Fulton Building, University of Dundee, Dundee DD1 4HN, UK
Copyright Elsevier

It is generally accepted that silicate-metal (‘rocky’) planet formation relies on coagulation from a mixture of sub-Mars sized planetary embryos and (smaller) planetesimals that dynamically emerge from the evolving circum-solar disc in the first few million years of our Solar System. Once the planets have, for the most part, assembled after a giant impact phase, they continue to be bombarded by a multitude of planetesimals left over from accretion. Here we place limits on the mass and evolution of these planetesimals based on constraints from the highly siderophile element (HSE) budget of the Moon. Outcomes from a combination of N-body and Monte Carlo simulations of planet formation lead us to four key conclusions about the nature of this early epoch. First, matching the terrestrial to lunar HSE ratio requires either that the late veneer on Earth consisted of a single lunar-size impactor striking the Earth before 4.45 Ga, or that it originated from the impact that created the Moon. An added complication is that analysis of lunar samples indicates the Moon does not preserve convincing evidence for a late veneer like Earth. Second, the expected chondritic veneer component on Mars is 0.06 weight percent. Third, the flux of terrestrial impactors must have been low (≲10−6 M⊕ Myr−1≲10−6 M⊕ Myr−1) to avoid wholesale melting of Earth’s crust after 4.4 Ga, and to simultaneously match the number of observed lunar basins. This conclusion leads to an Hadean eon which is more clement than assumed previously. Last, after the terrestrial planets had fully formed, the mass in remnant planetesimals was ∼10−3 M⊕∼10−3 M⊕, lower by at least an order of magnitude than most previous models suggest. Our dynamically and geochemically self-consistent scenario requires that future N-body simulations of rocky planet formation either directly incorporate collisional grinding or rely on pebble accretion.

^1,2Jérôme Gattacceca, ²Benjamin P. Weiss, ^3,4Matthieu Gounelle
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.09.008]
¹CNRS, Aix Marseille Univ, IRD, Coll France, CEREGE, Aix-en-Provence, France
²Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³Institut de Mineralogie de Physique des Materiaux et de Cosmochimie, CNRS & Museum National d’Histoire Naturelle, UMR 7202, 75005 Paris, France
⁴Institut Universitaire de France, 103 Boulevard Saint Michel, 75005 Paris, France
Copyright Elsevier

Recent paleomagnetic studies of Allende CV chondrite as well as thermal modeling suggest the existence of partially differentiated asteroids with outer unmelted and variably metamorphosed crusts overlying differentiated interiors. To further constrain the magnetic history of the CV parent body, we report here paleomagnetic results on Kaba CV chondrite. This meteorite contains 11 wt% pseudo-single domain magnetite, making it a rock with an excellent paleomagnetic recording capacity. Kaba appears to carry a stable natural remanent magnetization acquired on its parent body upon cooling in an internally generated magnetic field of about 3 μT3 μT from temperatures below 150 °C during thermal metamorphism about 10 to several tens of Myr after solar system formation. This strengthens the case for the existence of a molten advecting core in the CV parent body. Furthermore, we show that no significant magnetic field (i.e. lower than ∼0.3 μT∼0.3 μT) was present when aqueous alteration took place on the Kaba parent body around 4 to 6 Myr after solar system formation, suggesting a delay in the onset of the dynamo in the CV parent body and confirming that nebular fields had already decayed at that time.

¹Shichun Huang, ²Stein B. Jacobsen
Geochimica et Cosmochmica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.09.039]
¹Department of Geoscience, University of Nevada, Las Vegas
²Department of Earth and Planetary Sciences, Harvard University
Copyright Elsevier

We report mass-dependent and mass-independent Ca isotopic variations in nine chondrites from three groups: carbonaceous, ordinary and enstatite chondrites. There is about 0.25‰ per amu, i.e., ∼1‰ in 44Ca/40Ca, variation in chondrites: carbonaceous chondrites have the lightest Ca isotopes, enstatite chondrites have modeled bulk Earth like Ca isotopes, and ordinary chondrites are in between. The correlations between mass-dependent Ca isotopic variation and chemical variations in chondrites may reflect variable contributions from different endmembers, including refractory inclusions, in different chondrite groups. In detail, enstatite chondrites and the Earth share similar isotopic characteristics, but are very different in chemical compositions.

At the ±1 and ±2 ε-unit levels, respectively, there is no measurable 40Ca or 43Ca anomaly in bulk chondrites. Carbonaceous chondrites show several ε-units of 48Ca excess. That is, Ca exhibits both mass-dependent and mass-independent isotopic variations in chondrites, similar to O isotopes. The 48Ca anomaly in bulk chondrites is positively correlated with 50Ti anomaly, but does not form simple correlation with 54Cr anomaly, implying multiple supernova sources for these neutron-rich isotopes in the Solar System. Finally, all meteorites with negative Δ17O have either 48Ca deficits (differentiated meteorites) or 48Ca excess (carbonaceous chondrites), implying that the Sun with a very negative Δ17O is probably also characterized by 48Ca anomaly compared to the Earth. CAIs cannot be taken as representative of the initial isotopic compositions of refractory elements like Ca for the Earth-Moon system.

¹Glenn J. MacPherson, ²Kazuhide Nagashima, ²Alexander N. Krot, ³Patricia M. Doyle, ¹Marina A. Ivanova
Geochmica et Cosmochmica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.09.032]
¹US National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560, USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
³Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, RS91
Copyright Elsevier

High precision secondary ion mass-spectrometry (SIMS) analyses of kirschsteinite (CaFeSiO4) in the reduced CV3 chondrites Vigarano and Efremovka yield well resolved 53Cr excesses that correlate with 55Mn/52Cr, demonstrating in situ decay of the extinct short-lived radionuclide 53Mn. To ensure proper correction for relative sensitivities between 55Mn+ and 52Cr+ ions, we synthesized kirschsteinite doped with Mn and Cr to measure the relative sensitivity factor. The inferred initial ratio (53Mn/55Mn)0 in chondritic kirschsteinite is (3.71±0.50)×10–6. When anchored to 53Mn-53Cr relative and U-corrected 207Pb-206Pb absolute ages of the D’Orbigny angrite, this ratio corresponds to kirschsteinite formation View the MathML source3.2-0.7+08 Ma after CV Ca-, Al-rich inclusions. The kirschsteinite data are consistent within error with the data for aqueously-formed fayalite from the Asuka 881317 CV3 chondrite as reported by Doyle et al. (2015), supporting the idea that Ca-Fe silicates in CV3 chondrites are cogenetic with fayalite (and magnetite) and formed during metasomatic alteration on the CV3 parent body. Concentrically-zoned crystals of kirschsteinite and hedenbergite indicate that they initially formed as near end-member compositions that became more Mg-rich with time, possibly as a result of an increase in temperature.

¹J. Labidi, ²J. Farquhar, ³C.M.O’D. Alexander, ²D.L. Eldridge, ⁴H. Oduro
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.09.036]
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, USA
²Department of Geology, University of Maryland, College Park MD, 20740, USA
³Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, USA
⁴Department of Earth and Environmental Sciences, St Andrews University, Fife KY16 9AJ, United Kingdom
Copyright Elsevier

We have investigated the quadruple sulfur isotopic composition of inorganic sulfur-bearing phases from 13 carbonaceous chondrites of CM type. Our samples include 4 falls and 9 Antarctic finds. We extracted sulfur from sulfides, sulfates, and elemental sulfur (S0) from all samples. On average, we recover a bulk sulfur (S) content of 2.11±0.39 wt.% S (1σ). The recovered sulfate, S0 and sulfide contents represent 25±12%, 10±7% and 65±15% of the bulk S, respectively (all 1σ). There is no evidence for differences in the bulk S content between falls and finds, and there is no correlation between the S speciation and the extent of aqueous alteration. We report ranges of Δ33S and Δ36S values in CMs that are significantly larger than previously observed. The largest variations are exhibited by S0, with Δ33S values ranging between -0.104±0.012‰ and +0.256±0.018‰ (2σ). The Δ36S/View the MathML sourceω¯33S ratios of S0 are on average -3.1±1.0 (2σ). Two CMs show distinct Δ36S/View the MathML sourceω¯33S ratios, of +1.3±0.1 and +0.9±0.1. We suggest that these mass independent S isotopic compositions record H2S photodissociation in the nebula. The varying ΔΔ36S/Δ33S ratios are interpreted to reflect photodissociation that occurred at different UV wavelengths. The preservation of these isotopic features requires that the S-bearing phases were heterogeneously accreted to the CM parent body. Non-zero Δ33S values are also preserved in sulfide and sulfate, and are positively correlated with S0 values. This indicates a genetic relationship between the S-bearing phases: We argue that sulfates were produced by the direct oxidation of S0 (not sulfide) in the parent body. We describe two types of models that, although imperfect, can explain the major features of the CM S isotope compositions, and can be tested in future studies. Sulfide and S0 could both be condensates from the nebula, as the residue and product, respectively, of incomplete H2S photodissociation by UV light (wavelength < 150 nm). This idea requires that FeS formation and the S0 condensation co-occur. As an alternative, ice accretion to the CM parent body could allow the delivery of S-MIF in CMs. In that case, sulfides would have been the only S-bearing condensate in CM precursors, and S0 would have been derived from the oxidation of H2S trapped in ices, after its photodissociation at low temperature (< 500 K) in the nebula. In our models, the observations of H2S UV photodissociation is required to occur at the disk surface, and allowed in nebular environments with canonical C/O ratios. Vertical motions in the disk would redistribute phases that condensed at high altitude to the midplane, where they accreted in the phases that make up the chondritic matrix.