De¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

There is evidence that the peak brightness of a Type Ia supernova is affected by the electron fraction Y_e at the time of the explosion. The electron fraction is set by the aboriginal composition of the white dwarf and the reactions that occur during the pre-explosive convective burning. To date, determining the makeup of the white dwarf progenitor has relied on indirect proxies, such as the average metallicity of the host stellar population. In this paper, we present analytical calculations supporting the idea that the electron fraction of the progenitor systematically influences the nucleosynthesis of silicon group ejecta in Type Ia supernovae. In particular, we suggest the abundances generated in quasi-nuclear statistical equilibrium are preserved during the subsequent freeze-out. This allows potential recovery of Y_e at explosion from the abundances recovered from an observed spectra. We show that measurement of ²⁸Si, ³²S, ⁴⁰Ca, and ⁵⁴Fe abundances can be used to construct Y_e in the silicon-rich regions of the supernovae. If these four abundances are determined exactly, they are sufficient to recover Y_e to 6%. This is because these isotopes dominate the composition of silicon-rich material and iron-rich material in quasi-nuclear statistical equilibrium. Analytical analysis shows the ²⁸Si abundance is insensitive to Y_e, the ³²S abundance has a nearly linear trend with Y_e, and the ⁴⁰Ca abundance has a nearly quadratic trend with Y_e. We verify these trends with post-processing of one-dimensional models and show that these trends are reflected in the model’s synthetic spectra.

Reference
De et al. (2014) On Silicon Group Elements Ejected by Supernovae Type Ia. The Astrophysical Journal 787:149.
[doi:10.1088/0004-637X/787/2/149]

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Ellen R. Harju^a, Alan E. Rubin^b, Insu Ahn^c, Byeon-Gak Choi^d, Karen Ziegler^a,1, John T. Wasson^a,b,e

^aDepartment of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA
^bInstitute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA
^cKorea Polar Research Institute, Incheon, 406-840, Korea
^dDepartment of Earth Science Education, Seoul National University, Seoul, 151-748, Korea
^eDepartment of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1567, USA

The wide range in the degree of aqueous alteration of CR chondrites prompted us to formulate a numerical sequence for these rocks that ranges from petrologic type 2.0 to 2.8. (Hypothetical CR3.0 chondrites should be completely free of aqueous alteration effects.) About 70% of CR chondrites are slightly altered, type-2.8 rocks that exhibit heterogeneous alteration; these meteorites contain moderately abundant metallic Fe-Ni, no magnetite, and generally, a few chondrules with clear glassy mesostases. None of the chondrules in these rocks shows evidence of alteration of mafic silicate phenocrysts, but several chondrules are surrounded by phyllosilicate-rich rims that appear “smooth” when viewed by back-scattered-electron imaging. Matrix regions in slightly altered CR chondrites contain high S (~3 wt.%), but some matrix patches in the same thin sections record alteration effects and contain appreciably less S (<1.5 wt.%). In CR chondrites that have been more-significantly altered (e.g., Renazzo and Al Rais), metallic Fe-Ni has been partially replaced by magnetite±sulfide; mafic silicates have been partly altered to phyllosilicates, particularly along edges, fractures and twin boundaries. One of the most-altered CR chondrites (type-2.0 GRO 95577) contains abundant magnetite, additional oxide phases, iron carbonate, only very rare metallic Fe-Ni and essentially no mafic silicate grains. The whole-rock O-isotopic compositions of CR chondrites correlate with the degree of aqueous alteration: Δ¹⁷O ranges from ~-2.6‰ in type-2.8 samples to ~-0.4‰ in type 2.0.

Reference
Harju ER, Rubin AE, Ahn I, Choi B-G, Ziegler K and Wasson JT (in press) Progressive aqueous alteration of CR carbonaceous chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.048]
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Alessandro Maturilli^a, Jörn Helbert^a, James M. St. John^b, James W. Head III^c, William M. Vaughan^c, Mario D’Amore^a, Matthias Gottschalk^d, Sabrina Ferrari^a

^aInstitute for Planetary Research, German Aerospace Center DLR, Rutherfordstr. 2, Berlin–Adlershof, Germany
^bSchool of Earth Sciences, The Ohio State University at Newark, Newark, OH 43055, USA
^cDepartment of Geological Sciences, Brown University, Providence, RI 02912, USA
^dDepartment of Chemistry and Physics of Earth Materials, Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany

The elemental composition of Mercury’s surface, which has been recently measured by the NASA MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, suggests a mineralogy dominated by magnesium-rich orthopyroxene and feldspar. The most magnesium-rich and aluminium-poor regions of Mercury’s surface (which are presumably orthopyroxene-rich) have compositions, and possibly mineralogies, analogous to terrestrial boninites and basaltic komatiites. Unfortunately, little is known about the spectral properties of komatiites, especially at the high surface temperatures of Mercury. We therefore have collected three terrestrial komatiites with different compositions plus a synthetic komatiitic sample, and measured their reflectances in the visible and thermal infrared spectral ranges. Samples divided into four grain size ranges (when enough material was available) were measured fresh and after thermal processing in vacuum (10 Pa) at 500 °C, comparable to Mercury peak surface temperatures. Our measurements show that spectral changes between fresh and thermally processed samples occur in both spectral channels, but are stronger in the visible range, with reddening affecting all the samples, while darkening is more selective. It is important to note that darkening and reddening after thermally processing the samples are independent of the komatiites ferrous iron content. In fact the synthetic sample which is nearly iron-free is most strongly affected. From our study it turns out that thermally processing the samples in vacuum at Mercury surface temperature produces the removal of samples’ colour centres. The results of our study show also that the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument on MESSENGER orbiting Mercury currently cannot distinguish between different compositions of komatiites, while the future MErcury Radiometer and Thermal infrared Imaging Spectrometer (MERTIS) on the upcoming ESA BepiColombo mission will resolve their differences in the 7-14 µm spectral range.

Reference
Maturilli A, Helbert J, St. John JM, Head III JW, Vaughan WM, D’Amore M, Gottschalk M and Ferrari S (2014) Komatiites as Mercury surface analogues: Spectral measurements at PEL. Earth and Planetary Science Letters Volume 398:58–65.
[doi:10.1016/j.epsl.2014.04.035]
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I. Ramírez¹, A. T. Bajkova², V. V. Bobylev^2,3, I. U. Roederer⁴, D. L. Lambert¹, M. Endl¹, W. D. Cochran¹, P. J. MacQueen¹ and R. A. Wittenmyer^5,6

¹McDonald Observatory and Department of Astronomy, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, Texas 78712-1205, USA
²Central (Pulkovo) Astronomical Observatory of RAS, 65/1, Pulkovskoye Chaussee, St. Petersburg 196140, Russia
³Sobolev Astronomical Institute, St. Petersburg State University, Bibliotechnaya pl. 2, St. Petersburg 198504, Russia
⁴Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁵School of Physics, UNSW Australia, Sydney 2052, Australia
⁶Australian Centre for Astrobiology, University of New South Wales, UNSW Kensington Campus, Sydney 2052, Australia

Dynamical information along with survey data on metallicity and in some cases age have been used recently by some authors to search for candidates of stars that were born in the cluster where the Sun formed. We have acquired high-resolution, high signal-to-noise ratio spectra for 30 of these objects to determine, using detailed elemental abundance analysis, if they could be true solar siblings. Only two of the candidates are found to have solar chemical composition. Updated modeling of the stars’ past orbits in a realistic Galactic potential reveals that one of them, HD 162826, satisfies both chemical and dynamical conditions for being a sibling of the Sun. Measurements of rare-element abundances for this star further confirm its solar composition, with the only possible exception of Sm. Analysis of long-term high-precision radial velocity data rules out the presence of hot Jupiters and confirms that this star is not in a binary system. We find that chemical tagging does not necessarily benefit from studying as many elements as possible but instead from identifying and carefully measuring the abundances of those elements that show large star-to-star scatter at a given metallicity. Future searches employing data products from ongoing massive astrometric and spectroscopic surveys can be optimized by acknowledging this fact.

Reference
Ramírez I, Bajkova AT, Bobylev VV, Roederer IU, Lambert DL, Endl M, Cochran WD, MacQueen PJ and Wittenmyer RA (2014) Elemental Abundances of Solar Sibling Candidates. The Astrophysical Journal 787:154.
[doi:10.1088/0004-637X/787/2/154]

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Han Chi^a, Rajdeep Dasgupta^a, Megan Duncan^a and Nobumichi Shimizu^b

^aDepartment of Earth Science, Rice University, 6100 Main Street, MS 126, Houston, TX 77079, USA
^bDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

The budget and origin of carbon in Earth and other terrestrial planets are debated and one of the key unknowns is the fate of carbon during early planetary processes including accretion, core formation, and magma ocean (MO) crystallization. Here we determine, experimentally, the solubility of carbon in coexisting Fe-Ni alloy melt and basaltic silicate melt in shallow MO conditions, i.e., at 1-3 GPa, 1500-1800 °C. Oxygen fugacity of the experiments, estimated based on Fe (in metallic alloy melt)-FeO (in silicate melt) equilibrium, varied between ∼IW-0.4 and IW-1.0, where IW refers to the oxygen fugacity imposed by the coexistence of iron and wüstite. Four different starting mixes, each with 7:3 silicate:metal mass ratio and silicate melt NBO/T (estimated proportion of non-bridging oxygen with respect to tetrahedral cations; , where T = Si + Ti + Al + Cr + P) ranging from 0.81 to 1.54 were studied. Concentrations of carbon in the alloy melt were determined using electron microprobe whereas carbon contents of quenched basaltic glasses were determined using secondary ionization mass spectrometry (SIMS). Identification of carbon and hydrogen-bearing species in silicate glasses was performed using Raman and Fourier Transformed Infrared (FTIR) spectroscopy.

Our results show that carbon in the metallic melt varies between 4.39 and 7.43 wt.% and increases with increasing temperature and modestly with increasing pressure but decreases with increasing Ni content of the alloy melt. Carbon concentration in the silicate melts, on the other hand, varies from 11±1 ppm to 111±7 ppm and is negatively correlated with pressure but positively correlated with temperature, the NBO/T, the oxygen fugacity and the water content of the silicate melts. Raman and FTIR results show that at our experimental conditions, carbon in silicate melt is dissolved both as hydrogenated species and . The calculated carbon partition coefficient varies from 510±53 to 5369±217 and varies systematically as a function of P  , T  , f_O2, water content, the composition of the silicate melt (expressed using NBO/T), and Ni content of alloy melt (X  _Ni). The range of  measured in our study with carbonated and hydrogenated carbon species in silicate melt is similar to that reported in the literature for experiments where carbonyl complexes are the chief carbon species in silicate melts. An empirical parameterization was derived using the data from this and existing studies such as

where a   = -33510, b   = 1357, c   = -0.596, d   = -1.18, e   = 4.15, f   = 13.37,the temperature is in Kelvins, and the pressure is in gigapascals. Using this parameterization and the estimated conditions for the base of the MOs, the average  value for Earth, Mars, and the Moon can be predicted. The deep MO of Earth is predicted to cause the strongest depletion of its silicate carbon budget, closely followed by Mars with intermediate depth MO, and then the Moon with a shallow MO. We predict that the lunar mantle carbon budget, similar to that of the Earth’s present-day upper mantle, might have been set by equilibrium core-mantle fractionation in MO; whereas for Earth, later processes such as ingassing from a proto-atmosphere and late-stage accretion of volatile-rich material was necessary for delivery of carbon and other volatiles. Finally, the comparison of our measured and predicted value of for terrestrial MO with similar constraints on from the literature suggests that the apparent depletion of nitrogen relative to carbon for the bulk silicate Earth and the Earth’s upper mantle is unlikely to be caused by preferential partitioning of nitrogen to alloy melt during core formation.

Reference
Han Chi H, Dasgupta R, Duncan M and Shimizu N (in press) Partitioning of Carbon between Fe-rich Alloy Melt and Silicate Melt in a Magma Ocean – Implications for the Abundance and Origin of Volatiles in Earth, Mars, and the Moon. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.046]
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Harju ER, Rubin AE, Ahn I, Choi B-G, Ziegler K and Wasson JT (in press) Progressive aqueous alteration of CR carbonaceous chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.048]
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Julio A. Fernández, Andrea Sosa, Tabaré Gallardo, Jorge N. Gutiérrez

Departamento de Astronomí a, Facultad de Ciencias, Universidad de la República, Iguá 4225, 14000 Montevideo, Uruguay

We analyze a sample of 139 near-Earth asteroids (NEAs), defined as those that reach perihelion distancesq<1.3 au, and that also fulfill the conditions of approaching or crossing Jupiter’s orbit (aphelion distancesQ>4.8 au), having Tisserand parameters 2<T<3 and orbital periods P<20 yr. In order to compare the dynamics, we also analyze a sample of 42 Jupiter family comets (JFCs) in near-Earth orbits, i.e. with q<1.3au. We integrated the orbits of these two samples for 10⁴ yr in the past and in the future. We find that the great majority of the NEAs move on stable orbits during the considered period, and that a large proportion of them are in one of the main mean motion resonances with Jupiter, in particular the 2:1. We find a strong coupling between the perihelion distance and the inclination in the motion of most NEAs, due to Kozai mechanism, that generates many sungrazers. On the other hand, most JFCs are found to move on very unstable orbits, showing large variations in their perihelion distances in the last few 10²-10³ yr, which suggests a rather recent capture in their current near-Earth orbits. Even though most NEAs of our sample move in typical ’asteroidal’ orbits, we detect a small group of NEAs whose orbits are highly unstable, resembling those of the JFCs. These are: 1997 SE5, 2000 DN1, 2001 XQ, 2002 GJ8, 2002 RN38, 2003 CC11, 2003 WY25, 2009 CR2, and 2011 OL51. These objects might be inactive comets, and indeed 2003 WY25 has been associated with comet Blanpain, and it is now designed as comet 289P/Blanpain. Under the assumption that these objects are inactive comets, we can set an upper limit of ~0.17 to the fraction of NEAs with Q>4.8 au of cometary origin, but it could be even lower if the NEAs in unstable orbits listed before turn out to be bona fide asteroids from the main belt. This study strengthens the idea that NEAs and comets essentially are two distinct populations, and that periods of dormancy in comets must be rare. Most likely, active comets in near-Earth orbits go through a continuous erosion process in successive perihelion passages until disintegration into meteoritic dust and fragments of different sizes. In this scenario, 289P/Blanpain might be a near-devolatized fragment from a by now disintegrated parent comet.

Reference
Fernández JA, Sosa A, Gallardo T and Gutiérrez JN (in press) Assessing the physical nature of near-Earth asteroids through their dynamical histories. Icarus
[doi:10.1016/j.icarus.2014.04.048]
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Jemma Davidson^a, Henner Busemann^a,b, Larry R. Nittler^c, Conel M.O’D. Alexander^c, François-Régis Orthous-Daunay^d, Ian A. Franchi^a, Peter Hoppe^e

^aPlanetary and Space Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
^bSchool of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
^cDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington DC, 20015-1305, USA
^dInstitut de Planétologie et d’Astrophysique de Grenoble, UJF CNRS/INSU 38000 Grenoble, France
^eMax-Planck-Institut für Chemie, P.O. Box 3060, 55020 Mainz, Germany

It has been suggested that the matrices of all chondrites are dominated by a common material with Ivuna-like (CI) abundances of volatiles, presolar grains and insoluble organic matter (IOM) (e.g., Alexander, 2005). However, matrix-normalized abundances of presolar silicon carbide (SiC) grains estimated from their noble gas components show significant variations in even the most primitive chondrites (Huss and Lewis, 1995 and Huss et al., 2003), in contradiction to there being a common chondrite matrix material. Here we report presolar SiC abundances determined by NanoSIMS raster ion imaging of IOM extracted from primitive members of different meteorite groups. We show that presolar SiC abundance determinations are comparable between NanoSIMS instruments located at three different institutes, between residues prepared by different demineralization techniques, and between microtomed and non-microtomed samples. Our derived SiC abundances in CR chondrites are comparable to those found in the CI chondrites (~30 ppm) and are much higher than previously determined by noble gas analyses. The revised higher CR SiC abundances are consistent with the CRs being amongst the most primitive chondrites in terms of the isotopic compositions and disordered nature of their organic matter. Similar abundances between CR1, CR2, and CR3 chondrites indicate aqueous alteration on the CR chondrite parent body has not progressively destroyed SiC grains in them. A low SiC abundance for the reduced CV3 RBT 04133 can be explained by parent body thermal metamorphism at an estimated temperature of ~440°C. Minor differences between primitive members of other meteorite classes, which did not experience such high temperatures, may be explained by prolonged oxidation at lower temperatures under which SiC grains formed outer layers of SiO₂ that were not thermodynamically stable, leading to progressive degassing/destruction of SiC.

Reference
Davidson J, Busemann H, Nittler LR, Alexander CMO’D, Orthous-Daunay F-R, Franchi IA and Hoppe P (in press) Abundances of presolar silicon carbide grains in primitive meteorites determined by NanoSIMS. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.026]
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Graziella Caprarelli

Division of IT, Engineering and the Environment (DITEE), University of South Australia, GPO Box 2471, Adelaide, SA 5001, Australia

Impact craters on the surface of Mars are degraded by erosion and infilling due to combinations of geological processes. These result in modifications of relative crater dimensions, including diameter increase and reduction of rim-floor depths. In principle, the longer a crater is exposed to geological processes, the more pronounced the modifications. Visualization and analysis of these effects are achieved by plotting the measured depths (M) of impact craters versus the corresponding theoretical depths (predicted: P) calculated from the crater diameters using depth/Diameter power laws. This type of diagram is referred to as MPD (measured depth versus predicted depth diagram). The advantage of using the MPD representation consists in the fact that the data plot along linear regressions, more easily interpreted than standard depth vs. diameter diagrams.
As an example of application of the method, the MPD was used to discriminate different generations of impact craters in Terra Sabaea into four groups: T₀ (fresh craters), T₁, T₂ and T₃ (from younger to older), all located on the most ancient geological unit in the area (Npld). Other units in the area are Hpl₃ and Hr, impacted only by craters belonging to group T₀, suggesting that these units are stratigraphically correlated. The data of 5 craters in superposition relationships with the eastern reaches of Evros Vallis, one of the major valley networks in the area, were plotted in the diagram and assigned each to a regression depending on the location of their data points in relation to the prediction bands of the regressions. The craters superposed to the valley all belonged to T₀, indicating that Evros Vallis has the same relative age of units Hpl₃ and Hr.
A conceptual discussion of the results demonstrates that MPD statistics (a) are unaffected by the procedures used to acquire depths and diameters of impact craters and by the power laws used, and (b) can be interpreted irrespective of the sequence or combination of processes leading to modification of the crater morphometric data. These properties make the diagram a powerful statistical tool.

Reference
Caprarelli G (in press) Introducing and discussing a novel diagrammatic representation of impact crater dimensions. Icarus
[doi:10.1016/j.icarus.2014.04.051]
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R. Stevenson¹, J. M. Bauer^1,2, E. A. Kramer³, T. Grav⁴, A. K. Mainzer¹ and J. R. Masiero¹

¹Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 183-427, Pasadena, CA 91109, USA
²Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
³Department of Physics, University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA
⁴Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA

Comet 17P/Holmes underwent a massive outburst in 2007 October, brightening by a factor of almost a million in under 48 hr. We used infrared images taken by the Wide-Field Infrared Survey Explorer mission to characterize the comet as it appeared at a heliocentric distance of 5.1 AU almost 3 yr after the outburst. The comet appeared to be active with a coma and dust trail along the orbital plane. We constrained the diameter, albedo, and beaming parameter of the nucleus to 4.135 ± 0.610 km, 0.03 ± 0.01, and 1.03 ± 0.21, respectively. The properties of the nucleus are consistent with those of other Jupiter family comets. The best-fit temperature of the coma was 134 ± 11 K, slightly higher than the blackbody temperature at that heliocentric distance. Using Finson–Probstein modeling, we found that the morphology of the trail was consistent with ejection during the 2007 outburst and was made up of dust grains between 250 μm and a few cm in radius. The trail mass was ~1.2–5.3 × 10¹⁰ kg.

Reference
Stevenson R, Bauer JM, Kramer EA, Grav T, Mainzer AK and Masiero JR (2014) Lingering Grains of Truth around Comet 17P/Holmes. The Astrophysical Journal 787:116.
[doi:10.1088/0004-637X/787/2/116]

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Jon M. Friedrich^a,b, Grace C. Perrotta^a and Makoto Kimura^c,d

^aDepartment of Chemistry, Fordham University, Bronx, NY 10458 USA
^bDepartment of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024 USA
^cFaculty of Science, Ibaraki University, Mito 310-8512 Japan
^dNational Institute of Polar Research, Tokyo 190-8518, Japan

To examine compositional changes associated with high degrees of apparent thermal metamorphism among the LL chondrites, we have examined seven LL chondrites originally classified as being petrographic type 7. For comparison, we also analyzed the L6/7 chondrite Y-790124. We found that A-880933 is actually an LL4-6 genomict breccia and Y-790124 is best described as an L6 (S3) chondrite. The remaining six chondrites (EET 92013, Uden, Y-74160, Y-790144, Y-791067, Y-82067) are clearly of LL provenance, and each experienced temperatures high enough for them to have been recrystallized. In four of these samples (EET 92013, Uden, Y-74160, Y-790144) we find elemental patterns suggesting Fe(Ni)-FeS mobilization. Others (Y-791067, Y-82067) have compositions identical to average equilibrated LL chondrites. From our compositional data, we infer that EET 92013, Uden, Y-74160, Y-790144 experienced very low degrees of partial melting prior to recrystallization, but Y-791067 and Y-82067 experienced isochemical solid state recrystallization. The heat source responsible for the high degrees of thermal alteration of these meteorites is limited to either the decay of now extinct radionuclides (²⁶Al) or impact-related heating. To evaluate the nature of the heat source, we use ⁴⁰Ar-³⁹Ar literature data and petrographic examinations to infer the cooling history and shock history of these chondrites. We find that heating due to impact is the most likely heat source for the heating of the recrystallized chondrites. The potential impacts occurred well after the initial stages of LL chondrite thermal metamorphism, but still early in the LL parent body’s history, probably ~4.2-4.3 Ga ago. These rocks experienced mild shock histories following their recrystallization.

Reference
Friedrich JM, Perrotta GC and Kimura M (in press) Compositions, geochemistry, and shock histories of recrystallized LL chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.044]
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