^1,2M.E.I.Riebe et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12936]
¹Institute of Geochemistry and Petrology, ETH Zürich, Zurich, Switzerland
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., USA
Published by arrangement with John Wiley & Sons

The Almahata Sitta strewn field is dominated by ureilites, but contains a large fraction of chondritic fragments of various types. We analyzed stable isotopes of He, Ne, Ar, Kr, and Xe, and the cosmogenic radionuclides 10Be, 26Al, and 36Cl in six chondritic Almahata Sitta fragments (EL6 breccia, EL6, EL3-5, CB, LL4/5, R-like). The cosmic-ray exposure (CRE) ages of five of the six samples have an average of 19.2 ± 3.3 Ma, close to the average of 19.5 ± 2.5 Ma for four ureilites. The cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with Almahata Sitta. This corroborates that Almahata Sitta chondrite samples were part of the same asteroid as the ureilites. However, MS-179 has a lower CRE age of 11.0 ± 1.4 Ma. Further analysis of short-lived radionuclides in fragment MS-179 showed that it fell around the same time, and from an object of similar size as Almahata Sitta, making it almost certain that MS-179 is an Almahata Sitta fragment. Instead, its low CRE age could be due to gas loss, chemical heterogeneity that may have led to an erroneous 21Ne production-rate, or, perhaps most likely, MS-179 could represent the true 4π exposure age of Almahata Sitta (or an upper limit thereof), while all other samples analyzed so far experienced exposure on the parent body of similar lengths. Finally, MS-179 had an extraordinarily high activity of neutron-capture 36Cl, ~600 dpm kg−1, the highest activity observed in any meteorite to date, related to a high abundance of the Cl-bearing mineral lawrencite.

Christopher D.K.Herd et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.037]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, Canada
Copyright Elsevier

Northwest Africa (NWA) 8159 is an augite-rich shergottite, with a mineralogy dominated by Ca-, Fe-rich pyroxene, plagioclase, olivine, and magnetite. NWA 8159 crystallized from an evolved melt of basaltic composition under relatively rapid conditions of cooling, likely in a surface lava flow or shallow sill. Redox conditions experienced by the melt shifted from relatively oxidizing (with respect to known Martian lithologies, ∼QFM) on the liquidus to higher oxygen fugacity (∼QFM+2) during crystallization of the groundmass, and under subsolidus conditions. This shift resulted in the production of orthopyroxene and magnetite replacing olivine phenocryst rims. NWA 8159 contains both crystalline and shock-amorphized plagioclase (An50-62), often observed within a single grain; based on known calibrations we bracket the peak shock pressure experienced by NWA 8159 to between 15 and 23 GPa. The bulk composition of NWA 8159 is depleted in LREE, as observed for Tissint and other depleted shergottites; however, NWA 8159 is distinct from all other martian lithologies in its bulk composition and oxygen fugacity. We obtain a Sm-Nd formation age of 2.37 ± 0.25 Ga for NWA 8159, which represents an interval in Mars geologic time which, until recently, was not represented in the other martian meteorite types. The bulk rock 147Sm/144Nd value of 0.37 ± 0.02 is consistent with it being derived directly from its source and the high initial ε143Nd value indicates this source was geochemically highly depleted. Cr, Nd, and W isotopic compositions further support a unique mantle source. While the rock shares similarities with the 2.4-Ga NWA 7635 meteorite, there are notable distinctions between the two meteorites that suggest differences in mantle source compositions and conditions of crystallization. Nevertheless, the two samples may be launch-paired. NWA 8159 expands the known basalt types, ages and mantle sources within the Mars sample suite to include a second igneous unit from the early Amazonian.

¹Peter C. Stephenson,²Yangting Lin,¹Ingo Leya
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12950]
¹Physical Institute, Space Sciences and Planetology, University of Bern, Bern, Switzerland
²Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
Published by arrangement with John Wiley & Sons

Here we present the isotopic concentrations of He, Ne, Ar, Kr, and Xe for the three Martian meteorites, namely Grove Mountains 99027 (GRV 99027), Northwest Africa 7906 (NWA 7906), and Northwest Africa 7907 (NWA 7907). The cosmic ray exposure (CRE) age for GRV 99027 of 5.7 ± 0.4 Ma (1σ) is consistent with CRE ages for other poikilitic basaltic shergottites and suggests that all were ejected in a single event ~5.6 Ma ago. After correcting for an estimated variable sodium concentration, the CRE ages for NWA 7906 and NWA 7907 of 5.4 ± 0.4 and 4.9 ± 0.4 Ma (1σ), respectively, are in good agreement with the CRE age of ~5 Ma favored by Cartwright et al. (2014) for NWA 7034. The data, therefore, support the conclusion that all three basaltic regolith breccias are paired. The 40Ar gas retention age for NWA 7907 of ~1.3 Ga is in accord with Cartwright et al. (2014). For NWA 7906, we were unable to determine a 40Ar gas retention age. The 4He gas retention ages for NWA 7906 and 7907 are in the range of 200 Ma and are much shorter than the 40Ar gas retention age of NWA 7907, indicating that about 86–88% of the radiogenic 4He has been lost. The Kr and Xe isotopic concentrations in GRV 99027 are composed almost exclusively of Martian interior (MI) gases, while for NWA 7906 and NWA 7907, they indicate gases from the MI, elementally fractionated air, and possibly Martian atmosphere.

¹David McDougal,^1,2Daisuke Nakashima,^1,3Travis J. Tenner,¹Noriko T. Kita,¹John W. Valley,⁴Takaaki Noguchi
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12932]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA
²Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Sendai, Miyagi, Japan
³Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
⁴Faculty of Arts and Science, Kyushu University, Fukuoka, Japan
Published by arrangement with John Wiley & Sons

High-precision oxygen three-isotope ratios were measured for four mineral phases (olivine, low-Ca and high-Ca pyroxene, and plagioclase) in equilibrated ordinary chondrites (EOCs) using a secondary ion mass spectrometer. Eleven EOCs were studied that cover all groups (H, L, LL) and petrologic types (4, 5, 6), including S1–S4 shock stages, as well as unbrecciated and brecciated meteorites. SIMS analyses of multiple minerals were made in close proximity (mostly <100 μm) from several areas in each meteorite thin section, to evaluate isotope exchange among minerals. Oxygen isotope ratios in each mineral become more homogenized as petrologic type increases with the notable exception of brecciated samples. In type 4 chondrites, oxygen isotope ratios of olivine and low-Ca pyroxene are heterogeneous in both δ18O and Δ17O, showing similar systematics to those in type 3 chondrites. In type 5 and 6 chondrites, oxygen isotope ratios of the four mineral phases plot along mass-dependent fractionation lines that are consistent with the bulk average Δ17O of each chondrite group. The δ18O of three minerals, low-Ca and high-Ca pyroxene and plagioclase, are consistent with equilibrium fractionation at temperatures of 700–1000 °C. In most cases the δ18O values of olivine are higher than those expected from pyroxene and plagioclase, suggesting partial retention of premetamorphic values due to slower oxygen isotope diffusion in olivine than pyroxene during thermal metamorphism in ordinary chondrite parent bodies.

¹Nicholas E. Timms, ¹Timmons M. Erickson, ²Michael R. Zanetti, ³Mark A. Pearce, ⁴Cyril Cayron, ^1,5Aaron J. Cavosie, ¹Steven M. Reddy, ⁶Axel Wittmann, ⁷Paul K. Carpenter
Earth and Planetary Science Letters 477, 52-58 Link to Article [https://doi.org/10.1016/j.epsl.2017.08.012]
¹Department of Applied Geology, Curtin University, Perth, GPO Box U1987, Western Australia 6845, Australia
²University of Western Ontario, 1151 Richmond St, London, Ontario, N6A 3K7 Canada
³CSIRO Mineral Resources, Australian Resources Research Centre, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
⁴Laboratory of ThermoMechanical Metallurgy (LMTM), PX Group Chair, École Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71b, 2000 Neuchâtel, Switzerland
⁵NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin–Madison, Madison WI, USA
⁶LeRoy Eyring Center for Solid State Science, Arizona State University, 901 S Palm Walk, Tempe, AZ, 85287, USA
⁷Washington University in St Louis, Earth and Planetary Science Department and the McDonnell Center for Space Sciences; 1 Brookings Drive, St Louis MO, 63112, USA
Copyright Elsevier

Bolide impacts influence primordial evolution of planetary bodies because they can cause instantaneous melting and vaporization of both crust and impactors. Temperatures reached by impact-generated silicate melts are unknown because meteorite impacts are ephemeral, and established mineral and rock thermometers have limited temperature ranges. Consequently, impact melt temperatures in global bombardment models of the early Earth and Moon are poorly constrained, and may not accurately predict the survival, stabilization, geochemical evolution and cooling of early crustal materials. Here we show geological evidence for the transformation of zircon to cubic zirconia plus silica in impact melt from the 28 km diameter Mistastin Lake crater, Canada, which requires super-heating in excess of 2370 °C. This new temperature determination is the highest recorded from any crustal rock. Our phase heritage approach extends the thermometry range for impact melts by several hundred degrees, more closely bridging the gap between nature and theory. Profusion of >2370 °C superheated impact melt during high intensity bombardment of Hadean Earth likely facilitated consumption of early-formed crustal rocks and minerals, widespread volatilization of various species, including hydrates, and formation of dry, rigid, refractory crust.

¹Atsushi Takenouchi,¹Takashi Mikouchi,¹Toshihiro Kogure
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12949]
¹Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo, Japan
Published by arrangement with John Wiley & Sons

Martian meteorites, in particular shergottites, contain darkened olivine (so-called “brown olivine”) whose color is induced by iron nanoparticles formed in olivine during a shock event. The formation process and conditions of brown olivine have been discussed in the Northwest Africa 2737 (NWA 2737) chassignite. However, formation conditions of brown olivine in NWA 2737 cannot be applied to shergottites because NWA 2737 has a different shock history from that of shergottites. Therefore, this study observed brown olivine in the NWA 1950 shergottite and discusses the general formation process and conditions of brown olivine in shergottites. Our observation of NWA 1950 revealed that olivine is heterogeneously darkened between and within grains different from brown olivine in NWA 2737. XANES analysis showed that brown olivine contains small amounts of Fe3+ and TEM/STEM observation revealed that there is no SiO-rich phase around iron metal nanoparticles. These observations indicate that iron nanoparticles were formed by a disproportionation reaction of olivine (3Fe2+olivine → Fe0metal + 2Fe3+olivine + Volivine, where Volivine means a vacancy in olivine). Some parts of brown olivine show lamellar textures in SEM observation and Raman peaks in addition to those expected for olivine, implying that brown olivine experienced a phase transition (to e.g., ringwoodite). In order to induce heterogeneous darkening, heterogeneous high temperature of about 1500–1700 K and shock duration of at least ~90 ms are required. This heterogeneous high temperature resulted in high postshock temperature (>900 K) inducing back-transformation of most high-pressure phases. Therefore, in spite of lack of high-pressure phases, NWA 1950 (= Martian meteorites with brown olivine) experienced higher pressure and temperature compared to other highly shocked meteorite groups.

^1,2W. Zylberman,¹Y. Quesnel,¹P. Rochette,^2,3G. R. Osinski,²C. Marion,¹J. Gattacceca
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12917]
¹Aix-Marseille Univ, CNRS, IRD, Coll de France, CEREGE UM34, Aix-en-Provence, France
²Centre for Planetary Science and Exploration and Department Earth Sciences, University of Western Ontario, London, Ontario, Canada
³Centre for Planetary Science and Exploration and Department Earth Sciences, University of Western Ontario, London, Ontario, Canada
⁴Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

Haughton is a ~24 Myr old midsize (apparent diameter 23 km) complex impact structure located on Devon Island in Nunavut, Canada. The center of the structure shows a negative gravity anomaly of −12 mGal coupled to a localized positive magnetic field anomaly of ~900 nT. A field expedition in 2013 led to the acquisition of new ground magnetic field mapping and electrical resistivity data sets, as well as the first subsurface drill cores down to 13 m depth at the top of the magnetic field anomaly. Petrography, rock magnetic, and petrophysical measurements were performed on the cores and revealed two different types of clast-rich polymict impactites: (1) a white hydrothermally altered impact melt rock, not previously observed at Haughton, and (2) a gray impact melt rock with no macroscopic sign of alteration. In the altered core, gypsum is present in macroscopic veins and in the form of intergranular selenite associated with colored and zoned carbonate clasts. This altered core has a natural remanent magnetization (NRM) four to five times higher than materials from the other core but the same magnetic susceptibility. Their magnetization is still higher than the surrounding crater-fill impact melt rocks. X-ray fluorescence data indicate a similar proportion of iron-rich phases in both cores and an enrichment in silicates within the altered core. In addition, alternating-field demagnetization results show that one main process remagnetized the rocks. These results support the hypothesis that intense and possibly localized post-impact hydrothermal alteration enhanced the magnetization of the clast-rich impact melt rocks by crystallization of magnetite within the center of the Haughton impact structure. Subsequent erosion was followed by in situ concentration in the subsurface leading to large magnetic gradient on surface.

^1,2Dieter Stöffler,^1,3Christopher Hamann,⁴Knut Metzler
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12912]
¹Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
²Humboldt-Universität zu Berlin, Berlin, Germany
³Institute of Geological Sciences, Freie Universität Berlin, Berlin, Germany
⁴Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany
Published by arrangement with John Wiley &

We reevaluate the systematics and geologic setting of terrestrial, lunar, Martian, and asteroidal “impactites” resulting from single or multiple impacts. For impactites derived from silicate rocks and sediments, we propose a unified and updated system of progressive shock metamorphism. “Shock-metamorphosed rocks” occur as lithic clasts or melt particles in proximal impactites at impact craters, and rarely in distal impactites. They represent a wide range of metamorphism, typically ranging from unshocked to shock melted. As the degree of shock metamorphism, at a given shock pressure, depends primarily on the mineralogical composition and the porosity of a rock or sediment sample, different shock classification systems are required for different types of planetary rocks and sediments. We define shock classification systems for eight rock and sediment classes which are assigned to three major groups of rocks and sediments (1) crystalline rocks with classes F, M, A, and U; (2) chondritic rocks (class C); and (3) sedimentary rocks and sediments with classes SR, SE, and RE. The abbreviations stand for felsic (F), mafic (M), anorthositic (A), ultramafic (U), sedimentary rocks (SR), unconsolidated sediments (SE), and regoliths (RE). In each class, the progressive stages of shock metamorphism are denominated S1 to Sx. These progressive shock stages are introduced as: S1–S7 for F, S1–S7 for M, S1–S6 for A, S1–S7 for U, S1–S7 for C, S1–S7 for SR, S1–S5 for SE, and S1–S6 for RE. S1 stands for “unshocked” and Sx (variable between S5 and S7) stands for “whole rock melting.” We propose a sequence of symbols characterizing the degree of shock metamorphism of a sample, i.e., F-S1 to F-S7 with the option to add the tabulated pressure ranges (in GPa) in parentheses.