Jean-Alix Barrat¹ and Akira Yamaguchi^2,3

¹Université de Bretagne Occidentale and Institut Universitaire Européen de la Mer, CNRS UMR 6538, Plouzané Cedex, France
²National Institute of Polar Research, Tachikawa, Tokyo, Japan
³Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tachikawa, Tokyo, Japan

Mandler and Elkins-Tanton () recently proposed an upgraded magma ocean model for the differentiation history of the giant asteroid 4 Vesta. They show that a combination of both equilibrium crystallization and fractional crystallization processes can reproduce the major element compositions of eucritic melts and broadly the range of mineral compositions observed in diogenites. They assert that their model accounts for all the howardites, eucrites, and diogenites (HEDs), and use it to predict the crustal thickness and the proportions of the various lithologies. Here, we show that their model fails to explain the trace element diversity of the diogenites, contrary to their claim. The diversity of the heavy REE enrichment exhibited by the orthopyroxenes in diogenites is inconsistent with crystallization of these cumulates in either shallow magma chambers replenished by melts from a magma ocean or in a magma ocean. Thus, proportions of the various HED lithologies and the crustal thickness predicted from this model are not necessarily valid.

Reference
Barrat J-A and Yamaguchi A (in press) Comment on “The origin of eucrites, diogenites, and olivine diogenites: Magma ocean crystallization and shallow magma processes on Vesta” by B. E. Mandler and L. T. Elkins-Tanton. Meteoritics & Planetary Science
[doi:10.1111/maps.12250]
Published by arrangement with John Wiley & Sons

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A. V. Fisenko¹, A. B. Verchovsky², L. F. Semjonova¹

¹Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia
²Department of Physical Sciences, Open University, Milton Keynes, UK

The kinetics of the release of the Xe-P3 component from coarse-grained fractions of Orgueil (CI) meteorite nanodiamonds has been investigated using stepped and isothermal pyrolysis. It has been shown that a first-order chemical reaction diffusion model with a single activation energy cannot provide a satisfactory explanation for the observed retention of Xe-P3 during parent body thermal metamorphism and the kinetics of Xe-P3 release from nanodiamonds during isothermal pyrolysis. Using the activation energy and frequency factor calculated according to this model, it is shown that in the course of thermal metamorphism of the Orgueil meteorite almost the entire Xe-P3 component must have been lost in a very short time (<4 yr at approximately 100 °C). However, the calculated retention of Xe-P3 increases significantly if a diffusion model with a spectrum of activation energies is used. In this case, the model can explain not only a high retention of Xe-P3 in the Orgueil nanodiamonds but also the release pattern of the Xe-P3 from Semarkona and Bishunpur nanodiamonds that have experienced a significant gas loss during parent body metamorphism as well as the release of Xe-P3 during isothermal pyrolysis of the Orgueil nanodiamonds. The energetically complicated Xe-P3 distribution is most likely caused by structural damage to the nanodiamond grains or a complex phase composition of carbon in the surface layer of the diamond grains. It is supposed that the structural damage of the diamond grains can have a radiation origin, while the variations of the carbon phase composition in the grain’s mantle can be caused by the radiation-induced reactions and/or a thermal effect.

Reference
Fisenko AV, Verchovsky AB and Semjonova LF (in press) Kinetics of Xe-P3 release during pyrolysis of the coarse-grained fractions of Orgueil (CI) meteorite nanodiamonds. Meteoritics & Planetary Science
[doi:10.1111/maps.12278]
Published by arrangement with John Wiley & Sons

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Robert W. Hilts¹, Christopher D. K. Herd², Danielle N. Simkus³ and Greg F. Slater³

¹Department of Physical Sciences, MacEwan University, Edmonton, Alberta, Canada
²Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
³School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario, Canada

The C2 ungrouped Tagish Lake meteorite preserves a range of lithologies, reflecting variable degrees of parent-body aqueous alteration. Here, we report on soluble organic compounds, including aliphatic and aromatic hydrocarbons, monocarboxylic acids, and amino acids, found within specimens representative of the range of aqueous alteration. We find that differences in soluble organic compounds among the lithologies may be explained by oxidative, fluid-assisted alteration, primarily involving the derivation of soluble organic compounds from macromolecular material. In contrast, amino acids probably evolved from precursor molecules, albeit in parallel with other soluble organic compounds. Our results demonstrate the role of parent-body alteration in the modification of organic matter and generation of prebiotic compounds in the early solar system, and have implications for interpretation of the complement of soluble organic compounds in carbonaceous chondrites.

Reference
Hilts RW, Herd CDK, Simkus DN and Slater GF (in press) Soluble organic compounds in the Tagish Lake meteorite. Meteoritics & Planetary Science
[doi:10.1111/maps.12272]
Published by arrangement with John Wiley & Sons

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Matthias M. M. Meier^1,2, Birger Schmitz², Carl Alwmark¹, Reto Trappitsch³, Colin Maden⁴, Rainer Wieler⁴

¹Lund University, Department of Geology, Lund, Sweden
²Department of Physics, Lund University, Lund, Sweden
³Department of the Geophysical Sciences, University of Chicago and Chicago Center for Cosmochemistry, Chicago, Illinois, USA
⁴Department of Earth Sciences, ETH Zurich, Zurich, Switzerland

We analyzed He and Ne in chromite grains from the regolith breccia Ghubara (L5), to compare it with He and Ne in sediment-dispersed extraterrestrial chromite (SEC) grains from mid-Ordovician sediments. These SEC grains arrived on Earth as micrometeorites in the aftermath of the L chondrite parent body (LCPB) breakup event, 470 Ma ago. A significant fraction of them show prolonged exposure to galactic cosmic rays for up to several 10 Ma. The majority of the cosmogenic noble gases in these grains were probably acquired in the regolith of the LCPB (Meier et al. ). Ghubara, an L chondritic regolith breccia with an Ar-Ar shock age of 470 Ma, is a sample of that regolith. We find cosmic-ray exposure ages of up to several 10 Ma in some Ghubara chromite grains, confirming for the first time that individual chromite grains with such high exposure ages indeed existed in the LCPB regolith, and that the >10 Ma cosmic-ray exposure ages found in recent micrometeorites are thus not necessarily indicative of an origin in the Kuiper Belt. Some Ghubara chromite grains show much lower concentrations of cosmogenic He and Ne, indicating that the 4π (last-stage) exposure age of the Ghubara meteoroid lasted only 4–6 Ma. This exposure age is considerably shorter than the 15–20 Ma suggested before from bulk analyses, indicating that bulk samples have seen regolith pre-exposure as well. The shorter last-stage exposure age probably links Ghubara to a small peak of ⁴⁰Ar-poor L5 chondrites of the same exposure age. Furthermore, and quite unexpectedly, we find a Ne component similar to presolar Ne-HL in the chromite grains, perhaps indicating that some presolar Ne can be preserved even in meteorites of petrologic type 5.

Reference
Meier MMM, Schmitz B, Alwmark C, Trappitsch R, Maden C and Wieler R (in press) He and Ne in individual chromite grains from the regolith breccia Ghubara (L5): Exploring the history of the L chondrite parent body regolith.. Meteoritics & Planetary Science
[doi:10.1111/maps.12275]
Published by arrangement with John Wiley & Sons

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Gonzalo Tancredi

Departamento de Astronomí a, Facultad de Ciencias, Iguá 4225, 11400. Montevideo, URUGUAY

The classification criterion between asteroids and comets has evolved in recent decades, but the main distinction remains unchanged. Comets present gas and dust ejection from the surface at some point of their orbits, therefore, these objects are considered to be active. On the other hand, asteroids do not show any kind of large scale gas and dust ejection, they are inert. Nevertheless, this classification scheme is impractical when we have more than 500,000 asteroids already discovered. In addition, comets are not active all along their orbits. In order for a comet to display activity at present or in the recent past in the inner region of the Solar System (heliocentric distance <2AU), the cometary orbit must be unstable in the time scale on the order of ten thousands of years; otherwise, the object should have completely consumed its volatile component. Close encounters with the most massive planets is the only mechanism that could produce ”macroscopic” instabilities on a short time scale. The macroscopic changes in the orbital elements can be detected in a numerical integration of the dynamical evolution of the object over a time scale of several thousand years. This procedure to identify asteroids in cometary-like orbits is also impractical because it would require months of computing time. Therefore, a classification scheme based on the orbital elements to identify the border cases between the asteroid and comet populations is urgently required.
We present a criterion to classify asteroids and comets and to find the border case based on the Tisserand’s parameter, the Minimum Orbital Intersection Distance (MOID), and considering some information regarding the aphelion and perihelion distances. Objects in mean-motion are disregarded. After applying a filter to the sample of over half a million asteroids already discovered to select the precise orbits and to the sample of 487 short-period comets, we apply the proposed classification criterion. The resulting sample consists of ∼331 Asteroids in Cometary Orbits (ACOs). The ACOs are further classified in subclasses similar to the cometary classification. There are 436 Jupiter Family Comets and 203 ACOs of the Jupiter Family type. This new criterion is more strict that the criteria used by other authors to identify ACOs; nonetheless, with the new criterion we ensure that the ACOs have a chaotic dynamical evolution similar to the periodic comets. The discovered dormant or extinct comets seems, if they exist at all, to be a small fraction of the active comets.
We also analyse the available photometric data of ACOs to identify possible large brightness variations. Among the sample of ACOs, there is only one object with brightness variations typical of an active comet: 174P/(60558) Echeclus. But this object has already been double classified as asteroid and comet.

Reference
Tancredi G (in press) A criterion to classify asteroids and comets based on the orbital parameters. Icarus
[doi:10.1016/j.icarus.2014.02.013]
Copyright Elsevier

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Lauren A. Edgar^a, John P. Grotzinger^b, James F. Bell III^a and Joel A. Hurowitz^c

^aSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
^bDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
^cStony Brook University, Stony Brook, NY, USA

En route to Endeavour crater, the Mars Exploration Rover Opportunity embarked on a short but significant campaign at Santa Maria crater during sols 2450-2551. Santa Maria crater is a relatively young impact crater, approximately 100 m in diameter and 11-17 m deep. Opportunity performed detailed analyses on several ejecta blocks and completed an extensive imaging campaign around the crater. Many of the ejecta blocks are composed of sandstone with abundant wind ripple laminations suggestive of eolian deposition. However, other ejecta blocks are massive, fine-grained, and exhibit a nodular texture. These rocks are interpreted to be the first rocks of a grain size smaller than the Microscopic Imager can resolve, and may represent the first mudstones observed by the rover. Several depositional environments are considered for the origin of the fine-grained rocks, and the observations are best fit by a transient evaporitic lake. If the inferred mudstones were deposited in a lacustrine setting, then surface water may have been present in a broader range of surface environments than previously documented at Meridiani Planum.

Reference
Edgar LA, Grotzinger JP, Bell III JJ and Hurowitz JA (in press) Hypotheses for the origin of fine-grained sedimentary rocks at Santa Maria Crater, Meridiani Planum. Icarus
[doi:10.1016/j.icarus.2014.02.019]
Copyright Elsevier

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Kathryn A. Dyl^a,b, James S. Cleverley^b, Phil A. Bland^a, Chris G. Ryan^b, Louise A. Fisher^b and Robert M. Hough^b

^aDepartment of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
^bCSIRO Earth Sciences and Resource Engineering, 26 Dick Perry Avenue, Kensington, Perth, WA 6151, Australia

We present the application of a new synchrotron-based technique for rapid mapping of trace element distributions across large areas of the CV3 meteorites Allende and Vigarano. This technique utilizes the Australian Synchrotron X-ray Fluorescence Microscopy (XFM) beam line with its custom designed and built X-ray detector array called Maia. XFM with Maia allows data to be collected using a 2 μm spot size at very low dwell times (~0.1-0.5 ms), resulting in maps of entire thin sections in ~5 hours. Maia is an energy dispersive detector system with a large collection solid-angle, which allows full spectral acquisition and high sensitivity. Hence, there is no need to constrain the elements of interest a priori.
We collected whole section maps (~2 cm x 1 cm) from 3 thick sections of Allende and a single map (2 cm x 1.5 cm) from a thick section of Vigarano. Our experimental conditions provide data for elements with 20 ⩽ Z ⩽ 40 (K-shell, Ca through Zr) and the L-emissions of Os, Ir, Pt, Au, and Pb. We illustrate the unique capabilities of this technique by presenting observations across myriad length scales, from the centimeter-scale down to the detection of sub-micrometer particles within these objects. Our initial results show the potential of this technique to help decipher spatial and textural variations in trace element chemistry between CAIs, chondrules, matrix, and other chondritic components. We also illustrate how these datasets can be applied to understanding both nebular and parent-body processes within meteorites.

Reference
Dyl KA, Cleverley JS, Bland PA, Ryan CG, Fisher LA and Hough RM (in press) Quantified, whole section trace element mapping of carbonaceous chondrites by Synchrotron X-ray fluorescence microscopy: 1. CV meteorites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.020]
Copyright Elsevier

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L.J. Hallis^a,b, H.A. Ishii^c, J.P. Bradley^c and G.J. Taylor^a,b

^aNASA Astrobiology Institute, Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, Hawaii 96822-1839, United States
^bHawai’i Institute of Geophysics and Planetology, Pacific Ocean Science and Technology (POST) Building, University of Hawai’i, 1680 East-West Road, Honolulu, HI 96822, United States
^cInstitute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, L-415, Livermore, CA 94550, United States

The nakhlite group of martian meteorites found in the Antarctic contain varying abundances of both martian and terrestrial secondary alteration phases. The aim of this study was to use transmission electron microscopy (TEM) to compare martian and terrestrial alteration embodied within a single nakhlite martian meteorite find – MIL 090032. Martian alteration veins in MIL 090032 are composed of poorly ordered Fe-smectite phyllosilicate. This poorly-ordered smectite appears to be equivalent to the nanocrystalline phyllosilicate/hydrated amorphous gel phase previously described in the martian alteration veins of other nakhlites. Chemical differences in this nanocrystalline phyllosilicate between different nakhlites imply localised alteration, which occurred close to the martian surface in MIL 090032. Both structurally and compositionally the nakhlite nanocrystalline phyllosilicate shows similarities to the amorphous/poorly ordered phase recently discovered in martian soil by the Mars Curiosity Rover at Rocknest, Gale Crater.
Terrestrially derived alteration phases in MIL 090032 include jarosite and gypsum, amorphous silicates, and Fe-oxides and hydroxides. Similarities between the mineralogy and chemistry of the MIL 090032 terrestrial and martian alteration phases suggest the alteration conditions on Mars were similar to those in the Antarctic. At both sites a small amount of fluid at low temperatures infiltrated the rock and became acidic as a result of the conversion of Fe²⁺ to Fe³⁺ under oxidising conditions.

Reference
Hallis LJ, Ishii HA, Bradley JP and Taylor GJ (in press) Transmission Electron Microscope Analyses of Alteration Phases in Martian Meteorite MIL090032. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.007]
Copyright Elsevier

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Tsuyoshi Iizuka^a,b, Yuri Amelin^b, Angela Kaltenbach^c, Piers Koefoed^b and Claudine H. Stirling^c

^aDepartment of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
^bResearch School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
^cCentre for Trace Element Analysis and Department of Chemistry, University of Otago, PO Box 56, Union Place, Dunedin, New Zealand

Ibitira is an unbrecciated, equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites. We present the first combined high-precision U and Pb isotopic data for this unique meteorite. The ²³⁸U/²³⁵U value of 137.777 ± 0.013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites. This value results in corrections of -1.1 Ma for Pb-Pb dates calculated using the previously assumed invariant ²³⁸U/²³⁵U value of 137.88. Using the determined ²³⁸U/²³⁵U value, the 7 most radiogenic Pb isotopic analyses for acid-leached pyroxene-rich and whole rock fractions yield an isochron Pb-Pb age of 4556.75 ± 0.57 Ma, in excellent agreement with the results of Mn-Cr chronology which give the ages of 4557.4 ± 2.5 Ma and 4555.9 ± 3.2 Ma using the U-corrected Pb-Pb age of D’Orbigny as a time anchor. Along with the previously proposed thermal history of Ibitira and our closure temperature estimates for Pb diffusion, the Pb-Pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism. The metamorphism may have been caused by burial of Ibitira lava under successive lava flows and, if so, the Pb-Pb age should post-date the crystallization by a short time interval. The Pb isotopic data for acid leachates suggest partial re-equilibration of Pb between plagioclase and phosphate, perhaps during an impact event at 4.49 Ga, as recorded by K-Ar systematics. The whole rock²³⁸U/²⁰⁴Pb indicates that compared to CI chondrites, Ibitira is less depleted in Pb than in some alkali elements despite a lower condensation temperature of Pb than the alkali elements. The restricted Pb depletion may reflect preferential concentration of metals with high fluid/melt partition coefficients including Pb and Zn as a result of fluid exsolution and migration within the parent magma. We discuss the implications of the U-Pb systematics for the origin and differentiation of the parent body.

Reference
Iizuka T, Amelin Y, Kaltenbach A, Koefoed P and Stirling CH (in press) U-Pb systematics of the unique achondrite Ibitira: Precise age determination and petrogenetic implications. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.017]
Copyright Elsevier

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Tetsuo Yamamoto¹, Toshihiko Kadono², and Koji Wada³

¹Center for Planetary Science, Integrated Research Center of Kobe University, Minatojima, Chuo-ku, Kobe 650-0047, Japan
²School of Medicine, University of Occupational and Environmental Health, Yahatanishi-ku, Kitakyushu, Fukuoka 807-8555, Japan
³Planetary Exploration Research Center, Chiba Institute of Technology, Tsudanuma 2-17-1, Chiba 275-0016, Japan

N-body simulations of collisions of dust aggregates in protoplanetary disks performed so far have revealed that silicate aggregates suffer from catastrophic disruption if the collision velocities are higher than about 10 m s^-1, which is much lower than those expected in the disks. This is mainly due to the low surface energy of the quartz used in the simulations. We find a simple relation between the surface energy and melting temperature for various materials including those of astrophysical interest, and show that the surface energy of the quartz used in the previous simulations is much lower than the present estimate. This result may provide a way out of the difficulty of growing silicate dust inside the snowline in disks. We show that silicate dust can evade catastrophic disruption and grow even at high-velocity collisions expected in the disks if one takes the present estimate of the surface energy into account.

Reference
Yamamoto T, Kadono T and Wada K (2014) An examination of collisional growth of silicate dust in protoplanetary disks. The Astrophysical Journal – Letters 784:L36.
[doi:10.1088/2041-8205/783/2/L36]

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