¹Fatemeh Sedaghatpour, ²Fang-Zhen Teng
¹Isotope Laboratory, Department of Geosciences and Arkansas
²Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA

Magnesium isotopic compositions of 22 well-characterized differentiated meteorites including 7 types of achondrites and pallasite meteorites were measured to estimate the average Mg isotopic composition of their parent bodies and evaluate Mg isotopic heterogeneity of the solar system. The δ26Mg values are -0.236‰ and -0.190‰ for acapulcoite-lodranite and angrite meteorites, respectively and vary from -0.267‰ to -0.222‰ in the winonaite-IAB-iron silicate group, -0.369‰ to -0.292‰ in aubrites, -0.269‰ to -0.158‰ in HEDs, -0.299‰ to -0.209‰ in ureilites, -0.307‰ to -0.237‰ in mesosiderites, and -0.303‰ to -0.238‰ in pallasites. Magnesium isotopic compositions of most achondrites and pallasite meteorites analyzed here are similar and reveal no significant isotopic fractionation. However, Mg isotopic compositions of D′Orbigny (angrite) and some HEDs are slightly heavier than chondrites and the other achondrites studied here. The slightly heavier Mg isotopic compositions of angrites and some HEDs most likely resulted from either impact-induced evaporation or higher abundance of clinopyroxene with the Mg isotopic composition slightly heavier than olivine and orthopyroxene. The average Mg isotopic composition of achondrites (δ26Mg = -0.246 ± 0.082‰, 2SD, n = 22) estimated here is indistinguishable from those of the Earth (δ26Mg = -0.25 ± 0.07‰; 2SD, n = 139), chondrites (δ26Mg = -0.28 ± 0.06‰; 2SD, n = 38), and the Moon (δ26Mg = -0.26 ± 0.16‰) reported from the same laboratory. The chondritic Mg isotopic composition of achondrites, the Moon, and the Earth further reflects homogeneity of Mg isotopes in the solar system and the lack of Mg isotope fractionation during the planetary accretion process and impact events.

Reference
Sedaghatpour F, Teng F-Z (2015) Magnesium isotopic composition of achondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.016]
Copyright Elsevier

^1,2Alex Ruzicka, ¹Ryan Brown, ^3,4Jon Friedrich, ^1,2 Melinda Hutson, ² Richard Hugo, ⁵Mark Rivers
¹Cascadia Meteorite Laboratory, Portland State University, 1721 SW Broadway, Portland, Oregon 97207, U.S.A.
²Department of Geology, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland, Oregon 97207, U.S.A.
³Department of Chemistry, Fordham University, Bronx, New York 10458, U.S.A.
⁴Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, U.S.A.
⁵Consortium for Advanced Radiation Sources, University of Chicago, Argonne, Illinois 60439, U.S.A.

The conditions under which metal cores formed in silicate-metal planetary bodies in the early Solar System are poorly known. We studied the Buck Mountains 005 (L6) chondrite with serial sectioning, X-ray computed microtomography, and optical and electron microscopy to better understand how metal and troilite were redistributed as a result of a moderately strong (shock stage S4) shock event, as an example of how collisional processes could have contributed to differentiation. The chondrite was recovered on Earth in multiple small pieces, some of which have a prominent, 1.5–3 mm wide holocrystalline shock melt dike that forms a jointed, sheet-like structure, as well as an associated shock vein network. The data suggest that metal and troilite within the dike were melted, sheared, and transported as small parcels of melt, with metal moving out of the dike and along branching veins to become deposited as coarser nodules and veins within largely unmelted host. Troilite also mobilized but partly separated from metal to become embedded as finer-grained particles, vein networks, and emulsions intimately intergrown with silicates. Rock textures and metal compositions imply that shock melts cooled rapidly against relatively cool parent body materials, but that low-temperature annealing occurred by deep burial within the parent body. Our results demonstrate the ability of shock processes to create larger metal accumulations in substantially unmelted meteorite parent bodies, and they have implications for the formation of iron meteorites and for core formation within colliding planetesimals.

Reference
Ruzicka A, Brown R, Friedrich J, Hutson M, Hugo R, Rivers M(2015) Shock-induced mobilization of metal and sulfide in planetesimals: Evidence from the Buck Mountains 005 (L6 S4) dike-bearing chondrite. American Mineralogist 100, 2725-2738, Link to Article [doi:10.2138/am-2015-5225]
Copyright: The Mineralogical Society of America

¹Sandra Pizzarello, ¹Maitrayee Bose
¹Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA

Large isotopic anomalies are found in meteoritic insoluble organic materials (IOMs) and, for nitrogen, show 15N-excesses up to ${\delta }^{15}$N ~ 5000‰. These 15N-enrichments are commonly ascribed to presolar origins, but the attribution seems contradicted by available data on N-isotopes’ cosmic distribution. We report here that 15N hotspots in several IOMs are reduced by hydrothermal treatment and their loss correlates with 15N values of ammonia released upon treatment. Because released ammonia’s 15N-enrichments also relate with meteorites’ mineralogy, i.e., asteroidal processes, and no current models offer plausible explanations for the finding, we account for our data with a novel scenario whereby 15N-enriched ammonia produced in the solar nebula is incorporated by carbonaceous materials and delivered to early Earth by comets and meteorites. The proposal also implies that abundant reduced nitrogen, a required element in origins of life theories, could reach our nascent planet and other planetary systems affecting their habitability.

Reference
Pizzarello S, Bose M (2015) THE PATH OF REDUCED NITROGEN TOWARD EARLY EARTH: THE COSMIC TRAIL AND ITS SOLAR SHORTCUTS. The Astrophysical Journal 814, 2
Link to Article [http://dx.doi.org/10.1088/0004-637X/814/2/107]

¹Root, S., ¹Shulenburger, L., ¹Lemke, R.W., ¹Dolan, D.H., ¹Mattsson, T.R., ¹Desjarlais, M.P.
¹Sandia National Laboratories, Albuquerque, NM, United States

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Reference
Root S, Shulenburger L, Lemke RW, Dolan DH, Mattsson TR, Desjarlais MP (2015) Shock Response and Phase Transitions of MgO at Planetary Impact Conditions.
Physical Review Letters 115 (in Press)
Link to Article [DOI: 10.1103/PhysRevLett.115.198501]

¹Lorand, J.-P. et al. (>10)*
¹Laboratoire de Planétologie et Géodynamique à Nantes, CNRS UMR 6112, Université de Nantes, Nantes Cédex 3, France
*Find the extensive, full author and affiliation list on the publishers website

Martian regolith breccia NWA 7533 (and the seven paired samples) is unique among Martian meteorites in showing accessory pyrite (up to 1% by weight). Pyrite is a late mineral, crystallized after the final assembly of the breccia. It is present in all of the lithologies, i.e., the fine-grained matrix (ICM), clast-laden impact melt rocks (CLIMR), melt spherules, microbasalts, lithic clasts, and mineral clasts, all lacking magmatic sulfides due to degassing. Pyrite crystals show combinations of cubes, truncated cubes, and octahedra. Polycrystalline clusters can reach 200 μm in maximum dimensions. Regardless of their shape, pyrite crystals display evidence of very weak shock metamorphism such as planar features, fracture networks, and disruption into subgrains. The late fracture systems acted as preferential pathways for partial replacement of pyrite by iron oxyhydroxides interpreted as resulting from hot desert terrestrial alteration. The distribution and shape of pyrite crystals argue for growth at moderate to low growth rate from just-saturated near neutral (6 < pH<10), H2S-HS-rich fluids at minimum log fO2 of >FMQ + 2 log units. It is inferred from the maximum Ni contents (4.5 wt%) that pyrite started crystallizing at 400–500 °C, during or shortly after a short-duration, relatively low temperature, thermal event that lithified and sintered the regolith breccias, 1.4 Ga ago as deduced from disturbance in several isotope systematics.

Reference
Lorand J-P et al. (2015) Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12565]
Published by arrangement with John Wiley&Sons

^1,2Esther S. Posner, ¹Jibamitra Ganguly, ³Richard Hervig
¹Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA
²Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
³School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287, USA

The 53Mn-53Cr decay system, in which 53Mn decays to 53Cr (t1/2 = 3.7 Ma) has been widely used to construct 53Cr/52Cr vs. 55Mn/52Cr isochrons and thus determine relative ages of early solar system objects or events, assuming that the initial Cr isotopic ratio, (53Cr/52Cr)o, equals (53Mn/52Cr)o. With the primary objective of interpretation of these ages within a diffusion kinetic framework, we have determined the tracer diffusion coefficient of Cr in natural spinels, which are very close to the MgAl2O4 end-member composition, as a function of temperature and oxygen fugacity (f(O2)). It is found that the diffusion coefficient of Cr, D(Cr), in two stocks of spinels (referred to as cut-gems and gem-gravels) with very similar major element chemistry is consistently different, but the data in each stock yield well defined Arrhenius relations that show a difference of logD of 0.6 to 1.0, depending on temperature, with the D(Cr) in gem-gravel being higher than that in the cut-gem stock. The D(Cr) was found to have a positive dependence on f(O2) in the range of f(O2) of around ± 2 log units relative to that of the wüstite-magnetite buffer. The difference in the D(Cr) between the two stocks and the observed D(Cr) vs. f(O2) relation has been explained in terms of a change of point defect concentration resulting from heterovalent substitution of trace elements and equilibration with the imposed f(O2) conditions, respectively. Assuming a homogeneous semi-infinite matrix, the closure temperature (Tc) of Cr diffusion in spinel has been calculated as a function of grain size, cooling rate, peak temperature (To) and f(O2). Also the dependence of D(Cr) and Tc(Cr) on the Cr# (i.e. Cr/(Cr+Al) ratio) has been accounted for using available D(Cr) vs. Cr# data in Suzuki et al. (2008). We argue, on the basis of crystal chemical considerations and available diffusion kinetic data for minerals, that the Tc for Mn should be much lower than that for Cr in spinel, olivine and orthopyroxene, and discuss the potential implications of the anticipated disparity between Tc(Cr) and Tc(Mn) for the estimation of the (53Mn/55Mn)o ratio from an internal isochron defined by these minerals. Finally, we discuss the problem of determining the Tc for an internal isochron in relation to the individual Tc(Cr) for spinel, olivine and orthopyroxene.

Reference
Posner ES, Ganguly J, Hervig R (2015) Diffusion kinetics of Cr in spinel: Experimental studies and implications for 53Mn-53Cr cosmochronology. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.018]
Copyright Elsevier

¹Mark Tyra, ¹Adrian Brearley, ²Yunbin Guan
¹Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

We have determined the O and C isotope compositions of dolomite grains and the C isotope compositions of calcite grains in the highly altered CM1 chondrite, ALH 84049, using Secondary Ion Mass Spectrometry (SIMS). Chemically-zoned dolomite constitutes 0.8 volume percent (vol%) of the sample and calcite 0.9 vol%. Thirteen separate dolomite grains have δ13C values that range from 37 to 60 (± 2) ‰, δ18O values from 25 to 32 (± 3) ‰, and δ17O values from 10 to 16 (± 3) ‰ (VSMOW). Intragrain δ13C values in dolomite vary up to 10 ‰. The δ13C values of three calcite grains are distinct from those of dolomite and range from 10 to 13 (± 2) ‰ (PDB). Calcite and dolomite appear to record different precipitation episodes. Carbon isotope values of both dolomite and calcite in this single sample encompass much of the reported range for CM chondrites; our results imply that bulk carbonate C and O isotope analyses may oversimplify the history of carbonate precipitation. Multiple generations of carbonates with variable isotope compositions exist in ALH 84049 and, perhaps, in many CM chondrites. This work shows that one should exercise caution when using a clumped isotope approach to determine the original temperature and the isotopic compositions of water for CM chondrite carbonates. Less altered CM meteorites with more-homogeneous C isotope compositions, however, may be suitable for bulk-carbonate analyses, but detailed carbonate petrologic and isotopic characterization of individual samples is advised.

Reference
Tyra M, Brearley A, Guan Y (2015) Episodic carbonate precipitation in the CM chondrite ALH 84049: An ion microprobe analysis of O and C isotopes. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.10.034]
Copyright Elsevier

^1,2Aaron Z. Goldberg, ^1,3James E. Owen,^1,4Emmanuel Jacquet
¹Canadian Institute for Theoretical Astrophysics, 60 St. George Street, Toronto M5S 3H8, Canada
²Department of Physics and Astronomy, McMaster University, 1280 Main Street W., Hamilton, Ontario L8S 4M1, Canada
³Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
⁴Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Muséum National d’Histoire Naturelle, CP52, 57 rue Buffon, F-75005 Paris, France

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Reference
Goldberg AZ, Owen JE, Jacquet E (2015) Chondrule transport in protoplanetary discs. Monthly Notices of the Royal Astronomical Society 452, 4054-4069.
Link to Article [doi: 10.1093/mnras/stv1610]

^1,2Alan E. Rubin, ³John P. Breen, ^1,2,3John T. Wasson, ⁴Darryl Pitt
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA
³Department of Chemistry and Biochemistry, University of California, Los Angeles, California, USA
⁴Macovich Collection of Meteorites, New York City, New York, USA

A slab of the Willamette ungrouped iron contains elongated troilite nodules (up to ~2 × 10 cm) that were crushed and penetrated by wedges of crushed metal during a major impact event. What makes this sample unique is the contrast between the large amount of shock damage and the very small (~1%) amounts of shock melting in the large troilite nodules. The postshock temperature was low, probably ≾960 °C. The Widmanstätten pattern has been largely obscured by an episode of postshock annealing that caused recrystallization of the kamacite. The shock and thermal history of Willamette includes (1) initial crystallization and formation of multicentimeter-size troilite nodules from trapped melt, (2) impact-induced melting of metal-sulfide assemblages to form lobate taenite masses a few hundred micrometers in size, (3) impact-crushing of the nodules and jamming of metal wedges into them, (4) simultaneous crushing of metal grains adjacent to sulfide throughout the meteorite, (5) postshock annealing causing minor recrystallization of metal and troilite, and (6) a late-stage shock event (and additional annealing) producing Neumann lines in the kamacite.

Reference
Rubin AE, Breen JP, Wasson JT, Pitt D (2015) Shock effects in the Willamette ungrouped iron Meteorite. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12569]
Published by arrangement with John Wiley & Sons

^1,4Xiande Xie, ²Hexiong Yang, ³Xiangping Gu,²Robert T. Down
¹Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Guangzhou, 510640, China
²Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721-0077, U.S.A.
³School of Geosciences and Info-Physics, Central South University, Changsha, Hunan, 410083, China
⁴Guangdong Key Laboratory of Mineral Physics and Materials, Guangzhou, 510640 China

Merrillite, ideally Ca9NaMg(PO4)7, is an important accessory phosphate mineral in many different groups of meteorites, including martian meteorites, and a major carrier of rare earth elements (REE) in lunar rocks. By means of electron microprobe analysis, single-crystal X-ray diffraction, and Raman spectroscopy, we present the first structure determination of merrillite with a nearly ideal chemical composition, Ca9.00Na0.98(Mg0.95Fe0.06)∑1.01 (P1.00O4)7, from the Suizhou meteorite, a shock-metamorphosed L6-chondrite. Suizhou merrillite is trigonal with space group R3c and unit-cell parameters a = 10.3444(3), c = 37.0182(11) Å, and V = 3430.5(2) Å3. Its crystal structure, refined to R1 = 0.032, is characterized by a structural unit consisting of a [(Mg,Fe)(PO4)6]16− complex anion that forms a “bracelet-and-pinwheel” arrangement. Such structural units are linked by interstitial complexes with a formula of [Ca9Na(PO4)]16+, which differs from that of [Ca9(PO3[OH])]16+, [Ca9(PO3F)]16+, [Ca9(Ca0.5□0.5)(PO4)]16+, or [(Ca9−xREE)x(Na1−x□x)(PO4)]16+ in terrestrial whitlockite, terrestrial/extraterrestrial bobdownsite, meteoritic Ca-rich merrillite, or lunar REE-rich merrillite, respectively. The Suizhou merrillite is found to transform to tuite at high pressures, pointing to the likelihood of finding REE-bearing tuite on the Moon as a result of shock events on REE-merrillite.

Reference
Xie X, Yang H, Gu X, Downs RT (2015) Chemical composition and crystal structure of merrillite from the Suizhou meteorite. American Mineralogist 100, 2753-2756
Link to Article [doi:10.2138/am-2015-5488]
Copyright: The Mineralogical Society of America