¹Gioacchino Tempesta, ²Giorgio S.Senesi, ³Paola Manzari, ¹Giovanna Agrosì
Spectrochimica Acta Part B: Atomic Spectroscopy 144, 75-81 Link to Article [https://doi.org/10.1016/j.sab.2018.03.014]
¹Dipartimento di Scienze della Terra e Geoambientali (DiSTeGeo), University of Bari, Via E. Orabona 4, 70125 Bari, Italy
²CNR – Istituto di Nanotecnologia (NANOTEC), PLasMI Lab, Via Amendola 122/D, 70126 Bari, Italy
³Istituto Nazionale di Astrofisica, Istituto di Astrofisica e Planetologia Spaziali (INAF-IAPS), via Fosso del Cavaliere 100, Roma, Italy

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¹Manuel Raith,²Matthias Ebert,¹Katja Pinkert,¹Christian U. Grosse
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13113]
¹Chair of Non‐destructive Testing, Technical University of Munich, Munich, Germany
²Institute of Earth and Environmental Sciences, Geology, University of Freiburg, Freiburg, Germany
Published by arrangement with John Wiley & Sons

Since the 1960s, hypervelocity impact experiments have been conducted to study the complex deformation mechanisms which occur in the subsurface of meteorite craters. Here, we present ultrasound tomography measurements of the damage zone underneath seven experimentally produced impact craters in sandstone cubes. Within the framework of the Multidisciplinary Experimental and Modeling Impact Research Network and the NEOShield Project, decimeter‐sized sandstone targets were impacted by aluminum and steel projectiles with radii of 2.5, 4, and 5 mm at velocities between ~3.0 and ~7.4 km s−1. The 2‐D ultrasound tomography clearly shows a correlation between impact energy and the damaged volume within the target blocks. When increasing impact energies from 805 to 2402 J, a corresponding increase in the damage radius from ~13.1 cm to ~17.6 cm was calculated. p‐Wave velocity reductions up to 18.3% (for the highest impact energy) were observed in the vicinity of the craters. The reduction in seismic velocity decreased uniformly and linearly with increasing distance from the impact point. The damage intensities correspond to peak damage parameters of 0.4–0.51 compared to undamaged target blocks. In addition to the damage zone below the crater, we could identify weakened zones at the sandstone walls which represent precursors of spalling. The volume of the damaged subsurface beneath experimentally produced craters determined through ultrasound tomography is larger than that obtained from previously reported p‐wave velocity reductions or to microscopic and microcomputed tomography observations of crack densities in experimentally produced craters.

^1,2Atsushi Takenouchi,^1,3Takashi Mikouchi,^4,5Akira Yamaguchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13120]
¹Department of Earth and Planetary Science, Graduate School of Arts and Sciences, The University of Tokyo, Bunkyo‐ku, Tokyo, Japan
²Department of Basic Sciences, Graduate School of Science, The University of Tokyo, Meguro‐ku, Tokyo, Japan
³The University Museum, The University of Tokyo, Bunkyo‐ku, Tokyo, Japan
⁴National Institute of Polar Research, Tachikawa‐shi, Tokyo, Japan
⁵Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies)Tachikawa‐shi, Tokyo, Japan
Published by arrangement with John Wiley & Sons

Shergottite Martian meteorites are known to contain brown‐colored olivine (brown olivine), which is considered to form during a shock event on Mars. In order to constrain the formation conditions of brown olivine, four shergottites with brown olivine and four shergottites without brown olivine are analyzed in this study. Based on our observations, brown olivine is often accompanied by thin (<10 μm) melt veins indicating local temperature increase (1750–1870 K). Even in shergottites without brown olivine, olivine around shock melt veins/pockets is partly darkened and shows similar features to those of brown olivine. These observations support that brown olivine is formed under conditions similar to those around shock melt veins/pockets. Components of shock melt veins (vesicles, quench crystals, etc.) and the absence of high‐pressure phases in shergottites with brown olivine indicate that they have high postshock temperature (>1200–1170 K). Such high postshock temperature may indicate that shergottites with brown olivine experienced high pressure (around 55 GPa), while shergottites without brown olivine experienced lower shock pressure (>20–35 GPa). Therefore, brown olivine may be a good indicator for strong shock events (peak shock pressure: ~55 GPa; postshock temperature: >1200–1170 K) and such shock events could be induced by small but rapid projectiles onto Mars.

¹E. A. MacLagan,^1,2E. L. Walton,¹C. D. K. Herd,³M. Dence
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13122]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
²Department of Physical Sciences, MacEwan UniversityEdmonton, Alberta, Canada
³2602 – 38 Metropole Pvt.Ottawa, ON, Canada
Published by arrangement with John Wiley & Sons

The Steen River impact structure (SRIS) formed in mixed target rocks, with Devonian carbonates, shales, and evaporites overlying granitic basement rocks of the Canadian Shield. A detailed study of impact melt phases within a continuous sequence of polymict impact breccia, as intersected by drill core, evaluated the relationship of impact melt to the breccia, identified the target rocks that contributed to the melt, and calculated the amount of melt within the breccia. Impact melt in the SRIS breccia occurs in three main forms (1) as individual matrix‐supported clasts, (2) as rims enveloping granitic clasts, and (3) as layers of agglomerated melt. Major and minor element abundances of large impact melt clasts (>1 mm) are similar to granitic basement, aside from elevated CaO and K2O wt% oxides in these melt clasts from incorporation of carbonates and calcareous shales. In contrast, submillimeter‐sized impact melt clasts have a composition derived almost exclusively from melting of shales. The small size of the shale‐derived melt clasts is attributed to increased fragmentation and a wider dispersion due to the volatile‐rich nature of the shale protolith. The wide compositional range of impact‐melted target lithologies documented at the SRIS contradicts breccia clast formation by impact melts that merged into larger bodies but were subsequently disrupted. Our observations are consistent with disruption of impact melt early in its formation history, and the volatile‐rich nature of the target materials likely contributed to this disruption. Bimodal thin section scans provide an estimate of the proportion of impact melt phases in the SRIS breccias (~19 vol%). When compared to similarly sized, mixed‐target impact structures, our results are consistent with the estimated volume of impact melt clasts from Ries, Germany (21 vol%), but are roughly twice that observed at Haughton, Canada (<10 vol%).

^1,2,3Steven B. Simon,^1,4Stephen R. Sutton
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13123]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA
²The Field Museum of Natural History, Chicago, Illinois, USA
³Institute of Meteoritics, University of New Mexico Albuquerque, New Mexico, USA
⁴Center for Advanced Radiation Sources (CARS), The University of Chicago, Chicago, Illinois, USA
Published by arrangement with John Wiley & Sons

To assess the variability of redox states among mare basalt source regions, investigation of the valence of Ti, Cr, and V and the coordination environment of Ti in pyroxene and olivine in lunar rocks via XANES (X‐ray absorption near‐edge structure) spectroscopy has been extended to Apollo 17 basalts: two high‐Ti (70017 and 74275) hand samples, and three very low‐Ti (70006,371, 70007,289B, and 70007,296) basalt fragments from the Apollo 17 deep drill core. Valences of Ti in pyroxene of both suites range from 3.6 to 4, or from 40% to 0% Ti3+, averaging 15–20% Ti3+. Assuming Ti3+ is more compatible in pyroxene than Ti4+, then even lower Ti3+ proportions are indicated for the parental melts. The VLT pyroxene exhibits a slightly wider range of V valences (2.57–2.96) than the high‐Ti pyroxene (2.65–2.86) and a much wider range of Cr valences (2.32–2.80 versus 2.68–2.86); Cr is generally reduced in VLT pyroxene compared to high‐Ti pyroxene. Valences of Ti and Cr in VLT pyroxene become less reduced with increasing FeO contents, possibly indicating change in oxygen fugacity during crystallization. Olivine in all samples has very low (<20%) proportions of Ti3+, with no Ti3+ and higher proportions of Ti in tetrahedral coordination in the VLTs than in the high‐Ti basalts. Olivine in 74275, including that in a dunite clast, has much higher proportions of Cr2+ than the pyroxene in that sample, consistent with previous studies indicating that the olivine grains in this sample are xenocrysts and possibly indicating oxidation just prior to pyroxene crystallization. Results for this sample, the VLTs, and previously studied Apollo 14 and 15 basalts all indicate that mare magmas were in reducing environments at depth, as recorded in early crystallization products, and that later, presumably shallower environments, were relatively oxidizing; single, characteristic fO2s of formation cannot be assigned to these samples. A process likely to account for this feature seen in multiple samples is loss by degassing of a reducing, H‐rich vapor (probably H2) during ascent and/or eruption, causing oxidation of the residual melt, recorded in relatively late‐crystallized pyroxene.