¹Lidia Pittarello, ²Fabrizio Nestola, ³Cecilia Viti, ⁴Alvaro Penteado Crósta, ^1,5Christian Koeberl
¹ Department of Lithospheric Research, University of Vienna, Vienna, Austria
² Department of Geosciences, University of Padova, Padova, Italy
³ Department of Physics, Earth and Environmental Sciences, University of Siena, Siena, Italy
⁴ Institute of Geosciences, Campinas SP, Brazil
⁵ Natural History Museum, Vienna, Austria

Shatter cones are one of the most widely recognized pieces of evidence for meteorite impact events on Earth, but the process responsible for their formation is still debated. Evidence of melting on shatter cone surfaces has been rarely reported in the literature from terrestrial impact craters but has been recently observed in impact experiments. Although several models for shatter cones formation have been proposed, so far, no one can explain all the observed features. Shatter cones’ from the Vista Alegre impact structure, Brazil, formed in fine-grained basalt of the Jurassic-Cretaceous Serra Geral Formation (Paraná large igneous province). A continuous quenched melt film, consisting of a crystalline phase, mica, and amorphous material, decorates the striated surface. Ultracataclasites, containing subrounded pyroxene clasts in an ultrafine-grained matrix, occur subparallel to the striated surface. Several techniques were applied to characterize the crystalline phase in the melt, including Raman spectroscopy and transmission electron microscopy. Results are not consistent with any known mineral, but they do suggest a possible rare or new type of clinopyroxene. This peculiar evidence of melting and cataclasis in relation with shatter cone surfaces is interpreted as the result of tensile fracturing at the tip of a fast propagating shock-induced rupture, which led to the formation of shatter cones at the tail of the shock front, likely during the early stage of the impact events.

References
Pittarello L, Nestola F, Viti C, Crósta AP, Koeberl C (2015) Melting and cataclastic features in shatter cones in basalt from the Vista Alegre impact structure, Brazil. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12466]

Published by arrangement with John Wiley&Sons

¹Alan E. Rubin
¹Department of Earth, Planetary, and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA

Several aubrites (e.g., LAP 03719, Bishopville, Khor Temiki, ALH 83015) contain orthopyroxene grains that exhibit more-pronounced shock effects than associated olivine grains. The orthopyroxene grains in these samples have clinoenstatite lamellae on (100) and exhibit weak mosaic extinction, characteristic of shock stage S4; the olivine grains exhibit either sharp optical extinction, characteristic of shock stage S1 (as in LAP 03719), or undulose extinction (shock stage S2), as in Bishopville and ALH 83015. The Khor Temiki regolith breccia contains S1 and S2 olivine grains. Because literature data show that diffusion is much slower in orthopyroxene than in olivine, it seems likely that aubrites experienced postshock, impact-induced annealing. After differentiation, the aubrite parent asteroid suffered major collisions that caused extensive brecciation of near-surface materials and damaged orthopyroxene and olivine crystal lattices. As a result of these impact events, some aubrites were shocked and buried within warm ejecta blankets or beneath fallback debris under the crater floor. Entombed olivine crystal lattices healed (and became unstrained, reaching shock stage S1), but orthopyroxene lattices retained their S4-level shock-damaged features. Aubrites with S4 orthopyroxene and S2 olivine were probably very weakly shocked again after olivine was annealed to S1.

Reference
Rubin AE (2015) Shock and annealing in aubrites: Implications for parent-body history. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12464]
Published by arrangement with John Wiley&Sons

^1,2,3Katherine H. Joy, ⁴Channon Visscher, ⁵Michael E. Zolensky, ⁶Takashi Mikouchi, ⁷Kenji Hagiya, ⁸Kazumasa Ohsumi, ^1,2David A. Kring
¹Center for Lunar Science and Exploration, The Lunar and Planetary Institute—USRA, Houston, Texas, USA
² NASA Solar System Exploration Research Virtual Institute
³School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
⁴ Dordt College, Iowa, USA
⁵ARES, NASA Johnson Space Center, Houston, Texas, USA
⁶Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo, Japan
⁷Graduate School of Science, University of Hyogo, Ako-gun, Hyogo, Japan
⁸Japan Synchrotron Radiation Research Institute (JASRI), Sayo-gun, Hyogo, Japan

Lunar regolith breccias are temporal archives of magmatic and impact bombardment processes on the Moon. Apollo 16 sample 60016 is an “ancient” feldspathic regolith breccia that was converted from a soil to a rock at ~3.8 Ga. The breccia contains a small (70 × 50 μm) rock fragment composed dominantly of an Fe-oxide phase with disseminated domains of troilite. Fragments of plagioclase (An95-97), pyroxene (En74-75, Fs21-22,Wo3-4), and olivine (Fo66-67) are distributed in and adjacent to the Fe-oxide. The silicate minerals have lunar compositions that are similar to anorthosites. Mineral chemistry, synchrotron X-ray absorption near edge spectroscopy (XANES) and X-ray diffraction (XRD) studies demonstrate that the oxide phase is magnetite with an estimated Fe3+/ΣFe ratio of ~0.45. The presence of magnetite in 60016 indicates that oxygen fugacity during formation was equilibrated at, or above, the Fe-magnetite or wüstite–magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidization-driven mineral replacement of Fe-metal or desulphurisation from Fe-sulfides (troilite) at low temperatures (<570 °C) in equilibrium with H₂O steam/liquid or CO₂ gas. Oxidizing conditions may have arisen from vapor transport during degassing of a magmatic source region, or from a hybrid endogenic–exogenic process when gases were released during an impacting asteroid or comet Impact.

Reference
Joy KH, Visscher C, Zolensky ME, Mikouchi T, Hagiya K, Ohsumi K, Kring DA (2015) Identification of magnetite in lunar regolith breccia 60016: Evidence for oxidized conditions at the lunar surface. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12462]

Published by arrangement with John Wiley&Sons