During the last week of this year, Cosmochemistry Papers will be on Christmas break. Normal service will commence on January, 2nd, after we have recovered.

Merry Christmas & Happy New Year to everyone and see you in 2017 !

¹Sota Arakawa, ¹Taishi Nakamoto
The Astrophysical Journal Letters, 832, L19 Link to Article [http://dx.doi.org/10.3847/2041-8205/832/2/L19]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

Several pieces of evidence suggest that silicate grains in primitive meteorites are not interstellar grains but condensates formed in the early solar system. Moreover, the size distribution of matrix grains in chondrites implies that these condensates might be formed as nanometer-sized grains. Therefore, we propose a novel scenario for rocky planetesimal formation in which nanometer-sized silicate grains are produced by evaporation and recondensation events in early solar nebula, and rocky planetesimals are formed via aggregation of these nanograins. We reveal that silicate nanograins can grow into rocky planetesimals via direct aggregation without catastrophic fragmentation and serious radial drift, and our results provide a suitable condition for protoplanet formation in our solar system.

^1,2Mathieu Roskosz, ^2,3Boris Laurent, ²Hugues Leroux, ¹Laurent Remusat
The Astrophysical Journal 832, 55 Link to Article [http://dx.doi.org/10.3847/0004-637X/832/1/55]
¹IMPMC, CNRS UMR 7590, Sorbonne Universités, Université Pierre et Marie Curie, IRD, Muséum National d’Histoire Naturelle, CP 52, 57 rue Cuvier, Paris F-75231, France
²Unité Matériaux et Transformations, Université Lille 1, CNRS UMR 8207, Bâtiment C6, F-59655 Villeneuve d’Ascq, France
³Present address: Department of Earth and Environmental Sciences, University of St. Andrews, Irvine Building, KY16 9AL, Fife, Scotland, UK.

The origin of hydrogen in chondritic components is poorly understood. Their isotopic composition is heavier than the solar nebula gas. In addition, in most meteorites, hydrous silicates are found to be lighter than the coexisting organic matter. Ionizing irradiation recently emerged as an efficient hydrogen fractionating process in organics, but its effect on H-bearing silicates remains essentially unknown. We report the evolution of the D/H of hydrous silicates experimentally irradiated by electrons. Thin films of amorphous silica, amorphous “serpentine,” and pellets of crystalline muscovite were irradiated at 4 and 30 keV. For all samples, irradiation leads to a large hydrogen loss correlated with a moderate deuterium enrichment of the solid residue. The entire data set can be described by a Rayleigh distillation. The calculated fractionation factor is consistent with a kinetically controlled fractionation during the loss of hydrogen. Furthermore, for a given ionizing condition, the deuteration of the silicate residues is much lower than the deuteration measured on irradiated organic macromolecules. These results provide firm evidence of the limitations of ionizing irradiation as a driving mechanism for D-enrichment of silicate materials. The isotopic composition of the silicate dust cannot rise from a protosolar to a chondritic signature during solar irradiations. More importantly, these results imply that irradiation of the disk naturally induces a strong decoupling of the isotopic signatures of coexisting organics and silicates. This decoupling is consistent with the systematic difference observed between the heavy organic matter and the lighter water typically associated with minerals in the matrix of most carbonaceous chondrites.

¹N. G. Rudraswami, ¹M. Shyam Prasad, ^1,2S. Dey, ¹D. Fernandes, ³J. M. C. Plane, ³W. Feng, ⁴S. Taylor, ³J. D. Carrillo-Sánchez
The Astrophysical Journal,831 197 Link to Article [http://dx.doi.org/10.3847/0004-637X/831/2/197]
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403004, India
²Indian Institute of Technology, Roorkee, Uttarakhand 247667, India
³School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
⁴Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, New Hampshire 03755-1290, USA

Antarctica micrometeorites (~1200) and cosmic spherules (~5000) from deep sea sediments are studied using electron microscopy to identify Mg-rich olivine grains in order to determine the nature of the particle precursors. Mg-rich olivine (FeO < 5wt%) in micrometeorites suffers insignificant chemical modification during its history and is a well-preserved phase. We examine 420 forsterite grains enclosed in 162 micrometeorites of different types—unmelted, scoriaceous, and porphyritic—in this study. Forsterites in micrometeorites of different types are crystallized during their formation in solar nebula; their closest analogues are chondrule components of CV-type chondrites or volatile rich CM chondrites. The forsteritic olivines are suggested to have originated from a cluster of closely related carbonaceous asteroids that have Mg-rich olivines in the narrow range of CaO (0.1–0.3wt%), Al2O3 (0.0–0.3wt%), MnO (0.0–0.3wt%), and Cr2O3 (0.1–0.7wt%). Numerical simulations carried out with the Chemical Ablation Model (CABMOD) enable us to define the physical conditions of atmospheric entry that preserve the original compositions of the Mg-rich olivines in these particles. The chemical compositions of relict olivines affirm the role of heating at peak temperatures and the cooling rates of the micrometeorites. This modeling approach provides a foundation for understanding the ablation of the particles and the circumstances in which the relict grains tend to survive.

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

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

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

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

^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.