¹Michael P.Lucas,¹Nick Dygert,²Jialong Ren,^2,3Marc A.Hesse,²Nathaniel R.Miller,¹Harry Y.McSween
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.09.010]
¹Department of Earth & Planetary Sciences, University of Tennessee, 1621 Cumberland Ave., 602 Strong Hall, Knoxville, TN 37996
²Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX 78712
³Oden Institute of Computational Sciences and Engineering, University of Texas at Austin, 201 E 24th St., Austin, TX 78712
Copyright Elsevier

Ordinary chondrites (OCs) are variably thermally metamorphosed meteorites thought to originate from at least three different parent bodies (H, L, and LL) in the Main Belt of asteroids. The thermal evolutions of OC parent bodies are frequently explained by the onion shell model; however, a competing hypothesis is the fragmentation-reassembly model. The onion shell model proposes undisrupted, internally heated parent bodies with concentrically stratified thermal structure, and posits that OC petrologic types (i.e., 3 to 6) develop with increasing temperature and burial depth. In this model, petrologic types are inversely correlated with depth in the parent body, and cooling rate. The alternative fragmentation-reassembly model invokes catastrophic collisional disruption of parent bodies that initially possessed onion shell structures, followed by rapid reaccretion of hot fragments, forming rubble pile bodies. Fragmentation would result in fast cooling (quenching) of collisional fragments from the temperature experienced by the parent body at the time of collision. Discrimination between these two models may be possible via investigation of the thermal histories of OCs by application of geothermometry and geospeedometry, which are used to constrain the temperatures and rates through which igneous and metamorphic rock samples cool. Most published cooling rate data for OC parent bodies are based on methods that record rates through low closure temperatures (∼500-200 °C) rather than from peak metamorphic temperatures. Recently, a rare earth element (REE)-in-two-pyroxene thermometer has been shown to establish peak or magmatic temperatures (TREE; Liang et al. [2013]. GCA 102, 246-260) for rocks that cooled at moderate to fast geologic rates. We applied the REE-in-two-pyroxene method to determine peak temperatures for 18 OC samples (mostly type 6), in conjunction with conventional two-pyroxene thermometry (TBKN; Brey and Köhler [1990]. J. Pet. 31, 1353-1378) and Ca-in-olivine thermometry (TCa-Ol; Köhler and Brey [1990]. GCA 54, 2375-2388), to determine closure temperatures and estimate cooling rates for OC parent bodies. Inconsistent with slow cooling rates expected in an onion shell scenario, we obtain fast cooling at rates ≳0.5 °C/y from peak temperatures of ∼900 °C. Corroborating the TREE and TBKN measurements, TCa-Ol suggests that the OCs cooled through TCa-Ol closure temperatures (∼700 to 800 °C) at ∼10-2 to 10-1 °C/y. These cooling rates are three to six orders of magnitude faster than rates determined using methods sensitive to low temperature (≤500 °C) cooling (e.g., metallography, 40Ar–39Ar ages, 244Pu fission track). We developed a novel numerical thermal model that incorporates fragmentation of an initial onion shell body and reassembly into a rubble pile body that reproduces both the fast cooling from high temperatures and the slow cooling through low temperatures observed in chondritic meteorites. We hypothesize that OC parent bodies initially possessed onion shell thermal structures, but later experienced collisional breakup, then reaccreted rapidly to form thermally stable rubble-pile asteroids.

^1,2Daisuke Nakashima,^3,4Makoto Kimura,⁴Kouichi Yamada,⁵Takaaki Noguchi,^2,6Takayuki Ushikubo,²NorikoKita
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.09.003]
¹Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
²Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
³National Institute of Polar Research, Tokyo 190-8518, Japan
⁴Faculty of Science, Ibaraki University, Mito, Ibaraki, 310-8512, Japan
⁵Faculty of Arts and Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁶Kochi Institute for Core Sample Research, JAMSTEC, Monobe-otsu 200, Nankoku, Kochi 783-8502, Japan
Copyright Elsevier

We measured oxygen isotope ratios and major elemental compositions of non-porphyritic chondrules and lithic fragments with various textures and chemical compositions in the Asuka-881020 CH chondrite. The oxygen isotope ratios plot along the primitive chondrule mineral line with Δ17O (= δ17O – 0.52 × δ18O) values from ∼ –21‰ to +5‰. The Δ17O values increase with decreasing Mg# (= molar [MgO]/[MgO+FeO]%) from 99.6 to 58.5, similarly to the Δ17O-Mg# trends for the chondrules in other carbonaceous chondrites.
Most of the measured objects (non-porphyritic chondrules and lithic fragments) including chondrules analyzed in the previous studies are classified into three groups based on the Δ17O values and chemistry; the –2.3‰ group with FeO-poor compositions (the most abundant group), the +1.4‰ group with FeO-rich compositions, and the –6.3‰ group with FeO-poor compositions. Skeletal olivine and magnesian cryptocrystalline (MgCC) chondrules and MgCC chondrule fragments, which are the –2.3‰ group objects, may have formed via fractional condensation in the isotopically uniform gaseous environment with Δ17O of –2.3‰. When silica-normative materials condensed from gas at ∼ 1200 K, 16O-rich refractory solids, similar to Ca-Al-rich inclusions, were incorporated into the environment. The silica-normative materials that condensed onto the 16O-rich refractory solids were reheated at 1743 – 1968 K and formed cristobalite-bearing chondrules with Δ17O of ∼ –6‰. This scenario can explain the absence of silica-bearing chondrules in the –2.3‰ group and refractory element abundances in the cristobalite-bearing chondrules as high as those in the MgCC chondrules.
Refractory element abundances of the +1.4‰ group objects decrease from FeO-Al-rich and ferroan CC (FeCC) chondrules to FeCC chondrule fragments to FeNi metal-bearing to silica-bearing chondrules. This suggests the formation via fractional condensation in the isotopically uniform gaseous environment. The Δ17O values and FeO-rich compositions of this group could be explained by an addition of 16O-poor water ice as an oxidant to the relatively 16O-rich solids with Δ17O of –2.3‰, which may also explain existence of some MgCC chondrules and fragments with intermediate Δ17O values between –2.3‰ and +1.4‰. The immiscibility textures in the silica-bearing chondrules suggest a reheating event at a temperature of > 1968 K after condensation of silica-normative materials. Thus, the non-porphyritic chondrules and fragments in CH and CB chondrites, which are classified into three distinct Δ17O groups, require multiple chondrule-forming environments and heating events. Energy source for the heating events could be either impact plume and/or other dynamical processes in the protoplanetary disk, though a single heating event would not fully explain observed chemical and isotope signatures in these non-porphyritic chondrules.

¹Ruslan A.Mendybaev,^2,3Michiru Kamibayashi,⁴Fang-Zhen Teng,⁵Paul S.Savage,⁶R.Bastian Georg,¹Frank M.Richter,^2,3,7ShogoTachibana
Geochimica et Cosmochimca Acta (inPress) Link to Article [https://doi.org/10.1016/j.gca.2020.09.005]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637
²Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
³Department of Earth and Planetary Sciences, University of Tokyo, Tokyo 113-0033, Japan
⁴Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195
⁵School of Earth and Environmental Sciences, University of St. Andrews, St. Andrews, KY16 9AL, Scotland
⁶Water Quality Center, Trent University, Peterborough, ON K9L0G2, Canada
⁷Institute of Space and Astronautical Science, JAXA, Tokyo 252-0222, Japan
Copyright Elsevier

It is widely believed that the precursors of coarse-grained CAIs in chondrites are solar nebula condensates that were later reheated and melted to a high degree. Such melting under low-pressure conditions is expected to result in evaporation of moderately volatile magnesium and silicon and their mass-dependent isotopic fractionation. The evaporation of silicate melts has been extensively studied in vacuum laboratory experiments and a large experimental database on chemical and isotopic fractionations now exists. Nevertheless, it remains unclear if vacuum evaporation of CAI-like melts adequately describes the evaporation in the hydrogen-rich gas of the solar nebula. Here we report the results of a detailed experimental study on evaporation of a such melt at 1600°C in both vacuum and low-pressure hydrogen gas, using 1.5- and 2.5-mm diameter samples. The experiments show that although at 2×10−4 bar H2 magnesium and silicon evaporate ∼2.8 times faster than at 2×10−5 bar H2 and ∼45 times faster than in vacuum, their relative evaporation rates and isotopic fractionation factors remain the same. This means that the chemical and isotopic evolutions of all evaporation residues plot along a single evaporation trajectory regardless of experimental conditions (vacuum or low-PH2) and sample size. The independence of chemical and isotopic evaporation trajectories on PH2 of the surrounding gas imply that the existing extensive experimental database on vacuum evaporation of CAI-like materials can be safely used to model the evaporation under solar nebula conditions, taking into account the dependence of evaporation kinetics on PH2.
The experimental data suggest that it would take less than 25 minutes at 1600°C to evaporate 15–50% of magnesium and 5–20% of silicon from a 2.5-mm diameter sample in a solar nebula with PH2∼2×10−4 bar and to enrich the residual melt in heavy magnesium and silicon isotopes up to δ25Mg ∼ 5–10‰ and δ29Si ∼ 2–4‰. The expected chemical and isotopic features are compatible to those typically observed in coarse-grained Type A and B CAIs. Evaporation for ∼1 hour will produce δ25Mg ∼30–35‰ and δ29Si ∼10–15‰, close to the values in highly fractionated Type F and FUN CAIs. These very short timescales suggest melting and evaporation of CAI precursors in very short dynamic heating events. The experimental results reported here provide a stringent test of proposed astrophysical models for the origin and evolution of CAIs.

¹Mohammed, M.A.,²Saad, R.,²Ismail, N.A.,³Muhammad, S.B.,²Yusoh, R.,⁴Mokhtar, S.
Journal of the Earth and Space Physics 45, 77-87 Link to Article [DOI: 10.22059/jesphys.2019.266387.1007041]
¹Department of Physics, Faculty of Science, Federal University, Lafia, Nigeria
²Department of Geophysics, Faculty of Physics, University Sains Malaysia, Pinang, Malaysia
³Department of Physics, Faculty of Science, Usman Danfodio University, Sokoto, Nigeria
⁴Centre for Global Archeological Research, University Sains Malaysia, Pinang, Malaysia

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¹Emilie T. Dunham,¹Meenakshi Wadhwa,¹Steven J. Desch,¹Richard L. Hervig
Geostandards and Geoanalytical Research (in Press) Link to Article [https://doi.org/10.1111/ggr.12329]
¹School of Earth and Space Exploration, Arizona State University, 781 Terrace Mall, Tempe, AZ, 85287 USA

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

¹Bruce Hapke
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114105]
¹Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, PA, USA
Copyright Elsevier

The opposition effect is the sharp, narrow surge observed in the reflectance of a scattering medium near zero phase angle. Numerous observations and experiments have shown that the primary cause of the phenomenon in particulate media is coherent backscattering, in which wavelets traveling in opposite directions along chains of scatterers interfere constructively and generate the peak. A broader opposition surge caused by shadow hiding and preferential escape is also present, but is entangled with the incoherent continuum reflectance on which the coherent peak is superposed, making it difficult to identify and isolate. Theoretical models of media of independent scatterers predict that the angular width and shape of the coherent backscatter peak depend on the wavelength, porosity and particle size. It was hoped that remote measurements of the opposition effect would give information on the latter two quantities in planetary regoliths. However, observations and laboratory studies of media of large particles in contact with one another find little dependence on any of these quantities. Instead, these studies imply that the opposition effect in regolith-like media comes from reflection by short chains only a few scatterers long located on the surfaces of the particles of the medium, and that the lengths of these chains are proportional to the wavelength. Since the angular width of the peak is controlled by the ratio of the wavelength to the mean scattering chain length, the width is independent of wavelength. Because the wavelets never enter a particle, low albedo media can exhibit a strong coherent backscatter peak. Opposition effect peaks less than a degree wide on solar system bodies can imply an immature regolith; peaks several degrees wide imply a mature regolith.

^1,2,3S.Shkolyar,⁴E.Lalla,^4,5M.Konstantindis,⁶K.Cote,⁴M.G.Daly,⁷A.Steele
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114093]
¹Universities Space Research Association, Columbia, MD 21046, USA
²NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
³Blue Marble Space Institute of Science, Seattle, WA 98154, USA
⁴Centre for Research in Earth and Space Science, York University, Toronto M3J 1P3, ON, Canada
⁵Department of Mathematics and Statistics, York University, Toronto M3J 1P3, ON, Canada
⁶Department of Physics, University of Toronto, Toronto M5S 1A7, ON, Canada
⁷Earth and Planets Laboratory, Carnegie Institution of Washington, Washington, D.C 20015, USA
Copyright Elsevier

Combined UV Raman and laser-induced fluorescence (LIF) spectroscopy instruments will soon be launched onboard missions to planetary surfaces, including Mars, to search for biosignatures. However, the rare earth element Ce3+, found in many common and Mars-relevant minerals, can produce fluorescence features within the spectral window usually attributed to organic compounds in a LIF spectrum. This study explored the detection of Ce3+ as a biosignature mimicker using UV Raman-LIF mission instruments. We assessed how LIF spectra of a suite of synthetic CePO4 samples compare with those of organics, how varying concentrations of both Ce3+ and organics in Martian regolith simulant affect this comparison, and whether two additional data sets obtainable by combined UV Raman-LIF instruments, including time-resolved fluorescence decay lifetimes and Raman spectra, can distinguish Ce3+-containing samples from organics. Results showed that the dominant LIF features of Ce3+ (320 and 338 nm) are similar to those of the aromatic amino acid tryptophan (325 and 340 nm), even when Ce3+ samples were mixed in a Martian regolith simulant at a range of concentrations. Lifetimes were revealed to be 2–9 ns in Ce3+-containing samples, typical for organic fluorophores. These results support the erroneous interpretation that LIF spectra and lifetime values obtained on these samples constitute potential organic signatures. Raman spectroscopy results suggested that with UV laser excitation, Raman is unlikely to identify Ce-bearing samples due to strong absorption of Raman scattered energy by Ce3+. We conclude that biosignature searches using UV LIF and Raman spectroscopy instrumentation may encounter challenges with unambiguously distinguishing spectra of organic compounds from Ce-bearing compounds.

¹Alex M. Ruzicka,^2,3Jon M. Friedrich,¹Melinda L. Hutson,²Juliette W. Strasser,⁴Robert J. Macke,⁵Mark L. Rivers,⁶Richard C. Greenwood,⁷Karen Ziegler,¹Richard N. Pugh
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13567]
¹Department of Geology and Cascadia Meteorite Laboratory, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland, Oregon, 97201 USA
²Department of Chemistry, Fordham University, 441 East Fordham Road, Bronx, New York, 10458 USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, 79th Street at Central Park West, New York City, New York, 10024 USA
⁴Vatican Observatory, Vatican City, V‐00120 Italy
⁵Center for Advanced Radiation Sources, University of Chicago, Argonne, Illinois, 60439 USA
⁶Planetary Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
⁷Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87131 USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 8709 is a rare example of a type 3 ordinary chondrite melt breccia and provides critical information for the shock compaction histories of chondrites. An L3 protolith for NWA 8709 is inferred on the basis of oxygen isotope composition, elemental composition, diverse mineral chemistry, and overall texture. NWA 8709 is among the most strongly shocked type 3 chondrites known, and experienced complete melting of the matrix and partial melting of chondrules. Unmelted phases underwent FeO reduction and partial homogenization, with reduction possibly occurring by reaction of olivine and low‐Ca pyroxene with an S‐bearing gas that was produced by vaporization. Chondrules and metal grains became foliated by uniaxial compaction, and during compression, chondrules and fragments became attached to form larger clumps. This process, and possibly also melt incorporation into chondrules to cause “inflation,” may have contributed to anomalously large chondrule sizes in NWA 8709. The melt breccia character is attributed to strong shock affecting a porous precursor. Data‐model comparisons suggest that a precursor with 23% porosity that was impacted by a 3 km/s projectile could have produced the meteorite. The rarity of other type 3 ordinary chondrite melt breccias implies that the immediate precursors to such chondrites were lower in porosity than the NWA 8709 precursor, or experienced weaker shocks. Altogether, the data imply a predominantly “quiet” dynamical environment to form most type 3 ordinary chondrites, with compaction occurring in a series of relatively weak shock events.

¹Curtis D. Williams,¹Matthew E. Sanborn,²Céline Defouilloy,¹Qing-Zhu Yin,²Noriko T. Kita,³Denton S. Ebel,¹Akane Yamakawa,⁴Katsuyuki Yamashita
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [DOI:
https://doi.org/10.1073/pnas.2005235117]
¹Department of Earth and Planetary Sciences, University of California, Davis, CA 95616;
²WiscSIMS, Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706;
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024;
⁴Graduate School of Natural Science and Technology, Okayama University, Kita-ku, 700-8530 Okayama, Japan

Dynamic models of the protoplanetary disk indicate there should be large-scale material transport in and out of the inner Solar System, but direct evidence for such transport is scarce. Here we show that the ε50Ti-ε54Cr-Δ17O systematics of large individual chondrules, which typically formed 2 to 3 My after the formation of the first solids in the Solar System, indicate certain meteorites (CV and CK chondrites) that formed in the outer Solar System accreted an assortment of both inner and outer Solar System materials, as well as material previously unidentified through the analysis of bulk meteorites. Mixing with primordial refractory components reveals a “missing reservoir” that bridges the gap between inner and outer Solar System materials. We also observe chondrules with positive ε50Ti and ε54Cr plot with a constant offset below the primitive chondrule mineral line (PCM), indicating that they are on the slope ∼1.0 in the oxygen three-isotope diagram. In contrast, chondrules with negative ε50Ti and ε54Cr increasingly deviate above from PCM line with increasing δ18O, suggesting that they are on a mixing trend with an ordinary chondrite-like isotope reservoir. Furthermore, the Δ17O-Mg# systematics of these chondrules indicate they formed in environments characterized by distinct abundances of dust and H2O ice. We posit that large-scale outward transport of nominally inner Solar System materials most likely occurred along the midplane associated with a viscously evolving disk and that CV and CK chondrules formed in local regions of enhanced gas pressure and dust density created by the formation of Jupiter.

^1,2Anthony Gargano,^1,2Zachary Sharp,³Charles Shearer,⁴Justin I. Simon,⁵Alex Halliday,⁶Wayne Buckley
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [DOI:https://doi.org/10.1073/pnas.2014503117]
¹Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131-0001;
²Center for Stable Isotopes, University of New Mexico, Albuquerque, NM 87131-0001;
³Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131-0001;
⁴Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science Division, The Lyndon B. Johnson Space Center, National Aeronautics and Space Administration, Houston, TX 77058;
⁵The Earth Institute, Columbia University, New York, NY 10025;
⁶Jacob–Johnson Space Center Engineering, Technology and Science Contract, The Lyndon B. Johnson Space Center, National Aeronautics and Space Administration, Houston, TX 77058

Lunar mare basalts are depleted in F and Cl by approximately an order of magnitude relative to mid-ocean ridge basalts and contain two Cl-bearing components with elevated isotopic compositions relative to the bulk-Earth value of ∼0‰. The first is a water-soluble chloride constituting 65 ± 10% of total Cl with δ37Cl values averaging 3.0 ± 4.3‰. The second is structurally bound chloride with δ37Cl values averaging 7.3 ± 3.5‰. These high and distinctly different isotopic values are inconsistent with equilibrium fractionation processes and instead suggest early and extensive degassing of an isotopically light vapor. No relationship is observed between F/Cl ratios and δ37Cl values, which suggests that lunar halogen depletion largely resulted from the Moon-forming Giant Impact. The δ37Cl values of apatite are generally higher than the structurally bound Cl, and ubiquitously higher than the calculated bulk δ37Cl values of 4.1 ± 4.0‰. The apatite grains are not representative of the bulk rock, and instead record localized degassing during the final stages of lunar magma ocean (LMO) or later melt crystallization. The large variability in the δ37Cl values of apatite within individual thin sections further supports this conclusion. While urKREEP (primeval KREEP [potassium/rare-earth elements/phosphorus]) has been proposed to be the source of the Moon’s high Cl isotope values, the ferroan anorthosites (FANs) have the highest δ37Cl values and have a positive correlation with Cl content, and yet do not contain apatite, nor evidence of a KREEP component. The high δ37Cl values in this lithology are explained by the incorporation of a >30‰ HCl vapor from a highly evolved LMO.