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.

¹S. W. Lehner,²P. Németh,^3,4M. I. Petaev,¹P. R. Buseck
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12940]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
²Institute of Materials and Environmental Chemistry, Research Center for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary
³Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
⁴Solar, Stellar, and Planetary Sciences, Harvard-Smithsonian CfA, Cambridge, Massachusetts, USA
Published by arrangement with John Wiley & Sons

Two new occurrences of porous, S-bearing, amorphous silica are described within metal-sulfide nodules (MSN) and as interchondrule patches in EH3 chondrites SAH 97072 and ALH 84170. This porous amorphous material, which was first reported from sulfide-bearing chondrules, consists of sinewy SiO2-rich areas containing S with minor Na or Ca as well as Fe, Mg, and Al. Some pores contain minerals including pyrite, pyrrhotite, and anhydrite. Most pores appear vacant or contain unidentified material that is unstable under analytical conditions. Niningerite, olivine, enstatite, albite, and kumdykolite occur enclosed within porous silica patches. Porous silica is commonly interfingered with cristobalite suggesting its amorphous structure resulted from high-temperature quenching. We interpret the S-bearing porous silica to be a product of silicate sulfidation, and the Na, Ca, Fe, Mg, and Al detectable within this material are chemical residues of sulfidized silicates and metal. The occurrence of porous silica in the cores of MSN, which are considered to be pre-accretionary objects, suggests the sulfidizing conditions occurred prior to final parent-body solidification. Ubiquitous S-bearing porous silica among sulfide-bearing chondrules, MSN, and in the interchondrule clastic matrix, suggests that similar sulfidizing conditions affected all the constituents of these EH3 chondrites.

¹Jérome Gattacceca,^2,3Agata M. Krzesińska,⁴Yves Marrocchi,⁵Matthias M. M. Meier,⁶Michèle Bourot-Denise,⁷Rob Lenssen
Meteoritics & Planetary Science (in Press) Link top Article [DOI: 10.1111/maps.12942]
¹CNRS, Aix-Marseille Univ, IRD, Coll France, CEREGE, Aix-en-Provence, France
²Department of Earth Sciences, Natural History Museum, London, UK
³Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland
⁴CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-les-Nancy, France
⁵ETH Zurich, Institute of Geochemistry and Petrology, Zurich, Switzerland
⁶IMPMC, MNHN, UPMC, UMR CNRS 7590, Paris, France
⁷Private meteorite collector, The Netherlands
Published by agreement with John Wiley & Sons

Polymict chondritic breccias—rocks composed of fragments originating from different chondritic parent bodies—are of particular interest because they give insights into the mixing of asteroids in the main asteroid belt (occurrence, encounter velocity, transfer time). We describe Northwest Africa (NWA) 5764, a brecciated LL6 chondrite that contains a >16 cm3 L4 clast. The L clast was incorporated in the breccia through a nondestructive, low-velocity impact. Identical cosmic-ray exposure ages of the L clast and the LL host (36.6 ± 5.8 Myr), suggest a short transfer time of the L meteoroid to the LL parent body of 0.1 ± 8.1 Myr, if that meteoroid was no larger than a few meters. NWA 5764 (together with St. Mesmin, Dimmitt, and Glanerbrug) shows that effective mixing is possible between ordinary chondrite parent bodies. In NWA 5764 this mixing occurred after the peak of thermal metamorphism on the LL parent body, i.e., at least several tens of Myr after the formation of the solar system. The U,Th-He ages of the L clast and LL host, identical at about 2.9 Ga, might date the final assembly of the breccia, indicating relatively young mixing in the main asteroid belt as previously evidenced in St. Mesmin.

¹S. P. Beard, ¹T. D. Swindle
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12939]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

We use cosmic-ray exposure (CRE) ages of ureilites, combined with magnesium numbers of olivine, and oxygen isotopes, to search for evidence of specific source events initiating exposure for groups of ureilites. This technique can also be used to investigate the heterogeneity of the body from which the samples were derived. There are a total of 39 ureilites included in our work, which represents the largest collection of ureilite CRE age data used to date. Although we find some evidence of possible clusters, it is clear that most ureilites did not originate in one or two events on a homogeneous parent body.

¹Tasha L. Dunn, ^2,3,4Juliane Gross
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12946]
¹Department of Geology, Colby College, Waterville, Maine, USA
²Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
⁴Lunar and Planetary Institute, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

The single parent body model for the CV and CK chondrites (Greenwood et al. 2010) was challenged by Dunn et al. (2016a), who argued that magnetite compositions could not be reconciled by a single metamorphic sequence (i.e., CV3 → CK3 → CK4–6). Cr isotopic compositions, which are distinguishable between the CV and CK chondrites, also support two different parent bodies (Yin et al. 2017). Despite this, there are many petrographic and mineralogical similarities between the unequilibrated (petrologic type 3) CK chondrites and the CV chondrites (also type 3), which may result in misclassification of samples. Hart and Northwest Africa 6047 (NWA 6047) are an excellent example of this. In this study, we revisit the classification of Hart and NWA 6047 using magnetite compositions, petrography, and compositions of olivine, the most ubiquitous mineral in both CV and CK chondrites. Not only do our results suggest that NWA 6047 and Hart were misclassified, but our assessment of CV and CK3 chondrites has also led to the development of criteria that can be used to distinguish between CV and CK3 chondrites. These criteria include: abundances of Cr₂O₃, TiO₂, NiO, and Al₂O₃ in magnetite; Fa content and NiO abundance of matrix olivine; FeO content of chondrules; and the chondrule:matrix ratio. Classification as a CV chondrite is also supported by the presence of igneous chondrule rims, calcium-aluminum-rich inclusions, and an elongated petrofabric. However, none of these petrographic characteristics can be used conclusively to distinguish between CV and CK3 chondrites.

H. McSween Jr. et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12947]
¹Department of Earth & Planetary Sciences and Planetary Science Institute, University of Tennessee, Knoxville, Tennessee, USA
Published by arrangement with John Wiley & Sons

The mineralogy and geochemistry of Ceres, as constrained by Dawn’s instruments, are broadly consistent with a carbonaceous chondrite (CM/CI) bulk composition. Differences explainable by Ceres’s more advanced alteration include the formation of Mg-rich serpentine and ammoniated clay; a greater proportion of carbonate and lesser organic matter; amounts of magnetite, sulfide, and carbon that could act as spectral darkening agents; and partial fractionation of water ice and silicates in the interior and regolith. Ceres is not spectrally unique, but is similar to a few other C-class asteroids, which may also have suffered extensive alteration. All these bodies are among the largest carbonaceous chondrite asteroids, and they orbit in the same part of the Main Belt. Thus, the degree of alteration is apparently related to the size of the body. Although the ammonia now incorporated into clay likely condensed in the outer nebula, we cannot presently determine whether Ceres itself formed in the outer solar system and migrated inward or was assembled within the Main Belt, along with other carbonaceous chondrite bodies.

¹Masaaki Miyahara, ²Eiji Ohtani, ^3,4Akira Yamaguchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.034]
¹Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan
²Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
³National Institute of Polar Research, Tokyo 190-8518, Japan
⁴Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo 190-8518, Japan
Copyright Elsevier

An impact event recorded in the Northwest Africa (NWA) 8275 LL7 ordinary chondrite was investigated based on high-pressure mineralogy of pervasive shock-melt veins present in the rock. NWA 8275 consists of olivine, low-Ca pyroxene, plagioclase (albite–oligoclase composition), and minor high-Ca pyroxene, K-feldspar, phosphate minerals, metallic Fe–Ni and iron sulfide. Plagioclase and K-feldspar grains near the shock-melt veins have transformed to amorphous, although no high-pressure polymorphs of olivine and pyroxene were identified in or adjacent the shock-melt veins. Raman spectroscopy and focused ion beam (FIB)-assisted transmission electron microscopy (TEM) observations reveal that plagioclase entrained around the center portion of the shock-melt veins has dissociated into a jadeite + coesite assemblage. Alternately stacked jadeite and coesite crystals occurred in the original plagioclase. On approaching the host rock/shock-melt vein, only jadeite is present. Based on the high-pressure polymorph assemblage, the shock pressure and temperature conditions recorded in the shock-melt veins are ∼3–12 GPa and ∼1973–2373 K, respectively. Following a Rankine–Hugoniot relationship, the impact velocity was at least ∼0.45–1.54 km/s. The duration of high-pressure and high-temperature (HPHT) conditions required for the albite dissociation reaction is estimated a maximum of ∼4–5 s using the phase transition rate of albite, implying that a body of up to ∼9–12 km across collided with the parent body of NWA 8275. The coexistence of jadeite and coesite, the latter of which rarely accompanies jadeite in shocked ordinary chondrites, as a dissociation product of albite requires relatively long duration HPHT conditions. Thus, the impact event recorded in NWA 8275 was likely caused by a larger-than-typical projectile.