In situ analysis of opal in Gale crater, Mars

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¹W.Rapin et al. (>10)
Journal of Geophysical Research Planets (in Press) Link to Article [https://doi.org/10.1029/2017JE005483]
¹Division of Geological and Planetary Sciences, California Institute of TechnologyPasadena, CA, USA
Published by arrangement with John Wiley & Sons

Silica enrichments resulting in up to ~90 wt% SiO2 have been observed by the Curiosity rover’s instruments in Gale crater, Mars within the Murray and Stimson formations. Samples acquired by the rover drill revealed a significant abundance of an X‐ray amorphous silica phase. Laser induced breakdown spectroscopy (LIBS) highlights an overall correlation of the hydrogen signal with silica content for these Si‐enriched targets. The increased hydration of the high‐silica rocks compared to the surrounding bedrock is also confirmed by active neutron spectroscopy. Laboratory LIBS experiments have been performed to calibrate the hydrogen signal and show that the correlation observed on Mars is consistent with a silica phase containing on average 6.3 ± 1.4 wt% water. X‐ray diffraction and LIBS measurements indicate that opal‐A, amorphous hydrated silica, is the most likely phase containing this water in the rocks. Pyrolysis experiments were also performed on drilled samples by the Sample Analysis at Mars (SAM) instrument to measure volatile content, but the data suggests that most of the water was released during handling prior to pyrolysis. The inferred low‐temperature release of water helps constrain the nature of the opal. Given the geological context and the spatial association with other phases such as calcium sulfates, the opal was likely formed from multiple diagenetic fluid events and possibly represents the latest significant water‐rock interaction in these sedimentary rocks.

The reaction of carbonates in contact with laser‐generated, superheated silicate melts: Constraining impact metamorphism of carbonate‐bearing target rocks

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^1,2Christopher Hamann, ^1,2Saskia Bläsing, ^1,2Lutz Hecht, ⁴³Sebastian Schäffer, ⁴Alex Deutsch, ³Jens Osterholz, ³Bernd Lexow
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13133]
¹Museum für Naturkunde, Leibnitz‐Institut für Evolutions‐ und Biodiversitätsforschung, Berlin, Germany
²Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany
³Fraunhofer‐Institut für Kurzzeitdynamik, Ernst‐Mach‐Institut, Freiburg, Germany
⁴Institut für Planetologie, Westfälische Wilhelms‐Universität Münster, Münster, Germany
Published by arrangement with John Wiley & Sons

We simulated entrainment of carbonates (calcite, dolomite) in silicate impact melts by 1‐bar laser melting of silicate–carbonate composite targets, using sandstone, basalt, calcite marble, limestone, dolomite marble, and iron meteorite as starting materials. We demonstrate that carbonate assimilation by silicate melts of variable composition is extremely fast (seconds to minutes), resulting in contamination of silicate melts with carbonate‐derived CaO and MgO and release of CO₂ at the silicate melt–carbonate interface. We identify several processes, i.e., (1) decomposition of carbonates releases CO₂ and produces residual oxides (CaO, MgO); (2) incorporation of residual oxides from proximally dissociating carbonates into silicate melts; (3) rapid back‐reactions between residual CaO and CO₂ produce idiomorphic calcite crystallites and porous carbonate quench products; (4) high‐temperature reactions between Ca‐contaminated silicate melts and carbonates yield typical skarn minerals and residual oxide melts; (5) mixing and mingling between Ca‐ or Ca,Mg‐contaminated and Ca‐ or Ca,Mg‐normal silicate melts; (6) precipitation of Ca‐ or Ca,Mg‐rich silicates from contaminated silicate melts upon quenching. Our experiments reproduce many textural and compositional features of typical impact melts originating from silicate–carbonate targets. They reinforce hypotheses that thermal decomposition of carbonates, rapid back‐reactions between decomposition products, and incorporation of residual oxides into silicate impact melts are prevailing processes during impact melting of mixed silicate–carbonate targets. However, by comparing our results with previous studies and thermodynamic considerations on the phase diagrams of calcite and quartz, we envisage that carbonate impact melts are readily produced during adiabatic decompression from high shock pressure, but subsequently decompose due to heat influx from coexisting silicate impact melts or hot breccia components. Under certain circumstances, postshock conditions may favor production and conservation of carbonate impact melts. We conclude that the response of mixed carbonate–silicate targets to impact might involve melting anddecomposition of carbonates, the dominant response being governed by a complex variety of factors.

Meteorite reconnaissance in Saudi Arabia

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^1,2Beda A. Hofmann et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13132]
¹Natural History Museum Bern, Bern, Switzerland
²Institute for Geological Sciences, University of Bern, Bern, Switzerland
Published by arrangement with John Wiley & Sons

Meteorite searches in Saudi Arabia between 2008 and 2014 yielded 46 meteorites from the Yabrin area (23°N 49°E), 35 meteorites from the Rub’ al‐Khali sand desert (19°–20°N, 48°–51°E), and 1 meteorite from Al Haddar. No meteorites were found near Hafar al Batin (29°N 45°E). The 82 new meteorites represent ~57 falls comprising 43 ordinary chondrites, 4 carbonaceous chondrites, 2 enstatite chondrites, 3 ureilites, 3 eucrites, 1 acapulcoite, and 1 lunar meteorite. The median of 31 14C terrestrial ages is 6.2 ka, significantly younger than the Oman population (19.5 ka, n = 128). A further assessment of terrestrial 14C contamination is advised by a 11–15 ka 14C terrestrial age of heavily weathered meteorite Khawr al Fazra 014, geology indicating a terrestrial age >100 ka. Find densities of 0.4–2.8 km−2 for Yabrin and the western Rub’ al‐Khali are similar to ~0.5 km−2 observed in Oman. Higher find densities of ~135 km−2 (29 km−2 for masses >10 g) exist on small Pleistocene outcrops in blowouts in the south‐central Rub’ al‐Khali: 21 unpaired meteorites (four >10 g) were found in 11 blowouts with a combined area of 0.14 km2. The Rub’ al‐Khali meteorites show a relatively high degree of weathering (median W 3.6; 2.5 for Yabrin), low median mass (4.3/138 g), and a high H/L ratio (2.3/1.1). The high density of small meteorites is explained by prolonged sand protection and recent deflation. The high meteorite density and relatively high proportion of rare meteorite types render the Rub’ al‐Khali blowouts an interesting target for future exploration.

High‐resolution δ13Corg chemostratigraphy links the Decorah impact structure and Winneshiek Konservat‐Lagerstätte to the Darriwilian (Middle Ordovician) global peak influx of meteorites

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¹Stig M. Bergström, ²Birger Schmitz,³Huaibao P. Liu, ¹Fredrik Terfelt, ³Robert M. McKay
Lethaia (in Press) Link to Article [https://doi.org/10.1111/let.12269]
¹School of Earth Sciences, The Ohio State University, Columbus, OH, USA
²Division of Nuclear Physics, Department of Physics, Lund University, Lund, Sweden
³Iowa Geological Survey, IIHR‐Hydroscience & Engineering, University of Iowa, Iowa City, IA, USA