M. Weyrauch^a, J. Zipfel^b, S. Weyer^a
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.022]
^aInstitut für Mineralogie, Leibniz Universität Hannover, Callinstr. 3, 30167 Hannover, Germany
^bSenckenberg Forschungsinstitut und Naturmuseum Frankfurt, 60325 Frankfurt, Germany
Copyright Elsevier

The formation processes of the unusually metal-rich CB chondrites are a matter of debate. It is widely accepted that metal grains have formed by condensation. However, it is still debated whether they condensed directly from the solar nebula or from an impact-induced vapor plume. In this study, we present high precision Fe and Ni isotope and trace element composition of zoned and unzoned metal grains from the CBb chondrites Hammadah al Hamra 237, QUE 94411, and MAC 02675, and the CH/CBb breccia Isheyevo and unzoned metal from the CBa chondrites Bencubbin, Gujba, and NWA 4025. Data were obtained using femtosecond laser ablation (multicollector) inductively coupled plasma mass spectrometry (fs-LA-(MC)-ICP-MS). Zoned metal grains from CBb meteorites generally display parallel profiles of Ni and Fe isotope compositions with very low δ56Fe and δ60Ni, and elevated concentrations of refractory siderophile elements in their cores. These findings are consistent with dominantly kinetic isotope- and trace element fractionation during condensation from a confined and fast cooling gas reservoir. Tungsten and Mo are frequently depleted relative to other refractory elements, particularly in zoned metal grains, which is suggestive for elevated oxygen fugacities in the gas reservoir. Such conditions are indicative of the formation of these metal grains during an impact event.

Compared to zoned metal, unzoned metal grains are isotopically more homogeneous and more similar to the heavier rims of the zoned metal grains. This indicates that they formed under different conditions than the zoned metals, i.e., in a more slowly cooling environment. However, several unzoned grains still display significantly variable and correlated δ56Fe and δ60Ni, suggesting that their formation was related to that of the zoned metal grains. The kinetic fractionation-dominated isotopic signatures of the zoned metal grains strongly point to their formation during fast cooling, as may be expected for the exterior envelope of an impact plume. In contrast, the more homogenous isotopic signatures of the unzoned metal grains are more consistent with dominantly equilibrium-like isotope fractionation during condensation, as may be expected for the interior of an impact plume. In this scenario, the isotopically heavier rims of the zoned grains are best explained by a depletion of the outer plume gas reservoir in refractory elements and light isotopes. Accordingly, these findings indicate that zoned and unzoned metal grains likely formed during the same event. The compositional differences among individual unzoned metal grains, but also within some of the zoned grains, indicate turbulent gas mixing, also including movement of metals during their formation, between inner and outer regions of the impact plume.

Reggie L. Hudson
Astrophysical Journal 867, 160 Link to Article [DOI: 10.3847/1538-4357/aae584]
Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

As part of our work on nitrogen-rich ices, the IR spectra and band strengths used in a recent paper to identify and quantify radiation-induced changes in an N₂+H₂O ice near 15 K are examined, along with reports of (i) a chemical tracer for N₂+H₂O ices, (ii) a new IR feature of solid N₂, and (iii) a striking ¹⁵N isotopic enrichment. Problems are found for each IR band strength used and for each of the three claims made, to the extent that none are supported by the results presented to date. In contrast, new work presented here, combined with several older investigations, strongly supports the formation of di- and triatomic nitrogen oxides in irradiated N₂-rich ices. Observations and trends in the chemistry of N₂-rich icy solids are described, and conclusions are drawn. A considerable amount of material from previous chemical studies of N₂-rich systems, spanning more than a century, is brought together for the first time and used to examine the chemistry of N₂-rich ices in extraterrestrial environments. Needs are identified and suggestions made for future studies of N₂-rich interstellar and planetary ice analogs.

¹Reiners, P.W., ²Turchyn, A.V.
Geology 46, 863-866 Link to Article [DOI: 10.1130/G45040.1]
¹Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, United States
²Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, Cambridgeshire, CB2 3EQ, United Kingdom

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Devin L. Schrader^a, Roger R. Fu^b, Steven J. Desch^c, Jemma Davidson^a
Earth and Planetary Science Letters 504, 30-37 Link to Article [https://doi.org/10.1016/j.epsl.2018.09.030]
^aCenter for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287, United States of America
^bDepartment of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, United States of America
^cSchool of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287, United States of America
Copyright Elsevier

The background temperature of the protoplanetary disk is a fundamental but poorly constrained parameter that strongly influences a wide range of conditions and processes in the early Solar System, including the widespread process(es) by which chondrules originate. Chondrules, mm-scale objects composed primarily of silicate minerals, were formed in the protoplanetary disk almost entirely during the first four million years of Solar System history but their formation mechanism(s) are poorly understood. Here we present new constraints on the sub-silicate solidus cooling rates of chondrules at <873 K (600 °C) using the compositions of sulfide minerals. We show that chondrule cooling rates remained relatively rapid (∼10⁰ to 10¹ K/hr) between 873 and 503 K, which implies a protoplanetary disk background temperature of <503 K (230 °C) and is consistent with many models of chondrule formation by shocks in the solar nebula, potentially driven by the formation of Jupiter and/or planetary embryos, as the chondrule formation mechanism. This protoplanetary disk background temperature rules out current sheets and resulting short-circuit instabilities as the chondrule formation mechanism. More detailed modeling of chondrule cooling histories in impacts is required to fully evaluate impacts as a chondrule formation model. These results motivate further theoretical work to understand the expected thermal evolution of chondrules at ≤873 K under a variety of chondrule formation scenarios.

Iffat JABEEN^1,2, Minoru KUSAKABE^1,3, Keisuke NAGAO^1,4, and Arshad ALI^5,1
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13219]
¹Institute for Study of the Earth’s Interior (ISEI), Okayama University, Misasa, Tottori 682-0193, Japan
²Laboratory for Stable Isotope Science (LSIS), Earth Science Department, Western University, London, OntarioN6A 5B7, Canada
³University of Toyama, 3190 Gofuku, Toyama-shi 930-0855, Japan
⁴Division of Polar Earth-System Sciences, Korea Polar Research Institute (KOPRI), Incheon, Korea
⁵Earth Sciences Research Centre (ESRC), Sultan Qaboos University (SQU), Al-Khoudh, Muscat 123, Sultanate of Oman
Published by arrangement with John Wiley & Sons

Precise triple oxygen isotope compositions of 32 Allende bulk chondrules (ABCs) are determined using laser‐assisted fluorination mass spectrometry. Various chemically characterized chondrule types show ranges in δ¹⁸O that vary from −4.80‰ to +1.10‰ (porphyritic olivine; PO, N = 15), −3.10‰ to +1.50‰ (porphyritic olivine pyroxene; POP, N = 9), −3.40‰ to +2.60‰ (barred olivine; BO, N = 4), and −3.60‰ to +1.30‰ (porphyritic pyroxene; PP, N = 3). Oxygen isotope data of these chondrules yield a regression line referred to as the Allende bulk chondrule line (ABC line, slope = 0.86 ± 0.02). Most of our data fall closer to the primitive chondrule minerals line (PCM line, slope = 0.987 ± 0.013) and the carbonaceous chondrite anhydrous mineral line (CCAM line, slope = 0.94 ± 0.02) than the Allende anhydrous mineral line (AAML, slope = 1.00 ± 0.01) with a maximum δ¹⁸O value (+2.60‰) observed in a BO chondrule and a minimum δ¹⁸O value (−4.80‰) shown by a PO chondrule. Similarly, these chondrules depict variable ∆¹⁷O values that range from −5.65‰ to −3.25‰ (PO), −4.60‰ to −2.80‰ (POP), −4.95‰ to −3.00‰ (BO), −5.30‰ to −3.20‰ (PP), and −4.90‰ (CC). A simple model is proposed for the Allende CV3 chondrite with reference to the AAML and PCM line to illustrate the isotopic variations occurred due to the aqueous alteration processes. The estimated temperature ranging from 10 to 130 °C (mean ~60 °C) implies that the secondary mineralization in Allende happened in a warmer and relatively dry environment compared to Murchison. We further propose that thermal metamorphism could have dehydrated the Allende matrix at temperatures between >150 °C and <600 °C.

Tom G. Sharp^a, Erin L. Walton^b,c, Jinping Hu^d, Carl Agee^e
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.014]
^aArizona State University, School of Earth & Space Exploration, Tempe, AZ, 85287-1404, United States
^bMacEwan University, Department of Physical Sciences, Edmonton, AB, T5J 4S2, Canada
^cUniversity of Alberta, Department of Earth & Atmospheric Sciences, Edmonton, AB, T6G 2E3, Canada
^dCalifornia Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91106, United States
^eUniversity of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM, 87121-0001, United States
Copyright Elsevier

NWA 8159 is an augite-rich martian basalt, formed by cooling of a relatively evolved, Ca-rich, Ti-poor and LREE-depleted lava, under relatively oxidizing conditions, during the early Amazonian. In addition to its distinct igneous petrogenesis and high fO₂, NWA 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase (An_50-65). In this study, mineral transformations within and adjacent to shock veins in NWA 8159 were investigated using scanning electron microscopy, Raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite. The transformation of olivine to ahrensite (Fe-ringwoodite) along shock vein margins, and tissintite and coesite formed from igneous mineral (labradorite and silica) grains entrained as clasts within shock veins has been documented in this study. We report on a previously unidentified mineral assemblage of Ca-Na-majoritic garnet, sodic-clinopyroxene and stishovite crystallized from shock melt. This mineral assemblage indicates a crystallization pressure of approximately 16 GPa, which is within the range of previous shock pressure estimates for this meteorite (15–23 GPa). The presence of a majoritic garnet-bearing assemblage throughout veins up to 0.6 mm wide indicates that the sample remained at high-pressure throughout the melt vein quench. Based on thermal models, the sample must have remained at high pressure for ∼100 ms. This shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as Tissint or Zagami (>30 GPa; 10–20 ms) and would seem to imply a relatively large impact event. Recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact. The shock conditions experienced by NWA 8159 therefore do not require an impact event distinct from other shergottites. Rather, our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site. The shock history of NWA 8159 provides a picture of Mars consistent with that derived from remote observation; that of a random cratering process that samples a geologically long-lived and complex planet.

Axel WITTMANN¹, Randy L. KOROTEV¹, Bradley L. JOLLIFF¹, and Paul K. CARPENTER¹
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13217]
¹Eyring Materials Center, Arizona State University, 901 S. Palm Walk, PSA 213, Tempe, Arizona 85287–1704, USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri63130, USA
Published by arrangement with John Wiley & Sons

Magnesium‐rich spinel assemblages occur in the two lunar vitric breccia meteorites—Dhofar (Dho) 1528 and Graves Nunataks (GRA) 06157. Dho 1528 contains up to ~0.7 mm cumulate Mg‐rich spinel crystals associated with Mg‐rich olivine, Mg‐ and Al‐rich pyroxene, plagioclase, and rare cordierite. Using thermodynamic calculations of these mineral assemblages, we constrain equilibration depths and discuss an origin of these lithologies in the upper mantle of the Moon. In contrast, small, 10 to 20 μm spinel phenocryst assemblages in glassy melt rock clasts in Dho 1528 and GRA 06157 formed from the impact melting of Mg‐rich rocks. Some of these spinel phenocrysts match compositional constraints for spinel associated with “pink spinel anorthosites” inferred from remote sensing data. However, such spinel phenocrysts in meteorites and Apollo samples are typically associated with significant amounts of olivine ± pyroxene that exceed the compositional constraints for pink spinel anorthosites. We conclude that the remotely sensed “pink spinel anorthosites” have not been observed in the collections of lunar rocks. Moreover, we discuss impact‐excavation scenarios for the spinel‐bearing assemblages in Dhofar 1528 and compare the bulk rock composition of Dho 1528 to strikingly similar compositions of Luna 20 samples that contain ejecta from the Crisium impact basin.

Fred J. Ciesla¹, Sebastiaan Krijt¹, Reika Yokochi¹, and Scott Sandford²
Astrophysical Journal 867, 146 Link to Article [DOI: 10.3847/1538-4357/aae1a7]
¹Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL, USA
²NASA Ames Research Center, Moffett Field, CA, USA

Amorphous ice has long been invoked as a means for trapping extreme volatiles into solids, explaining the abundances of these species in comets and planetary atmospheres. Experiments have shown that this trapping is possible and has been used to estimate the abundances of each species in primitive ices after they have formed. However, these experiments have been carried out at deposition rates that exceed those expected in a molecular cloud or solar nebula by many orders of magnitude. Here, we develop a numerical model that reproduces the experimental results and apply it to those conditions expected in molecular clouds and protoplanetary disks. We find that two regimes of ice trapping exist: burial trapping, where the ratio of trapped species to water in the ice reflects that same ratio in the gas; and equilibrium trapping, where the ratio in the ice depends only on the partial pressure of the trapped species in the gas. The boundary between these two regimes is set by both the temperature and rate of ice deposition. These effects must be accounted for when determining the source of trapped volatiles during planet formation.

¹Christopher M.Mauney, ¹Davide Lazzati
Molecular Astrophysics 12, 1-9 Link to Article [https://doi.org/10.1016/j.molap.2018.03.002]
¹Oregon State University, USA

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MacArthur^a et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.026]
^aInstitute for Space & Earth Observation, Dept. of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK
Copyright Elsevier

Martian meteorite Northwest Africa (NWA) 8114 – a paired stone to NWA 7034 – provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe³⁺/ΣFe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 °C for at least 7 days in an oxidising regolith environment.

We report an approximate ⁴⁰Ar-³⁹Ar maximum age of 1.13 Ga to 1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe³⁺/ΣFe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket.

High temperatures, above 700 °C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days.

Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite.