During the last week of this year, Cosmochemistry Papers will be on Christmas break. Normal service will commence on January, 3nd, after we have recovered.

Merry Christmas & Happy New Year to everyone and see you in 2018 !

¹S. Holm-Alwmark,²L. Ferrière,¹C. Alwmark,³M. H. Poelchau
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13029]
¹Department of Geology, Lund University, Lund, Sweden
²Natural History Museum, Vienna, Austria
³Institute of Earth and Environmental Sciences, Universität Freiburg, Freiburg, Germany
Published by arrangement with John Wiley & Sons

Planar deformation features (PDFs) in quartz are the most widely used indicator of shock metamorphism in terrestrial rocks. They can also be used for estimating average shock pressures that quartz-bearing rocks have been subjected to. Here we report on a number of observations and problems that we have encountered when performing universal stage measurements and crystallographically indexing of PDF orientations in quartz. These include a comparison between manual and automated methods of indexing PDFs, an evaluation of the new stereographic projection template, and observations regarding the PDF statistics related to the c-axis position and rhombohedral plane symmetry. We further discuss the implications that our findings have for shock barometry studies. Our study shows that the currently used stereographic projection template for indexing PDFs in quartz might induce an overestimation of rhombohedral planes with low Miller–Bravais indices. We suggest, based on a comparison of different shock barometry methods, that a unified method of assigning shock pressures to samples based on PDFs in quartz is necessary to allow comparison of data sets. This method needs to take into account not only the average number of PDF sets/grain but also the number of high Miller–Bravais index planes, both of which are important factors according to our study. Finally, we present a suggestion for such a method (which is valid for nonporous quartz-bearing rock types), which consists of assigning quartz grains into types (A–E) based on the PDF orientation pattern, and then calculation of a mean shock pressure for each sample.

¹Devin L. Schrader, ²Kazuhide Nagashima, ³Scott R. Waitukaitis, ⁴Jemma Davidson, ⁵Timothy J. McCoy, ^6,7Harold C. Connolly Jr., ⁷Dante S. Lauretta
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.12.014]
¹Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
²Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
³Leiden University, Huygens Laboratory, Niels Bohrweg 2, 2333 CA Leiden, Netherlands
⁴Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, District of Columbia 20015–1305, USA
⁵Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10[th] & Constitution Avenue NW, Washington, DC 20560-0119, USA
⁶Department of Geology, School of Earth & Environment, Rowan University, 201 Mullica Hill Road, Glassboro, NJ 08028, USA
⁷Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA
Copyright Elsevier

By investigating the in situ chemical and O-isotope compositions of olivine in lightly sintered dust agglomerates from the early Solar System, we constrain their origins and the retention of dust in the protoplanetary disk. The grain sizes of silicates in these agglomeratic olivine (AO) chondrules indicate that the grain sizes of chondrule precursors in the Renazzo-like carbonaceous (CR) chondrites ranged from <1 to 80 µm. We infer this grain size range to be equivalent to the size range for dust in the early Solar System. AO chondrules may contain, but are not solely composed of, recycled fragments of earlier formed chondrules. They also contain ¹⁶O-rich olivine related to amoeboid olivine aggregates and represent the best record of chondrule-precursor materials.

AO chondrules contain one or more large grains, sometimes similar to FeO-poor (type I) and/or FeO-rich (type II) chondrules, while others contain a type II chondrule core. These morphologies are consistent with particle agglomeration by electrostatic charging of grains during collision, a process that may explain solid agglomeration in the protoplanetary disk in the micrometer size regime. The petrographic, isotopic, and chemical compositions of AO chondrules are consistent with chondrule formation by large-scale shocks, bow shocks, and current sheets.

The petrographic, isotopic, and chemical similarities between AO chondrules in CR chondrites and chondrule-like objects from comet 81P/Wild 2 indicate that comets contain AO chondrules. We infer that these AO chondrules likely formed in the inner Solar System and migrated to the comet forming region at least 3 Ma after the formation of the first Solar System solids. Observations made in this study imply that the protoplanetary disk retained a dusty disk at least ∼3.7 Ma after the formation of the first Solar System solids, longer than half of the dusty accretion disks observed around other stars.

¹Andreas T. Hertwig, ¹Céline Defouilloy, ¹Noriko T. Kita
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.12.013]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
Copyright Elsevier

Oxygen three-isotope analysis by secondary ion mass spectrometry of chondrule olivine and pyroxene in combination with electron microprobe analysis were carried out to investigate 24 FeO-poor (type I) and 2 FeO-rich (type II) chondrules from the Kaba (CV) chondrite. The Mg#’s of olivine and pyroxene in individual chondrules are uniform, which confirms that Kaba is one of the least thermally metamorphosed CV3 chondrites. The majority of chondrules in Kaba contain olivine and pyroxene that show indistinguishable Δ17O values (= δ17O – 0.52 × δ18O) within analytical uncertainties, as revealed by multiple spot analyses of individual chondrules. One third of chondrules contain olivine relict grains that are either 16O-rich or 16O-poor relative to other indistinguishable olivine and/or pyroxene analyses in the same chondrules. Excluding those isotopically recognized relicts, the mean oxygen isotope ratios (δ18O, δ17O, and Δ17O) of individual chondrules are calculated, which are interpreted to represent those of the final chondrule melt. Most of these isotope ratios plot on or slightly below the primitive chondrule mineral (PCM) line on the oxygen three-isotope diagram, except for the pyroxene-rich type II chondrule that plots above the PCM and on the terrestrial fractionation line. The Δ17O values of type I chondrules range from ∼ –8‰ to ∼ –4‰; the pyroxene-rich type II chondrule yields ∼0‰, the olivine-rich type II chondrule ∼ –2‰. In contrast to the ungrouped carbonaceous chondrite Acfer 094, the Yamato 81020 CO3, and the Allende CV3 chondrite, type I chondrules in Kaba only possess Δ17O values below –3‰ and a pronounced bimodal distribution of Δ17O values, as evident for those other chondrites, was not observed for Kaba.

Investigation of the Mg#-Δ17O relationship revealed that Δ17O values tend to increase with decreasing Mg#’s, similar to those observed for CR chondrites though data from Kaba cluster at the high Mg# (>98) and the low Δ17O end (–6‰ and –4‰). A mass balance model involving 16O-rich anhydrous dust (Δ17O = –8‰) and 16O-poor water ice (Δ17O = +2‰) in the chondrule precursors suggests that type I chondrules in Kaba would have formed in a moderately high dust enriched protoplanetary disk at relatively dry conditions (∼50-100× dust enrichment compared to Solar abundance gas and less than 0.6× ice enhancement relative to CI chondritic dust). The olivine-rich type II chondrule probably formed in a disk with higher dust enrichment (∼2000× Solar).

¹David G. Weisz, ¹Benjamin Jacobsen, ¹Naomi E. Marks, ¹Kim B. Knight, ¹Brett H. Isselhardt, ¹Jennifer E. Matzel
Geochimica et Cosmochimica Acta (in Press) link to Article [https://doi.org/10.1016/j.gca.2017.12.011]
¹Glenn T. Seaborg Institute, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
Copyright Elsevier

Aerodynamically-shaped glassy fallout is formed when vapor phase constituents from the nuclear device are incorporated into molten carriers (i.e. fallout precursor materials derived from soil or other near-field environmental debris). The effects of speciation and diffusive transport of condensing constituents are not well defined in models of fallout formation. Previously we reported observations of diffuse micrometer scale layers enriched in Na, Fe, Ca, and ²³⁵U, and depleted in Al and Ti, at the interfaces of agglomerated fallout objects. Here, we derive the timescales of uranium mass transport in such fallout as it cools from 2500 K to 1500 K by applying a 1-dimensional planar diffusion model to the observed ²³⁵U/³⁰Si variation at the interfaces. By modeling the thermal transport between the fireball and the carrier materials, the time of mass transport is calculated to be <0.6 seconds, <1 second, <2 seconds, and <3.5 seconds for fireball yields of 0.1 kt, 1 kt, 10 kt, and 100 kt respectively. Based on the calculated times of mass transport, a maximum temperature of deposition of uranium onto the carrier material of ∼2200 K is inferred (1σ uncertainty of ∼200 K). We also determine that the occurrence of micrometer scale layers of material enriched in relatively volatile Na-species as well as more refractory Ca-species provides evidence for an oxygen-rich fireball based on the vapor pressure of the two species under oxidizing conditions. These results represent the first application of diffusion-based modeling to derive material transport, thermal environments, and oxidation-speciation in near-surface nuclear detonation environments.

¹V. Vinogradoff, ¹S. Bernard, ²C. Le Guillou, ¹L. Remusata
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.12.019]
¹Institut de Minéralogie, Physique des Matériaux et Cosmochimie, IMPMC, Sorbonne Universités, CNRS UMR 7590, Muséum National d’Histoire naturelle, MNHN, UPMC, IRD UMR 206, Paris, France
²Unité Matériaux et Transformations, UMET, UMR CNRS 8207, Université Lille 1, France
Copyright Elsevier

Carbonaceous chondrites (CC) contain a diversity of organic compounds. No definitive evidence for a genetic relationship between these complex organic molecules and the simple organic molecules detected in the interstellar medium (ISM) has yet been reported. One of the many difficulties arises from the transformations of organic compounds during accretion and hydrothermal alteration on asteroids. Here, we report results of hydrothermal alteration experiments conducted on a common constituent of interstellar ice analogs, Hexamethylenetetramine (HMT – C₆H₁₂N₄). We submitted HMT to asteroidal hydrothermal conditions at 150°C, for various durations (up to 31 days) and under alkaline pH. Organic products were characterized by gas chromatography mass spectrometry, infrared spectroscopy and synchrotron-based X-ray absorption near edge structure spectroscopy. Results show that, within a few days, HMT has evolved into (1) a very diverse suite of soluble compounds dominated by N-bearing aromatic compounds (> 150 species after 31 days), including for instance formamide, pyridine, pyrrole and their polymers (2) an aromatic and N-rich insoluble material that forms after only 7 days of experiment and then remains stable through time. The reaction pathways leading to the soluble compounds likely include HMT dissociation, formose and Maillard-type reactions, e.g. reactions of sugar derivatives with amines. The present study demonstrates that, if interstellar organic compounds such as HMT had been accreted by chondrite parent bodies, they would have undergone chemical transformations during hydrothermal alteration, potentially leading to the formation of high molecular weight insoluble organic molecules. Some of the diversity of soluble and insoluble organic compounds found in CC may thus result from asteroidal hydrothermal alteration.

¹Dwijesh Ray, ¹Anil D.Shukla
Planetary and Space (in Press) Link to Article [https://doi.org/10.1016/j.pss.2017.11.005]
¹Physical Research Laboratory, Ahmedabad 380 009, India
Copyright Elsevier

Mukundpura is a new CM chondrite fell near Jaipur, Rajasthan, India on June 6, 2017 at 5:15 IST. The fall was observed by local villager. According to eyewitness, the meteorite was fragmented into several pieces once the object hit the ground. Based on petrography, mineralogy and bulk composition, Mukundpura is classified as CM2 chondrite. The chondrules are mainly similar to type I (Olivine: Fo99). Olivines are often found associated with pyroxene (Wo10-35En62-87Fs2-7) phenocryst. However, occurrences of forsteritic and fayalitic olivine (Fa58-71) as isolated mineral clast in matrix are not uncommon. Other types of chondrules include porphyritic pyroxene (En86Fs14) and barred olivine (Fa32.7±0.3) clast. Chondrules are commonly rimmed by fine-grained accretionary dust mantles. Phyllosilicates are the most dominant secondary mineral in matrix and largely associated with poorly characterised phases (PCP). FeO/SiO2 and S/SiO2 of PCP are 2.7 and 0.4 respectively. Other phases in matrix generally include calcite (pure CaCO3), Fe-Ni metal and sulphides. Spinel and perovskite occur occasionally as inclusions. The spherical or elliptical shaped metals (within chondrule or in isolated grains) are low-Ni type (kamacite <7.5 wt%) and resembles the solar Ni/Co ratio. However, Ni content in metal rarely exceeds 8.5 wt% (up to 23 wt%, taenite). Pyrrhotite (Fe ∼62 wt%; S ∼38 wt%) and pentlandite (Fe ∼31–33 wt%, Ni ∼28–32 wt%, S ∼33 wt%)) are the common sulphides occur as isolated grains within the matrix, however, the former is the most dominant. The bulk chemical composition of Mukundpura is largely similar to other CM type chondrite (e.g. Paris CM). Based on petrography, we infer a modest aqueous alteration stage for Mukundpura while the effect of thermal metamorphism was negligible.

¹O.Tschauner,²C. Ma,^3,4C. Prescher,³V. B. Prakapenka
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13009]
¹Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA
²Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
³Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, USA
⁴Institut für Geologie und Mineralogie, Universität zu Köln, Köln, Germany
Published by arrangement with John Wiley & Sons

Akimotoite (Mg,Fe)SiO₃ is one of the most common mineralogical indicators for high-level shock metamorphism in meteorites. First described 1997, its occurrence has been amply confirmed in a number of highly shocked chondrites. Yet, a thorough structure analysis of natural akimotoite has remained extant. Here we report accurate cell parameters, fractional atomic coordinates, and site occupancies for natural akimotoite from the holotype specimen based on synchrotron microdiffraction. The variation of unit cell shape and volume with Fe content define mixing volumes. Based on the mixing volume relation for akimotoite and hemleyite, we constrain the unit cell volume of endmember hemleyite to 273.8 ± 1.0 Å³. We show that mixing is nearly ideal for low Fe content but evolves to positive excess volume toward the Fe endmember. Based on this finding and the actual composition of akimotoite in Tenham, we show that this mineral has formed by solid–solid transformation prograde from enstatite, not by crystallization from melt.

^1,2Mario Trieloff,^1,2,3Ekaterina V. Korochantseva,^1,2,3Alexei I. Buikin,^1,2Jens Hopp,³Marina A. Ivanova,³Alexander V. Korochantsev
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13012]
¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Heidelberg, Germany
³Vernadsky Institute of Geochemistry, Moscow, Russia
Published by arrangement with John Wiley & Sons

We studied three lithologies (light and dark chondritic and impact melt rock) differing in shock stage from the LL5 chondrite Chelyabinsk. Using the 40Ar-39Ar dating technique, we identified low- and high-temperature reservoirs within all samples, ascribed to K-bearing oligoclase feldspar and shock-induced jadeite–feldspar glass assemblages in melt veins, respectively. Trapped argon components had variable 40Ar/36Ar ratios even within low- and high-temperature reservoirs of individual samples. Correcting for trapped argon revealed a lithology-specific response of the K-Ar system to shock metamorphism, thereby defining two distinct impact events affecting the Chelyabinsk parent asteroid (1) an intense impact event ~1.7 ± 0.1 Ga ago formed the light–dark-structured and impact-veined Chelyabinsk breccia. Such a one-stage breccia formation is consistent with petrological observations and was recorded by the strongly shocked lithologies (dark and impact melt) where a significant fraction of oligoclase feldspar was transformed into jadeite and feldspathic glass; and (2) a young reset event ~30 Ma ago particularly affected the light lithology due to its low argon retentivity, while the more retentive shock-induced phases were more resistant against thermal reset. Trapped argon with 40Ar/36Ar ratios up to 1900 was likely incorporated during impact-induced events on the parent body, and mixed with terrestrial atmospheric argon contamination. Had it not been identified via isochrons based on high-resolution argon extraction, several geochronologically meaningless ages would have been deduced.

¹C. Bu,¹G. Rodriguez Lopez,¹C. A. Dukes,²O. Ruesch,²L. A. McFadden,³J.-Y. Li
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13024]
¹Laboratory for Astrophysics and Surface Physics, University of Virginia, Charlottesville, Virginia, USA
²NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
³Planetary Science Institute, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

The formation of hydrated salts is an expected consequence of aqueous alteration of Main Belt objects, particularly for large, volatile-rich protoplanets like Ceres. Sulfates, present on water-bearing planetary bodies (e.g., Earth, Mars, and carbonaceous chondrite parent bodies) across the inner solar system, may contribute to Ceres’ UV and IR spectral signature along with phyllosilicates and carbonates. We investigate the presence and stability of hydrated sulfates under Ceres’ cryogenic, low-pressure environment and the consequent spectral effects, using UV–Vis–IR reflectance spectroscopy. H2O loss begins instantaneously with vacuum exposure, measured by the attenuation of spectral water absorption bands, and a phase transition from crystalline to amorphous is observed for MgSO4·6H2O by X-ray powder diffraction. Long-term (>40 h), continuous exposure of MgSO4·nH2O (n = 0, 6, 7) to low pressure (10−3–10−6 Torr) causes material decomposition and strong UV absorption below 0.5 μm. Our measurements suggest that MgSO4·6H2O grains (45–83 μm) dehydrate to 2% of the original 1.9 μm water band area over ~0.3 Ma at 200 K on Ceres and after ~42 Ma for 147 K. These rates, inferred from an Avrami dehydration model, preclude MgSO4·6H2O as a component of Ceres’ surface, although anhydrous and minimally hydrated sulfates may be present. A comparison between Ceres emissivity spectra and laboratory reflectance measurements over the infrared range (5–17 μm) suggests sulfates cannot be excluded from Ceres’ mineralogy.