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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