^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

¹Colin, A., ¹Moreira, M., ²Gautheron, C., ³Burnard, P.
¹Institut de Physique du Globe de Paris, Université Paris Diderot, Paris, France
²Faculté des Sciences d’Orsay, Université Paris Sud, Orsay, France
³CRPG-CNRS, Université de Lorraine, Vandœuvre-lès-Nancy, France

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Reference
Colin A, Moreira M, Gautheron C, Burnard P (2015) Constraints on the noble gas composition of the deep mantle by bubble-by-bubble analysis of a volcanic glass sample from Iceland. Chemical Geology 417, 173-183.
Link to Article [DOI: 10.1016/j.chemgeo.2015.09.020]

¹Qi He, ¹Long Xiao, ²J. Brian Balta, ³Ioannis P. Baziotis, ⁴Weibiao Hsu, ⁵Yunbin Guan
¹Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan, China
²Department of Geology and Planetary Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
³Agricultural University of Athens, Laboratory of Mineralogy and Geology, Athens, Greece
⁴Laboratory for Astrochemistry and Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
⁵Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA

We present a study of the petrology and geochemistry of basaltic shergottite Northwest Africa 2975 (NWA 2975). NWA 2975 is a medium-grained basalt with subophitic to granular texture. Electron microprobe (EMP) analyses show two distinct pyroxene compositional trends and patchy compositional zoning patterns distinct from those observed in other meteorites such as Shergotty or QUE 94201. As no bulk sample was available to us for whole rock measurements, we characterized the fusion crust and its variability by secondary ion mass spectrometer (SIMS) measurements and laser ablation inductively coupled plasma spectroscopy (LA-ICP-MS) analyses as a best-available proxy for the bulk rock composition. The fusion crust major element composition is comparable to the bulk composition of other enriched basaltic shergottites, placing NWA 2975 within that sample group. The CI-normalized REE (rare earth element) patterns are flat and also parallel to those of other enriched basaltic shergottites. Merrillite is the major REE carrier and has a flat REE pattern with slight depletion of Eu, parallel to REE patterns of merrillites from other basaltic shergottites. The oxidation state of NWA 2975 calculated from Fe-Ti oxide pairs is NNO-1.86, close to the QFM buffer. NWA 2975 represents a sample from the oxidized and enriched shergottite group, and our measurements and constraints on its origin are consistent with the hypothesis of two distinct Martian mantle reservoirs: a reduced, LREE-depleted reservoir and an oxidized, LREE-enriched reservoir. Stishovite, possibly seifertite, and dense SiO2 glass were also identified in the meteorite, allowing us to infer that NWA 2975 experienced a realistic shock pressure of ~30 GPa.

Reference
He Q, Xiao L, Balta JB, Baziotis IP, Hsu W, Guan Y (2015) Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12571]
Published by arrangement with John Wiley & Sons

^1,2Jisun Park et al. (>10)*
¹Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey, USA
²Lunar and Planetary Institute, Houston, Texas, USA
*Find the extensive, full author and affiliation list on the publishers Website

The Hayabusa mission to asteroid 25143, Itokawa, brought back 2000 small particles, which most closely resemble material found in LL4-6 chondrites. We report an 40Ar/39Ar age of 1.3 ± 0.3 Ga for a sample of Itokawa consisting of three grains with a total mass of ~2 μg. This age is lower than the >4.0 Ga ages measured for 75% of LL chondrites but close to one for Y-790964 and its pairs. The flat 40Ar/39Ar release spectrum of the sample suggests complete degassing 1.3 Ga ago. Recent solar heating in Itokawa’s current orbit does not appear likely to have reset that age. Solar or impact heating 1.3 Ga ago could have done so. If impact heating was responsible, then the 1.3 Ga age sets an upper bound on the time at which the Itokawa rubble pile was assembled and suggests that rubble pile creation was an ongoing process in the inner solar system for at least the first 3 billion years of solar system history.

Reference
Park J et al. (2015) 40Ar/39Ar age of material returned from asteroid 25143 Itokawa. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12564]
Published by arrangement with John Wiley & Sons

¹Díaz Michelena, M., ²Kilian, R.
¹Payloads and Space Sciences Department, INTA, Ctra. Torrejón – Ajalvir km 4, Torrejón de Ardoz, Spain
²Geology Department, University of Trier, Behringstrasse, Trier, Germany

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Reference
Díaz Michelena M, Kilian R (2015) Magnetic signatures of the orogenic crust of the Patagonian Andes with implication for planetary Exploration. Physics of the Earth and Planetary Interiors 248, 35-54
Link to Article [DOI: 10.1016/j.pepi.2015.08.005]

¹Erokhin, Y.V., ¹Koroteev, V.A., ¹Khiller, V.V., ²Burlakov, E.V., ¹Ivanov, K.S., ²Kleimenov, D.A.
¹Zavaritskii Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg, Russian Federation
²Ural State Mining University, Yekaterinburg, Russian Federation

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Reference
Erokhin YV, Koroteev VA, Khiller VV, Burlakov EV, Ivanov KS, Kleimenov DA (2015) The Kunashak meteorite: New data on mineralogy. Doklady Earth Sciences 464, 1058-1061
Link to Article [DOI: 10.1134/S1028334X15100128]