¹Kerry Sieh,¹Jason Herrin,²Brian Jicha, ¹Dayana Schonwalder Angel,¹James D. P. Moore, ¹Paramesh Banerjee,³Weerachat Wiwegwin, ⁴Vanpheng Sihavong,²Brad Singer,³Tawachai Chualaowanich,⁵Punya Charusiri
Proceedings of the National Academy of Sciences of the United States of America (PNAS)(In Press) Link to Article [DOI:https://doi.org/10.1073/pnas.1904368116]
¹Earth Observatory of Singapore, Nanyang Technological University, 639798 Singapore;
²Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706;
³Department of Mineral Resources, Ministry of Natural Resources and Environment, Ratchatewi, 10400 Bangkok, Thailand;
⁴Department of Geology and Mines, Ministry of Energy and Mines, Vientiane, Lao People’s Democratic Republic;
⁵Department of Geology, Chulalongkorn University, Khet Pathumwan, 10330 Bangkok, Thailand

The crater and proximal effects of the largest known young meteorite impact on Earth have eluded discovery for nearly a century. We present 4 lines of evidence that the 0.79-Ma impact crater of the Australasian tektites lies buried beneath lavas of a long-lived, 910-km³ volcanic field in Southern Laos: 1) Tektite geochemistry implies the presence of young, weathered basalts at the site at the time of the impact. 2) Geologic mapping and ⁴⁰Ar-³⁹Ar dates confirm that both pre- and postimpact basaltic lavas exist at the proposed impact site and that postimpact basalts wholly cover it. 3) A gravity anomaly there may also reflect the presence of a buried ∼17 × 13-km crater. 4) The nature of an outcrop of thick, crudely layered, bouldery sandstone and mudstone breccia 10–20 km from the center of the impact and fractured quartz grains within its boulder clasts support its being part of the proximal ejecta blanket.

¹A.B.Verchovsky,¹M.Anand,¹S.J.Barber,¹S.Sheridan,¹G.H.Morgan
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2019.104830]
¹School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

^1,2Leonard F. Henrichs,²Agnes Kontny,²Boris Reznik,³Uta Gerhards,⁴Jörg Göttlicher,²Tim Genssle,²Frank Schilling
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13422]
¹Karlsruhe Institute of Technology, Institute of Nanotechnology, Hermann‐von‐Helmholtz‐Platz 1, 76344 Eggenstein‐Leopoldshafen, Germany
²Karlsruhe Institute of Technology, Institute of Applied Geosciences, Adenauerring 20, 76131 Karlsruhe, Germany
³Karlsruhe Institute of Technology, Institute for Micro Process Engineering, Hermann‐von‐Helmholtz‐Platz 1, 76344 Eggenstein‐Leopoldshafen, Germany
⁴Karlsruhe Institute of Technology, Institute for Photon Science and Synchrotron Radiation (IPS), Hermann‐von‐Helmholtz‐Platz 1, 76344 Eggenstein‐Leopoldshafen, Germany
Published by arrangement with John Wiley & Sons

Projectile–target interactions as a result of a large bolide impact are important issues, as abundant extraterrestrial material has been delivered to the Earth throughout its history. Here, we report results of shock‐recovery experiments with a magnetite‐quartz target rock positioned in an ARMCO iron container. Petrography, synchrotron‐assisted X‐ray powder diffraction, and micro‐chemical analysis confirm the appearance of wüstite, fayalite, and iron in targets subjected to 30 GPa. The newly formed mineral phases occur along shock veins and melt pockets within the magnetite‐quartz aggregates, as well as along intergranular fractures. We suggest that iron melt formed locally at the contact between ARMCO container and target, and intruded the sample causing melt corrosion at the rims of intensely fractured magnetite and quartz. The strongly reducing iron melt, in the form of μm‐sized droplets, caused mainly a diffusion rim of wüstite with minor melt corrosion around magnetite. In contact with quartz, iron reacted to form an iron‐enriched silicate melt, from which fayalite crystallized rapidly as dendritic grains. The temperatures required for these transformations are estimated between 1200 and 1600 °C, indicating extreme local temperature spikes during the 30 GPa shock pressure experiments.

¹Friedrich Hörz,²Mark J. Cintala,¹Kathie L. Thomas‐Keprta,¹Daniel K. Ross,¹Simon J. Clemett
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13424]
¹Jacobs‐JETS, 2224 Bay Area Boulevard, Houston, Texas, 77058 USA
²Code XI3, NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

We shocked calcite in an unconfined environment by launching small marble cylinders at 0.8–5.5 km s−1 into aluminum or copper plates, producing shock stresses between 5 and 79 GPa. The resulting 5–20 mm craters contained intimately mixed clastic and molten projectile residues over the entire pressure range, with melting commencing already at 5 GPa. Stoichiometrically pure calcite melts were not observed as all melts contained target metal. Some of these residues were distinctly depleted in CO2 and some contained even tiny CaO crystals, thus illustrating partial to complete loss of CO2. We interpret a thin seam of finely crystalline calcite to be the product of back reactions between CaO and CO2. The amount of carbonate residue in these craters, especially those at low velocities (<2 km s−1), is dramatically less than that of silicate impactors in similar cratering experiments, and we suggest that this is due to substantial outgassing of CO2. Similarly, the volume of carbonate melts relative to the volume of limestone or dolomite in many terrestrial crater structures seems insignificant as well, as is the volume of carbonate melt compared to the volume of impact melts derived from silicates. These volume considerations suggest that volatilization of CO2 is the dominant process in carbonate‐containing targets. Because we have difficulties in explaining naturally occurring calcite melts by shock processes in dolomite‐dominated targets, we speculate—essentially via process of elimination—that such carbonate melt blebs might be condensation products from an impact‐produced vapor cloud.

^1,2N. G. Lunning,³A. Bischoff,²J. Gross,³M. Patzek,¹C. M. Corrigan,¹T. J. McCoy
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13430]
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, 20560‐0119 USA
²Rutgers, Department of Earth and Planetary Sciences, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey, 08854‐8066 USA
³Institut für Planetologie, Westfälische Wilhelms‐Universität Münster, Wilhelm‐Klemm‐Str. 10, D‐48149 Münster, Germany
Published by arrangement with John Wiley & Sons

Ancient, SiO₂‐rich achondrites have previously been proposed to have formed by disequilibrium partial melting of chondrites. Here, we test the alternative hypothesis that these achondrites formed by fractional crystallization of impact melts of Rumuruti (R) chondrites. We identified two new melt clasts in R chondrites, one in Pecora Escarpment (PCA) 91241 and one in LaPaz Icefield (LAP) 031275. We analyzed major, minor, and trace element concentrations, as well as oxygen isotopes, of these two clasts and a third one that had been previously recognized (Bischoff et al. 2011) as an impact melt in Dar al Gani (DaG) 013. The melt clast in PCA 91241 is an R chondrite impact melt closely resembling the one previously recognized in DaG 013. The melt clast in LAP 031275 has an L chondrite provenance. We show that SiO₂‐rich melts could form from the mesostases of R chondrite impact melts. However, their CI‐normalized rare earth element patterns are flat, whereas those of ancient SiO₂‐rich achondrites (Day et al. 2012; Srinivasan et al. 2018) and those of disequilibrium partial melts of chondrites (Feldstein et al. 2001) have positive Eu anomalies from preferential melting of plagioclase. Thus, we conclude that ancient SiO₂‐rich achondrites were probably formed by disequilibrium partial melting (due to an internal heat source on their parent bodies), rather than from impact melts.