¹Y. Ricard, ²D. Bercovici, ¹F. Albarède
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.12.020]
¹Université de Lyon, Ens de Lyon, CNRS, Université Lyon 1, Laboratoire de Sciences de la Terre, 15 parvis René Descartes, 69007, France
²Department of Geology & Geophysics, Yale University, PO Box 208109, New Haven, Connecticut, 06520-8109, USA
Copyright Elsevier

Although the mass distribution of planetesimals during the early stages of planetary formation has been discussed in various studies, this is not the case for their temperature distribution. Mass and temperature distributions are closely linked, since the ability of planetesimals to dissipate the heat produced by both radioactive decay and impacts is related to their size and hence mass. Here, we propose a simple model of the evolution of the joint mass-temperature distribution through a formalism that encompasses the classic statistical approach of Wetherill (1990). We compute the statistical distribution of planetesimals by using simple rules for aggregation. Although melting temperatures can be easily reached, the formation of molten planetary embryos requires that they be formed in only a few 100 kyr. Our aggregation model, which even ignores fragmentation during collision, predicts that planetesimals with radii less than approximately 20 km will not melt during their formation.

^1,2S. Maruyama, ^3,4,5M. Santosh, ¹S. Azuma
Geoscience Frontiers (in Press) Link to Article [http://dx.doi.org/10.1016/j.gsf.2016.11.009]
¹Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1, Ookayama-Meguro-ku, Tokyo 152-8550, Japan
²Institute for Study of the Earth’s Interior, Okayama University, 827 Yamada, Misasa, Tottori 682-0193, Japan
³Centre for Tectonics, Resources and Exploration, Department of Earth Sciences, University of Adelaide, SA 5005, Australia
⁴School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China
⁵Faculty of Science, Kochi University, Kochi 780-8520, Japan

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¹Q. B. Zhang, ²C. H. Braithwaite, ¹J. Zhao
Philosophical Transactions of the Royal Society A, 375 Link to Article [https://doi.org/10.1098/rsta.2016.0169]
¹Department of Civil Engineering, Monash University, Clayton, Victoria 3800, Australia
²Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK

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^1,2A. Agarwal, ²B. Reznik, ¹L. M. Alva-Valdivia, ³D. C. Srivastava
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12806]
¹Laboratorio de Paleomagnetismo, Instituto de Geofisica, Universidad Nacional Autónoma de México, Mexico DF, Mexico
²Division of Structural Geology and Tectonophysics, Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, Germany
³Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee, Uttar Pradesh, India
Published by arrangement with John Wiley & Sons

This paper reports peculiar alternating augite-plagioclase wedges in basement dolerites of Lockne impact structure, Sweden. The combined microscopic and spectroscopic studies of the micro/nanoscale wedges reveal that these are deformation-induced features. First, samples showing wedges, 12 out of 18 studied, are distributed in the impact structure within a radius of up to 10 km from the crater center. Second, the margins between the augite and labradorite wedges are sharp and the {110} prismatic cleavage of augite develops into fractures and thereafter into wedges. The fractures are filled with molten labradorite pushed from the neighboring bulk labradorite grain. Third, compared to the bulk labradorite, the dislocation density and the residual strain in the labradorite wedges are significantly higher. A possible mechanism of genesis of the wedges is proposed. The mechanism explains that passing of the shock waves in the basement dolerite induced (i) formation of microfractures in augite and labradorite; (ii) development of the augite prismatic cleavages into the wedges, which overprint the microfracture in the labradorite wedges; and (iii) thereafter, infilling of microfractures in the augite wedges by labradorite.

¹C. Lantz, ²R. Brunetto, ¹M.A. Barucci, ¹S. Fornasier, ²D. Baklouti, ³J. Bourçois, ³M. Godard
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.12.019]
¹Laboratoire d’Étude Spatiales et d’Instrumentation en Astrophysique (LESIA) – Observatoire de Paris, PSL Research University, CNRS (UMR 8109) / UPMC, Sorbonne Universités / Univ. Paris Diderot, Sorbonne Paris Cité, 92195 Meudon Cedex, France
²Institut d’Astrophysique Spatiale (IAS), UMR 8617 CNRS / Univ. Paris Sud, Univ. Paris-Saclay, Bâtiment 104, 91405 Orsay Cedex, France
³Centre de Sciences Nucléaire et de Sciences de la Matière (CSNSM), UMR 8609 CNRS/IN2P3 – Univ. Paris Sud, Univ. Paris-Saclay, Bâtiment 104, 91405 Orsay Cedex, France
Copyright Elsevier

We present an experimental study on ion irradiation of carbonaceous chondrites, simulating solar wind irradiation on primitive asteroids, to better constrain the space weathering processes of low albedo objects. The irradiations were performed on pressed pellets of the CV Allende, CO Frontier Mountain 95002 and Lancé, CM Mighei, CI Alais, and ungrouped Tagish Lake meteorites, as well as on some silicate samples (olivine and diopside). We used 40 keV He++ with fluences up to 6· 1016 ions/cm2 corresponding to timescales of 103-104 years for an object in the Main Belt. Reflectance spectra were acquired ex situ before and after irradiations in the visible to mid-infrared range (0.4 – 16 μm). Several spectral modifications are observed. In the MIR range, we observe a shift of the phyllosilicates (near 3 and 10 μm) and silicates (near 10 μm) bands toward longer wavelength. In the visible-NIR range, spectral darkening and reddening are observed for some samples, while others show spectral brightening and blueing. Results are also compared with previous irradiation on ordinary and carbonaceous chondrites. We find that the spectral modifications in the visible range are correlated with the initial albedo/composition. We propose a model for space weathering effects on low albedo objects, showing that those with initial albedo between 5 and 9 % shall not suffer SpWe effects in the visible range. These experiments provide new clues on spectroscopic features modifications within the visible-infrared ranges that could be detected in situ by future sample return missions (Hayabusa-2/JAXA and OSIRIS-REx/NASA).

¹Andreas Morlok, ¹Addi Bischoff, ¹Markus Patzek, ²Martin Sohn, ¹Harald Hiesinger
Icarus 284, 431-442 Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.11.030]
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, Münster 48149, Germany
²Hochschule Emden/Leer, Constantiaplatz 4, Emden 26723, Germany
Copyright Elsevier

In order to provide spectral ground truth data for remote sensing applications, we have measured mid-infrared spectra (2–18 µm) of three typical, well-defined lithologies from the Chelyabinsk meteorite that fell on February 15, 2013, near the city of Chelyabinsk, southern Urals, Russia. These lithologies are classified as (a) moderately shocked, light lithology, (b) shock-darkened lithology, and (c) impact melt lithology. Analyses were made from bulk material in four size fractions (0–25 µm, 25–63 µm, 63–125 µm, and 125–250 µm), and from additional thin sections.

Characteristic infrared features in the powdered bulk material of the moderately shocked, light lithology, dominated by olivine, pyroxene and feldspathic glass, are a Christiansen feature (CF) between 8.5 and 8.8 µm; a transparency feature (TF) in the finest size fraction at ∼13 µm, and strong reststrahlen bands (RB) at ∼9.1 µm, 9.5 µm, 10.3 µm, 10.8 µm, 11.2–11.3 µm, 12 µm, and between 16 and 17 µm. The ranges of spectral features for the micro-FTIR spots show a wider range than those obtained in diffuse reflectance, but are generally similar.

With increasing influence of impact shock from ‘pristine’ LL5 (or LL6) material (which have a low or moderate degree of shock) to the shock-darkened lithology and the impact melt lithology as endmembers, we observe the fading/disappearing of spectral features. Most prominent is the loss of a ‘twin peak’ feature between 10.8 and 11.3 µm, which turns into a single peak. In addition, in the ‘pure’ impact melt “endmember lithology” features at ∼9.6 µm and ∼9.1 µm are also lost. These losses are most likely correlated with decreasing amounts of crystal structure as the degree of shock melting increases. These changes could connect mid-infrared features with stages for shock metamorphism (Stöffler et al., 1991): Changes up to shock stage S4 would be minor, the shock darkened lithology could represent S5 and the impact melt lithology S6 and higher.

Similarities of the Chelyabinsk spectra to those of other LL chondrites indicate that the findings of this study could be related to this group of meteorites in general.

¹Philipp Gleißner, ¹Harry Becker
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.12.017]
¹Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstr. 74-100, 12249 Berlin, Germany
Copyright Elsevier

Fe-Ni metal-schreibersite-troilite intergrowths in Apollo 16 impact melt rocks and new highly siderophile element (HSE) and S abundance data indicate that millimeter-scale closed-system fractional crystallization processes during cooling of impactor-derived metal melt droplets in impact-melts are the main reason for compositional variations and strong differences in abundances and ratios of HSE in multiple aliquots from Apollo 16 impact melt rocks. Element ratios obtained from linear regression of such data are therefore prone to error, but weighted averages take into account full element budgets in the samples and thus represent a more accurate estimate of their impactor contributions. Modeling of solid metal-liquid metal partitioning in the Fe-Ni-S-P system and HSE patterns in impactites from different landing sites suggest that bulk compositions of ancient lunar impactites should be representative of impact melt compositions and that large-scale fractionation of the HSE by in situ segregation of solid metal or sulfide liquid in impact melt sheets most likely did not occur. The compositional record of lunar impactites indicates accretion of variable amounts of chondritic and non-chondritic impactor material and the mixing of these components during remelting of earlier ejecta deposits. The non-chondritic composition appears most prominently in some Apollo 16 impactites and is characterized by suprachondritic HSE/Ir ratios which increase from refractory to moderately volatile HSE and exhibit a characteristic enrichment of Ru relative to Pt. Large-scale fractional crystallization of solid metal from sulfur and phosphorous rich metallic melt with high P/S in planetesimal or embryo cores is currently the most likely process that may have produced these compositions. Similar materials or processes may have contributed to the HSE signature of the bulk silicate Earth (BSE).

¹G.J. MacPherson, ^1,2E.S. Bullock, ^3,4T.J. Tenner, ^3,5D. Nakashima, ³N.T. Kita, ^1,6M.A. Ivanova, ⁷A.N. Krot, ⁸M.I. Petaev, ⁸S.B. Jacobsen
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.12.006]
¹Dept. of Mineral Sciences, Museum of Natural History, Smithsonian Institution, Washington, DC, USA, 20560
²Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015
³WiscSIMS, University of Wisconsin, Madison, WI 53706, USA
⁴Los Alamos National Laboratory, Los Alamos, NM, 87545
⁵Tohoku University, Miyagi 980-8578, Japan
⁶Vernadsky Institute, Moscow, Kosygin St. 119991, Russia
⁷University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822, USA
⁸Harvard University, Cambridge, Massachusetts 02138, USA
Copyright Elsevier

In order to further elucidate possible temporal relationships between different varieties of calcium-, aluminum-rich inclusions (CAIs), we measured the aluminum-magnesium isotopic systematics of seven examples of the rare type known as forsterite-bearing Type B (FoB) inclusions from four different CV3 carbonaceous chondrites: Allende, Efremovka, NWA 3118, and Vigarano. The primary phases (forsterite, Al-Ti-rich diopside, spinel, melilite, and anorthite) in each inclusion were analyzed in situ using high-precision secondary ion mass-spectrometry (SIMS). In all cases, minerals with low Al/Mg ratios (all except anorthite) yield well-defined internal Al-Mg isochrons, with a range of initial 26Al/27Al ratios [(26Al/27Al)0] ranging from (5.30±0.22)×10−5 down to (4.17±0.43)×10−5. Anorthite in all cases is significantly disturbed relative to the isochrons defined by the other phases in the same CAIs, and in several cases contains no resolved excesses of radiogenic 26Mg (δ26Mg∗) even at 27Al/24Mg ratios greater than 1000. The fact that some FoBs preserve (26Al/27Al)0 of ∼ 5.2×10−5, close to the canonical value of (5.23±0.13)×10−5 inferred from bulk magnesium-isotope measurements of CV CAIs (Jacobsen et al., 2008), demonstrates that FoBs began forming very early, contemporaneous with other more-refractory CAIs. The range of (26Al/27Al)0 values further shows that FoBs continued to be reprocessed over ∼200,000 years of nebular history, consistent with results obtained for other types of igneous CAIs in CV chondrites. The absence of any correlation between of CAI+FoB formation or reprocessing times with bulk composition or CAI type means that there is no temporal evolutionary sequence between the diverse CAI types. The initial δ26Mg∗ value in the most primitive FoB (SJ101) is significantly lower than the canonical solar system value of −0.040±0.029‰.

^1,2Michelle S. Thompson,¹Thomas J. Zega,^3,4Jane Y. Howe
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12798]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
²NASA Johnson Space Center, Houston, Texas, USA
³Hitachi High-Technologies Canada Inc., Rexdale, Ontario, Canada
⁴Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario, Canada
Published by arrangement with John Wiley & Sons

We report the results of the first dynamic, in situ heating of lunar soils to simulate micrometeorite impacts on the lunar surface. We performed slow- and rapid-heating experiments inside the transmission electron microscope to understand the chemical and microstructural changes in surface soils resulting from space-weathering processes. Our slow-heating experiments show that the formation of Fe nanoparticles begins at ~575 °C. These nanoparticles also form as a result of rapid-heating experiments, and electron energy-loss spectroscopy measurements indicate the Fe nanoparticles are composed entirely of Fe0, suggesting this simulation accurately mimics micrometeorite space-weathering processes occurring on airless body surfaces. In addition to Fe nanoparticles, rapid-heating experiments also formed vesiculated textures in the samples. Several grains were subjected to repeated thermal shocks, and the measured size distribution and number of Fe nanoparticles evolved with each subsequent heating event. These results provide insight into the formation and growth mechanisms for Fe nanoparticles in space-weathered soils and could provide a new methodology for relative age dating of individual soil grains from within a sample population.

¹Heather Charles, ²Timothy Titus, ²Rosalyn Hayward, ²Christopher Edwards, ³Caitlin Ahrens
Earth and Planetary Science Letters 458, 152-160 Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.10.022]
¹USGS/NAU, United States
²USGS, United States
³University of Arkansas, United States
Copyright Elsevier

The composition of two dune fields, Ogygis Undae and the NE–SW trending dune field in Gale crater (the “Bagnold Dune Field” and “Western Dune Field”), were analyzed using thermal emission spectra from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and the Mars Odyssey Thermal Emission Imaging System (THEMIS). The Gale crater dune field was used as a baseline as other orbital compositional analyses have been conducted, and in situ sampling results will soon be available.

Results from unmixing thermal emission spectra showed a spatial variation between feldspar mineral abundances and pyroxene mineral abundances in Ogygis Undae. Other datasets, including nighttime thermal inertia values, also showed variation throughout the dune field. One explanation proposed for this variation is a bimodal distribution of two sand populations. This distribution is seen in some terrestrial dune fields.

The two dune fields varied in both mineral types present and in uniformity of composition. These differences point to different source lithologies and different distances travelled from source material. Examining these differences further will allow for a greater understanding of aeolian processes on Mars.