¹A.P.Crósta,^2,3W.U.Reimold,⁴M.A.R.Vasconcelos,²N.Hauser,¹G.J.G.Oliveira,¹M.V.Maziviero,⁵A.M.Góes
Chemie der Erde (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2018.09.002]
¹State University of Campinas, Brazil
²University of Brasília, Brazil
³Natural History Museum – Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
⁴Federal University of Bahia, Brazil
⁵University of São Paulo, Brazil
Copyright Elsevier

In the first part of this review of the impact record of South America, we have presented an up-to-date introduction to impact processes and to the criteria to identify/confirm an impact structure and related deposits, as well as a comprehensive examination of Brazilian impact structures. The current paper complements the previous one, by reviewing the impact record of other countries of South America and providing current information on a number of proposed impact structures. Here, we also review those structures that have already been discarded as not being formed by meteorite impact. In addition, current information on impact-related deposits is presented, focusing on impact glasses and tektites known from this continent, as well as on the rare K–Pg boundary occurrences revealed to date and on reports of possible large airbursts. We expect that this article will not only provide systematic and up-to-date information on the subject, but also encourage members of the South American geoscientific community to be aware of the importance of impact cratering and make use of the criteria and tools to identify impact structures and impact deposits, thus potentially contributing to expansion and improvement of the South American impact record.

¹A.P.Crósta,^2,3W.U.Reimold,⁴M.A.R.Vasconcelos,²N.Hauser,¹G.J.G.Oliveira,¹M.V.Maziviero,⁵A.M.Góes
Chemie der Erde (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2018.06.001]
¹State University of Campinas, Brazil
²University of Brasília, Brazil
³Natural History Museum – Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
⁴Federal University of Bahia, Brazil
⁵University of São Paulo, Brazil
Copyright Elsevier

The Earth’s impact record is known to be rather limited in both time and space. There are ca. 190 impact structures currently known on Earth, representing a minor fraction of all the impact events that contributed to the initial formation of our protoplanet, and then to formation and modification of the surface of the planet. Moreover, the distribution of impact structures on Earth is manifestly uneven. One continent that stands out for its relatively small number of confirmed impact structures and impact ejecta occurrences is South America. The limited impact record for this large continent makes a robust case that there is a significant potential for further discoveries. Significant information on the impact record of South America is dispersed in different types of publications (journal articles, books, conferences abstracts, etc.), and in several languages, making it difficult to access and disseminate it among the geoscientific community. We aim to present a summary of the current knowledge of the impact record of this continent, encompassing the existing literature on the subject. It is published in two parts, with the first one covering an up-to-date introduction to impact cratering processes and to the criteria to identify/confirm an impact structure and related deposits. This is followed by a comprehensive analysis of the Brazilian impact structures. The Brazilian impact record accounts for the totality of the large structures of this kind currently confirmed in South America. The second part will examine the impact record of other countries in South America, provide information about a number of proposed impact structures, and review those that already have been discarded as not being formed by impact.

¹E. P. Gurov,² V. V. Permiakov,^2,3 C. Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13241]
¹Institute of Geological Sciences, National Academy of Sciences of Ukraine, , Kiev, Ukraine
²Department of Lithospheric Research, University of Vienna, , A‐1090 Vienna, Austria
³Natural History Museum, , A‐1010 Vienna, Austria
Published by arrangement with John Wiley & Sons

Remnants of paleoflora were discovered in impact melt rocks from the El’gygytgyn crater, Chukotka, Russia. El’gygytgyn is a 3.58 Ma, 18 km diameter impact structure in Chukotka, northeastern Russia. A circular crater basin is surrounded by an uplifted rim. The crater floor is occupied by the El’gygytgyn Lake, 12 km in diameter, surrounded by lacustrine terraces up to 80 m in height. Impactites found at the El’gygytgyn crater include impact melt rocks, glass bombs, and shock metamorphosed volcanic rocks. Most impact melt rocks occur only in redeposited state in the terrace lake deposits. Floral remnants were discovered in impact melt rocks from various locations in the terrace deposits. The floral remnants include fragments of leaves, cell tissue, and undetermined organic matter that occur in vesicles within glassy melt rocks and impact melt breccias. After the discovery of floral remnants in impact melt breccias from upper Miocene strata in Argentina, and the description of floral imprints in the Dakhleh Glass of proposed impact origin in Egypt, the detection of paleoflora remnants in impact melt rocks of the El’gygytgyn structure is the first such occurrence in a confirmed impact crater on Earth.

¹Jens Barosch,^1,2Dominik C.Hezel,^3,4,5Denton S.Ebel,^1,6Pia Friend
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.01.018]
¹University of Cologne, Department of Geology and Mineralogy, Zülpicher Str. 49b, 50674 Köln, Germany
²Natural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD, London, UK
³American Museum of Natural History, Department of Earth and Planetary Sciences, NY 10024, New York, USA
⁴Department of Earth and Environmental Sciences, Columbia University, New York, NY
⁵Graduate School and Graduate Center of the City University of New York
⁶University of Wuppertal, Faculty of Mathematics and Natural Sciences, Gaußstraße 20, 42119 Wuppertal, Germany
Copyright Elsevier

Chondrules are a major component of chondritic meteorites. Understanding their formation conditions provides fundamental insights about how the early solar system formed and evolved. We studied the textures of ∼650 chondrules from all three groups (H, L, LL) of ordinary chondrites, in 2-dimensional (2D) sections through the meteorites. About 40% of the chondrules are mineralogically zoned. They consist of an olivine-rich core, which is surrounded by a low-Ca pyroxene-rich rim. Chondrules sectioned through their low-Ca pyroxene rim do not appear as zoned chondrules, hence, considering such sectioning effects, their true fraction might be as high as ∼50%. Mineralogical zonation is, therefore, a typical chondrule texture in basically all ordinary chondrites, and records a fundamental process during chondrule formation. Chondrules were open systems, and initially olivine-rich chondrules reacted with their surrounding gas to form low-Ca pyroxene rims. Zoned and unzoned chondrules have the same range of bulk compositions, thus ordinary chondrite chondrules were likely affected by two sequential episodes: in the first episode, gaseous SiO was added to all chondrules, thereby forming low-Ca pyroxene rims around all chondrules. In the second episode, only a portion of the chondrules were reheated, thereby remelting and homogenizing their initial pyroxene rims, but retaining their bulk compositions. It is therefore likely that all chondrules in ordinary chondrites were affected by gas-melt interactions during their formation. Open system exchange is consistent with previous studies of chondrule formation and can explain many chondrule textures and bulk chondrule compositional variations in single meteorites. Hence, the open system behaviour recorded in zoned chondrules provides a pivotal constraint on chondrule formation conditions.

^1,2Peter Jenniskens et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13235]
¹SETI Institute, Carl Sagan Center, , Mountain View, California, 94043 USA
²NASA Ames Research Center, , Moffett Field, California, 94035 USA
Published by arrangement with John Wiley & Sons

It has been proposed that all L chondrites resulted from an ongoing collisional cascade of fragments that originated from the formation of the ~500 Ma old asteroid family Gefion, located near the 5:2 mean‐motion resonance with Jupiter in the middle Main Belt. If so, L chondrite pre‐atmospheric orbits should be distributed as expected for that source region. Here, we present contradictory results from the orbit and collisional history of the October 24, 2015, L6 ordinary chondrite fall at Creston, CA (here reclassified to L5/6). Creston’s short 1.30 ± 0.02 AU semimajor axis orbit would imply a long dynamical evolution if it originated from the middle Main Belt. Indeed, Creston has a high cosmic ray exposure age of 40–50 Ma. However, Creston’s small meteoroid size and low 4.23 ± 0.07° inclination indicate a short dynamical lifetime against collisions. This suggests, instead, that Creston originated most likely in the inner asteroid belt and was delivered via the ν6 resonance. The U‐Pb systematics of Creston apatite reveals a Pb‐Pb age of 4,497.1 ± 3.7 Ma, and an upper intercept U‐Pb age of 4,496.7 ± 5.8 Ma (2σ), circa 70 Ma after formation of CAI, as found for other L chondrites. The K‐Ar (age ~4.3 Ga) and U,Th‐He (age ~1 Ga) chronometers were not reset at ~500 Ma, while the lower intercept U‐Pb age is poorly defined as 770 ± 320 Ma. So far, the three known L chondrites that impacted on orbits with semimajor axes a <2.0 AU all have high (>3 Ga) K‐Ar ages. This argues for a source of some of our L chondrites in the inner Main Belt. Not all L chondrites originate in a continuous population of Gefion family debris stretching across the 3:1 mean‐motion resonance.

¹Seda Ozdemir, ^1,2Toni Schulz, ³David van Acken, ⁴Ambre Luguet, ⁵W. Uwe Reimold, ^1,6Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13234]
¹Department of Lithospheric Research, University Vienna, , 1090 Vienna, Austria
²Institut für Geologie und Mineralogie, University Cologne, , 50674 Cologne, Germany
³Irish Centre for Research in Applied Geosciences (iCRAG), UCD School of Earth Sciences, University College Dublin, , Dublin 4, Ireland
⁴Steinmann‐Institut of Geology, Mineralogy and Palaeontology, University of Bonn, , 53115 Bonn, Germany
⁵Geochronology Laboratory, Instituto de Geociências, Universidade de Brasília, , 70910‐900 Brasília, DF, Brazil
⁶Natural History Museum, , A‐1010 Vienna, Austria
Published by arrangement with John Wiley & Sons

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re‐Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present‐day ¹⁸⁷Os/¹⁸⁸Os isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re‐Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes.

¹E.D.Young,²A.Shahar,³F.Nimmo,¹H.E.Schlichting,¹E.A.Schauble,¹H.Tang, ¹J.Labidi
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.012]
¹Department of Earth, Planetary, and Space Sciences, UCLA, Los Angeles, CA, USA
²Geophysical Laboratory, Carnegie Institution for Science, Washington, DC, USA
³Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA, USA
Copyright Elsevier

Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controlled attendant isotope fractionations.

Energy balance at the surface of a body accreted within ~1 Myr of CAI formation and heated from within by ²⁶Al decay results in internal temperatures exceeding the silicate solidus, producing a transient magma ocean with a thin surface boundary layer of order <1 m that would be subject to foundering. Bodies that are massive enough to form magma oceans by radioisotope decay (≥0.1% M_⊕) can retain hot rock vapor even in the absence of ambient nebular gas. We find that a steady-state rock vapor forms within minutes to hours and results from a balance between rates of magma evaporation and atmospheric escape. Vapor pressure buildup adjacent to the surfaces of the evaporating magmas would have inevitably led to an approach to equilibrium isotope partitioning between the vapor phase and the silicate melt. Numerical simulations of this near-equilibrium evaporation process for a body with a radius of ~700 km yield a steady-state far-field vapor pressure of 10⁻⁸ bar and a vapor pressure at the surface of 10⁻⁴ bar, corresponding to 95% saturation. Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation due to the formation of steady-state rock vapor atmospheres and/or the presence of protostellar gas.

We model the Si and Mg isotopic composition of bulk Earth as a consequence of accretion of planetesimals that evaporated subject to the conditions described above. The results show that the best fit to bulk Earth is for a carbonaceous chondrite-like source material with about 12% loss of Mg and 15% loss of Si resulting from near-equilibrium evaporation into the solar protostellar disk of H₂ on timescales of 10⁴ to 10⁵ years.

¹Christian Liebske, ²Amir Khan 
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.014]
¹Institute of Geochemistry and Petrology, ETH Zürich, Switzerland
²Institute of Geophysics, ETH Zürich, Switzerland
Copyright Elsevier

The terrestrial planets are believed to have formed from primitive material sampling a broad region of the inner solar system. Several meteoritic mixing models attempting to reconcile isotopic characteristics of Mars and Earth have recently been proposed, but, because of the inherent non-uniqueness of these solutions, additional independent observations are required to resolve the question of the primary building blocks of the terrestrial planets. Here, we consider existing isotopic measurements of <span id="MathJax-Element-1-Frame" class="MathJax_SVG" role="presentation" data-mathml="Δ′17″>Δ′17O, ϵ⁴⁸Ca, ϵ⁵⁰Ti, ϵ⁵⁴Cr, ϵ⁶²Ni, and ϵ⁸⁴Sr for primitive chondrites and differentiated achondrites and mix these stochastically to reproduce the isotopic signatures of Mars and Earth. For both planets we observe ∼ 10⁵ unique mixing solutions out of 10⁸ random meteoritic mixtures, which are categorised into distinct clusters of mixtures using principle component analysis. The large number of solutions implies that isotopic data alone are insufficient to resolve the building blocks of the terrestrial planets. To further discriminate between isotopically valid mixtures, each mixture is converted into a core and mantle component via mass balance for which geophysical properties are computed and compared to observations. For Mars, the geophysical parameters include mean density, mean moment of inertia, and tidal response, whereas for Earth upper mantle Mg/(Mg+Fe) ratio and core size are employed. The results show that Mars requires an oxidised, FeO-rich differentiated object next to chondritic material as main building blocks. In contrast, Earth’s origin remains enigmatic. From a redox perspective, it appears inescapable that enstatite chondrite-like matter constitutes a dominant proportion of the building blocks from which Earth is made. The apparent need for compositionally distinct building blocks for Mars and Earth suggests that dissimilar planetesimal reservoirs were maintained in the inner Solar System during accretion.

¹Alex M.Ruzicka, ²Richard C.Greenwood, ^1,3Katherine Armstrong, ¹Kristy L.Schepker,²Ian A.Franchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.01.017]
¹Portland State University, Department of Geology and Cascadia Meteorite Laboratory, 17 Cramer Hall, 1721 SW Broadway, Portland, OR, USA
²Planetary Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
³Universität Bayreuth, Bayrisches Geoinstitut, D-95440, Bayreuth, DE
Copyright Elsevier

Large (>3.5 mm and up to 4 cm across) igneous inclusions poor in metal and sulfide are a minor but not uncommon component in ordinary chondrites, and have implications for the nature of physiochemical and melting processes in the early solar system. We obtained petrographic-chemical data for forty-two large igneous inclusions in ordinary chondrites of various groups (H, L, LL) and petrographic types (3-6) and oxygen isotope data for a subset of twelve of these inclusions and their host chondrites. Different inclusions formed both before and after the thermal metamorphism experienced by their host chondrites. The bulk chemical compositions of the inclusions vary broadly around whole-rock chondrite composition, comprise four main chemical types and some other variants, and show little evidence of having formed as igneous differentiates. Oxygen isotope compositions overlap ordinary chondrite compositions and are related to inclusion chemical type. Most prevalent in type 3 and 4 chondrites are inclusions, often droplets, of the vapor-fractionated (Vfr) chemical type, either enriched in refractory lithophile elements, or depleted in volatile lithophile elements, or both. These inclusions have low Δ¹⁷O (∼0.1-0.6‰) and high δ¹⁸O (∼4-8‰) values and formed in reservoirs with Δ¹⁷O lower than their hosts, primarily as evaporative melts and mixtures that probably experienced kinetic isotopic fractionation. Another chemical type (Unfr+K) has unfractionated abundances of lithophile elements except for being strongly enriched in K, a signature also found in some impact melts from melt rocks and melt breccias. These inclusions formed by impact melting of chondritic material and accompanying K enrichment. Inclusions with unfractionated (Unfr) lithophile element abundances are present in type 3-6 chondrites and are prevalent in type 5 and 6. Some are spatially associated with coarse metal-sulfide nodules in the chondrites and likely formed by in situ impact melting. Others were melted prior to thermal metamorphism and were chemically but not isotopically homogenized during metamorphism; they are xenoliths that formed in oxygen reservoirs different than the hosts in which they were metamorphosed. The latter inclusions provide evidence for nebular or collisional mixing of primitive materials prior to thermal metamorphism of asteroid bodies, including transport of H-like source materials to the L body, LL-like source materials to the L body, and low-Δ¹⁷O materials to the LL body. Feldspar-rich (FldR) inclusions have compositions similar to melt pockets and could have formed by disequilibrium melting and concentration of feldspar during an impact event to form large droplets or large masses. Overall, the results of this study point to important and varied roles for both “planetary” impact melting and “nebular” evaporative melting processes to form different large igneous inclusions in ordinary chondrites. Chondrules may have formed by processes similar to those inferred for large inclusions, but there are important differences in the populations of these objects.

¹Sara Mazrouei, ^1,2Rebecca R. Ghent, ³William F. Bottke, ³Alex H. Parker, ⁴Thomas M. Gernon
Science 363, 253-257 Link to Article [DOI: 10.1126/science.aar4058]
¹Department of Earth Sciences, University of Toronto, Toronto, ON, Canada.
²Planetary Science Institute, Tucson, AZ, USA.
³Southwest Research Institute, Boulder, CO, USA.
⁴School of Ocean and Earth Science, University of Southampton, Southampton, UK.
Reprinted with permission from AAAS

The terrestrial impact crater record is commonly assumed to be biased, with erosion thought to eliminate older craters, even on stable terrains. Given that the same projectile population strikes Earth and the Moon, terrestrial selection effects can be quantified by using a method to date lunar craters with diameters greater than 10 kilometers and younger than 1 billion years. We found that the impact rate increased by a factor of 2.6 about 290 million years ago. The terrestrial crater record shows similar results, suggesting that the deficit of large terrestrial craters between 300 million and 650 million years ago relative to more recent times stems from a lower impact flux, not preservation bias. The almost complete absence of terrestrial craters older than 650 million years may indicate a massive global-scale erosion event near that time.