^1,2Brendan A. Haas,^1,2Christine Floss,³Rhonda M. Stroud,^1,2Ryan C. Ogliore
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13438]
¹Laboratory for Space Sciences, Washington University, St. Louis, Missouri, 63130 USA
²Physics Department, Washington University, St. Louis, Missouri, 63130 USA
³Materials Science and Technology Division, Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, District of Columbia, 20375 USA
Published by arrangement with John Wiley & Sons

Aluminum foils from the Stardust cometary dust collector contain impact craters formed during the spacecraft’s encounter with comet 81P/Wild 2 and retain residues that are among the few unambiguously cometary samples available for laboratory study. Our study investigates four micron‐scale (1.8–5.2 μm) and six submicron (220–380 nm) diameter craters to better characterize the fine (<1 μm) component of comet Wild 2. We perform initial crater identification with scanning electron microscopy, prepare the samples for further analysis with a focused ion beam, and analyze the cross sections of the impact craters with transmission electron microscopy (TEM). All of the craters are dominated by combinations of silicate and iron sulfide residues. Two micron‐scale craters had subregions that are consistent with spinel and taenite impactors, indicating that the micron‐scale craters have a refractory component. Four submicron craters contained amorphous residue layers composed of silicate and sulfide impactors. The lack of refractory materials in the submicron craters suggests that refractory material abundances may differentiate Wild 2 dust on the scale of several hundred nanometers from larger particles on the scale of a micron. The submicron craters are enriched in moderately volatile elements (S, Zn) when normalized to Si and CI chondrite abundances, suggesting that, if these craters are representative of the Wild 2 fine component, the Wild 2 fines were not formed by high‐temperature condensation. This distinguishes the comet’s fine component from the large terminal particles in Stardust aerogel tracks which mostly formed in high‐temperature events.

¹Winfried H. Schwarz,¹Michael Hanel,¹Mario Trieloff
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13437]
¹Klaus‐Tschira‐Labor für Kosmochemie, Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234‐236, D‐69120 Heidelberg, Germany
Published by arrangement with John Wiley & Sons

In situ U‐Pb measurements on zircons of the Ries impact crater are presented for three samples from the quarry at Polsingen. The U‐Pb data of most zircons plot along a discordia line, leading to an upper intercept of Carboniferous age (331 ± 32 Ma [2σ]). Four zircons define a concordia age of 313.2 ± 4.4 Ma (2σ). This age most probably represents the age of a granite from the basement target rocks. From granular textured zircon grains (including baddeleyite and anatase/Fe‐rich phases, first identified in the Ries crater), most probably recrystallized after impact (13 analyses, 4 grains), a concordia age of 14.89 ± 0.34 Ma (2σ) and an error weighted mean ²⁰⁶Pb*/²³⁸U age of Ma 14.63 ± 0.43 (2σ) is derived. Including the youngest concordant ages of five porous textured zircon grains (24 spot analyses), a concordia age of 14.75 ± 0.22 Ma (2σ) and a mean ²⁰⁶Pb*/²³⁸U age of 14.71 ± 0.26 Ma (2σ) can be calculated. These results are consistent with previously published ⁴⁰Ar/³⁹Ar ages of impact glasses and feldspar. Our results demonstrate that even for relatively young impact craters, reliable U‐Pb ages can be obtained using in situ zircon dating by SIMS. Frequently the texture of impact shocked zircon grains is explained by decomposition at high temperatures and recrystallization to a granular texture. This is most probably the case for the observed granular zircon grains having baddeleyite/anatase/Fe‐rich phases. We also observe non‐baddeleyite/anatase/Fe‐rich phase bearing zircons. For these domains, reset to crater age is more frequently for high U,Th contents. We tentatively explain the higher susceptibility to impact resetting of high U,Th domains by enhanced Pb loss and mobilization due to higher diffusivity within former metamict domains that were impact metamorphosed more easily into porous as well as granular textures during decomposition and recrystallization, possibly supported by Pb loss during postimpact cooling and/or hydrothermal activity.

^1,2Toni Schulz,³Pavel P. Povinec,⁴Ludovic Ferrière,^5,6A. J. Timothy Jull,³Andrej Kováčik,³Ivan Sýkora,²Jonas Tusch,²Carsten Münker,⁴Dan Topa,^1,4Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13435]
¹Department of Lithospheric Research, University Vienna, Althanstrasse 14, 1090 Vienna, Austria
²Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Strasse 49b, 50674 Köln, Germany
³Faculty of Mathematics, Physics and Informatics, Department of Nuclear Physics and Biophysics, Comenius University, SK‐84248 Bratislava, Slovakia
⁴Natural History Museum, Burgring 7, 1010 Vienna, Austria
⁵Department of Geosciences, University of Arizona, Tucson, Arizona, 85721 USA
⁶Hungarian Academy of Sciences, Institute for Nuclear Research, ICER, 4026 Debrecen, Hungary
⁷MicroStep‐MIS, 84104 Bratislava, Slovakia
Published by arrangement with John Wiley & Sons

The Tissint meteorite fell on July 18, 2011 in Morocco and was quickly recovered, allowing the investigation of a new unaltered sample from Mars. We report new high‐field strength and highly siderophile element (HSE) data, Sr‐Nd‐Hf‐W‐Os isotope analyses, and data for cosmogenic nuclides in order to examine the history of the Tissint meteorite, from its source composition and crystallization to its irradiation history. We present high‐field strength element compositions that are typical for depleted Martian basalts (0.174 ppm Nb, 17.4 ppm Zr, 0.7352 ppm Hf, and 0.0444 ppm W), and, together with an extended literature data set for shergottites, help to reevaluate Mars’ tectonic evolution in comparison to that of the early Earth. HSE contents (0.07 ppb Re, 0.92 ppb Os, 2.55 ppb Ir, and 7.87 ppb Pt) vary significantly in comparison to literature data, reflecting significant sample inhomogeneity. Isotope data for Os and W (¹⁸⁷Os/¹⁸⁸Os = 0.1289 ± 15 and an ε¹⁸²W = +1.41 ± 0.46) are both indistinguishable from literature data. An internal Lu‐Hf isochron for Tissint defines a crystallization age of 665 ± 74 Ma. Considering only Sm‐Nd and Lu‐Hf chronometry, we obtain, using our and literature values, a best estimate for the age of Tissint of 582 ± 18 Ma (MSWD = 3.2). Cosmogenic radionuclides analyzed in the Tissint meteorite are typical for a recent fall. Tissint’s pre‐atmospheric radius was estimated to be 22 ± 2 cm, resulting in an estimated total mass of 130 ± 40 kg. Our cosmic‐ray exposure age of 0.9 ± 0.2 Ma is consistent with earlier estimations and exposure ages for other shergottites in general.

¹A. J. G. Jurewicz et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13439]
¹Center for Meteorite Studies, Arizona State University, m/c 6004, Tempe, Arizona, 85287 USA
Published by arrangement with John Wiley & Sons

NASA’s Genesis Mission returned solar wind (SW) to the Earth for analysis to derive the composition of the solar photosphere from solar material. SW analyses control the precision of the derived solar compositions, but their ultimate accuracy is limited by the theoretical or empirical models of fractionation due to SW formation. Mg isotopes are “ground truth” for these models since, except for CAIs, planetary materials have a uniform Mg isotopic composition (within ≤1‰) so any significant isotopic fractionation of SW Mg is primarily that of SW formation and subsequent acceleration through the corona. This study analyzed Mg isotopes in a bulk SW diamond‐like carbon (DLC) film on silicon collector returned by the Genesis Mission. A novel data reduction technique was required to account for variable ion yield and instrumental mass fractionation (IMF) in the DLC. The resulting SW Mg fractionation relative to the DSM‐3 laboratory standard was (−14.4‰, −30.2‰) ± (4.1‰, 5.5‰), where the uncertainty is 2ơ SE of the data combined with a 2.5‰ (total) error in the IMF determination. Two of the SW fractionation models considered generally agreed with our data. Their possible ramifications are discussed for O isotopes based on the CAI nebular composition of McKeegan et al. (2011).

¹A.‐J. Soini,¹I. T. Kukkonen,¹T. Kohout,²A. Luttinen
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13441]
¹Department of Geosciences and Geography, University of Helsinki, PO Box 64, FI‐00014 Helsinki, Finland
²Finnish Museum of Natural History, University of Helsinki, PO Box 44, FI‐00014 Helsinki, Finland
Published by arrangement with John Wiley & Sons

We report direct measurements of thermal diffusivity and conductivity at room temperature for 38 meteorite samples of 36 different meteorites including mostly chondrites, and thus almost triple the number of meteorites for which thermal conductivity is directly measured. Additionally, we measured porosity for 34 of these samples. Thermal properties were measured using an optical infrared scanning method on samples of cm‐sizes with a flat, sawn surface. A database compiled from our measurements and literature data suggests that thermal diffusivities and conductivities at room temperature vary largely among samples even of the same petrologic and chemical type and overlap among, for example, different ordinary chondrite classes. Measured conductivities of ordinary chondrites vary from 0.4 to 5.1 W m⁻¹ K⁻¹. On average, enstatite chondrites show much higher values (2.33–5.51 W m⁻¹ K⁻¹) and carbonaceous chondrites lower values (0.5–2.55 W m⁻¹ K⁻¹). Mineral composition (silicates versus iron‐nickel) and porosity control conductivity. Porosity shows (linear) negative correlation with conductivity. Variable conductivity is attributed to heterogeneity in mineral composition and porosity by intra‐ and intergranular voids and cracks, which are important in the scale of typical meteorite samples. The effect of porosity may be even more significant for thermal properties than that of the metal content in chondrites.

^1,2A. Losiak,³A. Jõeleht,³J. Plado,⁴M. Szyszka,³K. Kirsimäe,⁵E. M. Wild,⁵P. Steier,²C. M. Belcher,¹A. M. Jazwa,³R. Helde
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13431]
¹Planetary Geology Lab, Institute of Geological Sciences, Polish Academy of Sciences, Warsaw, Poland
²wildFIRE Lab, Hatherly Laboratories, University of Exeter, Exeter, UK
³Department of Geology, University of Tartu, Tartu, Estonia
⁴Institute of Geology, Adam Mickiewicz University in Poznań, Poznań, Poland
⁵VERA Laboratory, Faculty of Physics—Isotope Physics, University of Vienna, Vienna, Austria
Published by arrangement with John Wiley & Sons

The Ilumetsa site, in Estonia, consists of two round, rimmed structures that are 725 m apart. The structures are listed as proven impact craters in the Impact Earth database, despite lack of commonly accepted, unequivocal proof of extraterrestrial collision identified at this location. We excavated trenches though the Ilumetsa Large and Ilumetsa Small structures and found small pieces of charcoal within the putative proximal ejecta in both structures, in a similar geological setting as previously identified charcoal in Kaali (Losiak et al. 2016) and Morasko craters (see Szokaluk et al. 2019). Our ¹⁴C dating of charcoal allowed us to conclude that these crater‐like features formed simultaneously between 7170 and 7000 cal. years bp, about 7 ka after deglaciation of this area. A ground penetrating radar survey of the nearby bog shows that no additional Ilumetsa structures bigger than 40 m exist. Geochemical studies of the ejecta and a search using a metal detector did not reveal any clear indication of extraterrestrial material. This suggests Ilumetsa may have been formed by an impact of stony‐iron or stony body, which got significantly weathered in a wet‐temperate climate. The mystery of the formation of the structures at Ilumetsa remains; however, due to significant circumstantial evidence discussed herein, we are confident to call it a “probable” impact site.

¹Billy P. Glass,²Luigi Folco,²Matteo Masotta,^2,3Fabrizio Campanale
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13433]
¹Department of Geological Sciences, University of Delaware, Newark, Delaware, 19716 USA
²Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
³Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia (IIT), Piazza San Silvestro 12, 56127 Pisa, Italy
Published by arrangement with John Wiley & Sons

We examined 16 white opaque inclusions exposed on two polished slices of a Muong Nong‐type Australasian tektite from Muong Phin, Laos. The inclusions usually consist of a core, surrounded by a froth layer, and a quartz neoblast layer. The cores are composed primarily of a mixture of silica glass, coesite, and quartz in varying proportions. A thin (up to ~4 μm) layer of SiO₂‐poor glass enriched in FeO, MgO, CaO, Al₂O₃, and TiO₂ is observed as a bright halo in backscattered electron images around the quartz neoblasts and in places contains μm‐sized crystals, which may be Fe,Mg‐rich spinel. The distribution and textural relationships between the coesite‐bearing inclusions and the tektite matrix point to an in situ formation of the coesite due to an impact, rather than to infall, from a nearby impact, into tektite melt produced by the aerial burst of a bolide. The quartz neoblasts probably formed by crystallization of silica melt squeezed out of the inclusion core during the development of the froth layer. The bright halo may be the result of silica diffusing from the adjacent tektite melt into the growing quartz neoblasts. We propose that the survival of coesite was possible due to the froth layer that acted as a heat sink during bubble expansion and then as a thermal insulator.

^1,2Ryota Moriwaki,^3,4Tomohiro Usui,²Minato Tobita,²Tetsuya Yokoyama
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.014]
¹Planetary Exploration Research Center, Chiba Institute of Technology, Address: 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Address: 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
³Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Address: 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
⁴Earth-Life Science Institute, Tokyo Institute of Technology, Address: 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
Copyright Elsevier

Radiogenic isotopic compositions of shergottite meteorites suggest that early planetary differentiation processes, which are related to the crystallization of the Martian Magma Ocean (MMO), resulted in the geochemically heterogeneous Martian mantle. In order to understand the early geochemical evolution of Mars, we investigated the Pb isotope systematics in the depleted Martian mantle on the basis of the analyses of two geochemically depleted shergottites, Dar al Gani (DaG) 476 and Yamato 980459 (Y-980459). Their initial Pb isotopic compositions were estimated from geochemical analyses of highly leached acid residues and age-correction calculations using reference crystallization ages. This yielded μ-values (²³⁸U/²⁰⁴Pb) for the DaG 476 and Y-980459 source reservoirs of 2.33 ± 0.07 and 2.32 ± 0.06, respectively. These μ-values are distinct from those of other depleted shergottite source reservoirs (e.g., 1.4 ± 0.1 for the Tissint meteorite) and show a negative correlation with corresponding ¹⁴⁷Sm/¹⁴⁴Nd, ¹⁷⁶Lu/¹⁷⁷Hf, ɛ¹⁸²W, and ε¹⁴²Nd compositions. Such correlations between long- and short-lived isotopic signatures suggest that a geochemically heterogeneous depleted shergottite source mantle was formed on the early Mars. This geochemical heterogeneity would have been formed by variable mixing of depleted and enriched end-member components that originally formed by fractional crystallization in the MMO. Local remelting in the geochemically depleted Martian mantle after the crystallization of the MMO is another possible explanation for the formation of a geochemically heterogeneous depleted shergottite source mantle.

^1,2F.Jourdan,^1,2T.Kennedy,²G.K.Benedix,³E.Eroglu,¹C.Mayer
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.036]
¹Western Australian Argon Isotope Facility, John de Laeter Centre, TIGeR, Curtin University, Australia
²Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Australia
³Discipline of Chemical Engineering, WA School of Mines: Minerals, Energy and Chemical Engineering, Curtin University, Perth, WA 6845, Australia
Copyright Elsevier

Eucrites are extraterrestrial basalts and cumulate gabbros formed, and subsequently more or less metamorphosed, at the crustal level of the HED (Howardite-Eucrite-Diogenite) parent body, thought to be the asteroid 4 Vesta. Unbrecciated eucrites offer the best way to understand the igneous, metamorphic and cooling processes occurring in the crust of Vesta following accretion since they were not substantially affected/altered by secondary impact processes. The ⁴⁰Ar/³⁹Ar system of unbrecciated eucrites should be in a relatively pristine state, and thus can inform us on the early volcanic and thermal history of the HED parent body, and, in particular, the cooling history of various crustal parts below the ∼300 °C isotherm, which represents the average closure temperature of the Ar diffusion in plagioclase.

We analysed plagioclase and pyroxene (± groundmass) separates of two cumulate (Moore County and Moama), and five (Caldera, BTN 00300, EET 90020, GRA 98098, QUE 97053) equilibrated basaltic eucrites with the ⁴⁰Ar/³⁹Ar technique using a Thermo© ARGUS VI multi-collection mass spectrometer. The two cumulate unequilibrated gabbros also gave cooling ages of 4531 ± 11 Ma and 4533 ± 12 Ma and combined with a fast cooling rate estimated from lamella thicknesses, suggest that magmatic activity persisted up to 4533 ± 11 and 4535 ± 12 Ma and that the plutons were intruded in a relatively shallow part of the crust, above the metamorphosed regions. Four equilibrated eucrites yielded a well-defined cluster of ages between 4523 ± 8 Ma to 4514 ± 6 Ma. Those ages indicate when the part of the upper crust, where those eucrites probably resided (∼10-15 km deep), cooled below ∼300°C at a rate of 17.3 ± 3.6°C/Ma (2σ). Such a slow cooling rate combined with available global thermal models, supports the hypothesis of a global crustal metamorphism by burial and reheating of lava flows. Finally, an age of 4531 ± 5 Ma was obtained for metamorphosed eucrite EET 90020 and, combined with petrographic observations, indicates the age of a major crustal excavation event by impact. ⁴⁰Ar diffusion models suggest that it is possible to differentiate impact vs crustal cooling provided that a sufficient quantity of pyroxene is measured by ⁴⁰Ar/³⁹Ar.

¹K. Metzler,²D. C. Hezel,³J. Nellesen
The Astrophysical Journal 887, 230 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab58d0]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany
²Institut für Geologie und Mineralogie, University of Cologne, Zülpicher Straße 49a, D-50674 Köln, Germany
³Fakultät Maschinenbau, Technische Universität Dortmund, Leonhard-Euler-Str. 2, D-44227 Dortmund, Germany

Chondrules are approximately millimeter-sized beads of crystallized silicate melt. They formed mainly in the first ~3 Ma of the Sun’s protoplanetary disk and are the main constituents of chondritic asteroids. Here we report on the size–frequency distributions (2D and 3D) of chondrules in the brecciated ordinary chondrite (OC) Northwest Africa (NWA) 5205. We investigated three large (centimeter- to decimeter-sized) chondritic lithic clasts of a particular textural type (“cluster chondrite”) with eye-catching different chondrule sizes. One clast shows the largest mean chondrule size (~1.5 mm) ever measured in a chondrite. As in the other OCs, we find a positive correlation between the minimum and mean chondrule size, which we consider as an argument for chondrule size sorting. Chondrule size–frequency distributions in the clasts are distinctly more symmetric than the about log-normal distributions in other OCs. Furthermore, we find a co-enrichment of chondrule types with a priori small mean sizes (type I, porphyritic) in clasts with overall small mean chondrule sizes. We consider this as the fingerprint of an additional/second size-sorting process, which acted later on these chondrule populations. This process possibly subdivided a typical LL-type chondrule population into several subpopulations with different mean chondrule sizes. We speculate that this second sorting occurred in a unidirectional gas stream or headwind, e.g., by settling of chondrules through an asteroidal atmosphere or interaction with an expanding impact plume. Possibly, fine-grained matrix was almost completely removed by this, and the size-sorted chondrule subpopulations accreted in a hot state separately in different regions of the asteroid.