Cartier Camille^a,b,c, Hammouda Tahar^a,b,c, Doucelance Régis^a,b,c, Boyet Maud^a,b,c, Devidal Jean-Luc^a,b,c, Moine Bertrand^b,c,d

^aClermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France
^bCNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France
^cIRD, R 163, LMV, F-63038 Clermont-Ferrand, France
^dDépartement de Géologie, Université Jean Monnet, 23 rue du Dr P. Michelon, 42023 Saint-Etienne, Cedex 02, France

In order to investigate the influence of very reducing conditions, we report enstatite-melt trace element partition coefficients (D) obtained on enstatite chondrite material at 5 GPa and under oxygen fugacities (fO₂) ranging between 0.8 and 8.2 log units below the iron-wustite (IW) buffer. Experiments were conducted in a multianvil apparatus between 1580 and 1850°C, using doped (Sc, V, REE, HFSE, U, Th) starting materials. We used a two-site lattice strain model and a Monte-Carlo-type approach to model experimentally determined partition coefficient data. The model can fit our partitioning data i.e. trace elements repartition in enstatite, which provides evidence for the attainment of equilibrium in our experiments. The precision on the lattice strain model parameters obtained from modelling does not enable determination of the influence of intensive parameters on crystal chemical partitioning, within our range of conditions (fO₂, P, T, composition). We document the effect of variable oxygen fugacity on the partitioning of multivalent elements. Cr and V, which are trivalent in the pyroxene at around IW-1 are reduced to 2+ state with increasingly reducing conditions, thus affecting their partition coefficients. In our range of redox conditions Ti is always present as a mixture between 4+ and 3+ states. However the Ti³⁺/Ti⁴⁺ ratio increases strongly with increasingly reducing conditions. Moreover in highly reducing conditions, Nb and Ta, that usually are pentavalent in magmatic systems, appear to be reduced to lower valence species, which may be Nb²⁺ and Ta³⁺. We propose a new proxy for fO₂ based on D(Cr)/D(V) ratio. Our new data extend the redox range covered by previous studies and allows this proxy to be used in the whole range of redox conditions of the solar system objects. We selected trace-element literature data of six chondrules on the criterion of their equilibrium. Applying the proxy to opx-matrix systems, we estimated that three type I chondrules have equilibrated at IW-7±1, one type I chondrule at IW-4±1, and two type II chondrules at IW+3±1. This first accurate estimation of enstatite-melt fo₂for type I chondrules is very close to CAI values.

Reference
Camille C, Tahar H, Régis D, Maud B, Jean-Luc D, Bertrand M (in press) Experimental study of trace element partitioning between enstatite and melt in Enstatite-Chondrites at low oxygen fugacities and 5 GPa. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.002]
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Hiroshi Kimura

Graduate School of Science, Kobe University, c/o CPS (Center for Planetary Science), Chuo-ku Minatojima Minamimachi 7-1-48, Kobe 650-0047, Japan

We model dust in comets, protoplanetary disks, and debris disks as aggregates consisting of submicron-sized grains with a silicate core and an organic-rich carbonaceous mantle. By computing the infrared (IR) spectra of the aggregates, we show that the degree of carbonization determines the positions of infrared peaks characteristic of magnesium-rich crystalline silicates. We discuss our results in terms of processing of organic materials by ultraviolet irradiation, ion bombardments, and thermal devolatilization. A comparison between the model IR spectra of the aggregates and the observed spectra of dust in circumstellar disks reveals that at least one third of the organic refractory component has suffered from carbonization in a very short timescale.

Reference
Kimura H (in press) The Organic-Rich Carbonaceous Component of Dust Aggregates in Circumstellar Disks: Effects of Its Carbonization on Infrared Spectral Features of Its Magnesium-Rich Olivine Counterpart. Icarus
[doi:10.1016/j.icarus.2014.01.009]
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F. Moreno¹, J. Licandro^2,3, C. Álvarez-Iglesias^2,3,4, A. Cabrera-Lavers^2,3,4 and F. Pozuelos¹

¹Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
²Instituto de Astrofísica de Canarias, c/Vía Láctea s/n, E-38200 La Laguna, Tenerife, Spain
³Departamento de Astrofísica, Universidad de La Laguna (ULL), E-38205 La Laguna, Tenerife, Spain
⁴GTC Project, E-38205 La Laguna, Tenerife, Spain

We present observations and models of the dust environment of activated asteroid P/2013 P5 (PANSTARRS). The object displayed a complex morphology during the observations, with the presence of multiple tails. We combined our own observations, all made with instrumentation attached to the 10.4 m Gran Telescopio Canarias on La Palma, with previously published Hubble Space Telescopeimages to build a model aimed at fitting all the observations. Altogether, the data cover a full three month period of observations which can be explained by intermittent dust loss. The most plausible scenario is that of an asteroid rotating with the spinning axis oriented perpendicular to the orbit plane and losing mass from the equatorial region, consistent with rotational break-up. Assuming that the ejection velocity of the particles (v ~0.02–0.05 m s^-1) corresponds to the escape velocity, the object diameter is constrained to ~30–130 m for bulk densities 3000–1000 kg m^-3.

Reference
Moreno F, Licandro J, Álvarez-Iglesias C, Cabrera-Lavers A and Pozuelos F (2014) Intermittent Dust Mass Loss from Activated Asteroid P/2013 P5 (PANSTARRS). The Astrophysical Journal 781:118.
[doi:10.1088/0004-637X/781/2/118]

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M. E. Schmidt¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Earth Sciences, Brock University, St. Catharines, Ontario, Canada

The first four rocks examined by the Mars Science Laboratory Alpha Particle X-ray Spectrometer indicate that Curiosity landed in a lithologically diverse region of Mars. These rocks, collectively dubbed the Bradbury assemblage, were studied along an eastward traverse (sols 46–102). Compositions range from Na- and Al-rich mugearite Jake_Matijevic to Fe-, Mg-, and Zn-rich alkali-rich basalt/hawaiite Bathurst_Inlet and span nearly the entire range in FeO* and MnO of the data sets from previous Martian missions and Martian meteorites. The Bradbury assemblage is also enriched in K and moderately volatile metals (Zn and Ge). These elements do not correlate with Cl or S, suggesting that they are associated with the rocks themselves and not with salt-rich coatings. Three out of the four Bradbury rocks plot along a line in elemental variation diagrams, suggesting mixing between Al-rich and Fe-rich components. ChemCam analyses give insight to their degree of chemical heterogeneity and grain size. Variations in trace elements detected by ChemCam suggest chemical weathering (Li) and concentration in mineral phases (e.g., Rb and Sr in feldspars). We interpret the Bradbury assemblage to be broadly volcanic and/or volcaniclastic, derived either from near the Gale crater rim and transported by the Peace Vallis fan network, or from a local volcanic source within Gale Crater. High Fe and Fe/Mn in Et_Then likely reflect secondary precipitation of Fe³⁺ oxides as a cement or rind. The K-rich signature of the Bradbury assemblage, if igneous in origin, may have formed by small degrees of partial melting of metasomatized mantle.

Reference
Schmidt ME et al. (in press) Geochemical diversity in first rocks examined by the Curiosity Rover in Gale Crater: Evidence for and significance of an alkali and volatile-rich igneous source. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004481]
Published by arrangement with John Wiley & Sons

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Andrew R. Poppe

Space Sciences Laboratory, 7 Gauss Way, University of California at Berkeley, Berkeley, CA, 94720, USA

The influx of interplanetary dust grains (IDPs) to the Pluto-Charon system is expected to drive several physical processes, including the formation of tenuous dusty rings and/or exospheres, the deposition of neutral material in Pluto’s atmosphere through ablation, the annealing of surface ices, and the exchange of ejecta between Pluto and its satellites. The characteristics of these physical mechanisms are dependent on the total incoming mass, velocity, variability, and composition of interplanetary dust grains; however, our knowledge of the IDP environment in the Edgeworth-Kuiper Belt has, until recently, remained rather limited. Newly-reported measurements by the New Horizons Student Dust Counter combined with previous Pioneer 10 meteoroid measurements and a dynamical IDP tracing model have improved the characterization of the IDP environment in the outer solar system, including at Pluto-Charon. Here we report on this modeling and data comparison effort, including a discussion of the IDP influx to Pluto and its moons, and the implications thereof.

Reference
Poppe AR (in press) Interplanetary dust influx to the Pluto-Charon system. Icarus
[doi:10.1016/j.icarus.2013.12.029]
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Alex Ruzicka

Cascadia Meteorite Laboratory, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland, OR 97207-0751, United States

Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (~50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by ²⁶Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.

Reference
Ruzicka R (in press) Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies. Chemie der Erde
[doi:10.1016/j.chemer.2013.10.001]
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M. A. Siegler and S. E. Smrekar

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

We reexamine the Apollo Heat Flow Experiment in light of new orbital data. Using three-dimensional thermal conduction models, we examine effects of crustal thickness, density, and radiogenic abundance on measured heat flow values at the Apollo 15 and 17 sites. These models show the importance of regional context on heat flux measurements. We find that measured heat flux can be greatly altered by deep subsurface radiogenic content and crustal density. However, total crustal thickness and the presence of a near-surface radiogenic-rich ejecta provide less leverage, representing only minor (<1.5 mW m⁻²) perturbations on surface heat flux. Using models of the crust implied by Gravity Recovery and Interior Laboratory results, we found that a roughly 9–13 mW m⁻² mantle heat flux best approximate the observed heat flux. This equates to a total mantle heat production of 2.8–4.1 × 10¹¹ W. These heat flow values could imply that the lunar interior is slightly less radiogenic than the Earth’s mantle, perhaps implying that a considerable fraction of terrestrial mantle material was incorporated at the time of formation. These results may also imply that heat flux at the crust-mantle boundary beneath the Procellarum potassium, rare earth element, and phosphorus (KREEP) Terrane (PKT) is anomalously elevated compared to the rest of the Moon. These results also suggest that a limited KREEP-rich layer exists beneath the PKT crust. If a subcrustal KREEP-rich layer extends below the Apollo 17 landing site, required mantle heat flux can drop to roughly 7 mW m⁻², underlining the need for future heat flux measurements outside of the radiogenic-rich PKT region.

Reference
Siegler MA and Smrekar SE (in press) Lunar heat flow: Regional prospective of the Apollo landing sites. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004453]
Published by arrangement with John Wiley & Sons

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Elizabeth Gibney

We still seek a copyright agreement with Nature to display abstracts of their cosmochemistry related publications.

Reference
Gibney E (2014) Comet craft ready to wake. Nature 505:269–270.
[doi:10.1038/505269a]

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Sune G. Nielsen^a,b, Julie Prytulak^a,c, Bernard J. Wood^a and Alex N. Halliday^a

^aDepartment of Earth Sciences, University of Oxford, South Parks Road, OX1 3AN, Oxford, UK
^bDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
^cDepartment of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK

It has been argued that the stable isotopic composition of the element vanadium (V) provides a potential indicator of the effects high-energy irradiation early in Solar System development. Such irradiation would produce enrichment in the minor isotope, ⁵⁰V compared with the 400 times more abundant ⁵¹V (Gounelle et al., 2001 and Lee et al., 1998). Here we show that the vanadium isotopic composition of the silicate Earth is enriched in ⁵¹V by ~0.8‰ compared with carbonaceous and ordinary chondrites as well as achondrites from Mars and the asteroid 4 Vesta. Although V is depleted by core formation, experiments reveal no isotopic fractionation between metal and silicate that could account for the observed difference in V isotope composition between terrestrial and extraterrestrial materials. Nucleosynthetic provenance of the terrestrial vanadium isotope offset is inconsistent with anomalies of other nucleosynthetically produced isotopes in bulk meteorites, which are more variable than vanadium (Burkhardt et al., 2011, Carlson et al., 2007 and Trinquier et al., 2009). Furthermore, V isotopes are unlikely to have been affected by volatilization, parent body alteration or impact erosion of Earthʼs surface. Therefore, the cause of the isotopic difference is unclear. One possibility is that Earthʼs isotopically heavier V reflects a deficit in material irradiated during the initial stages of Solar System formation. Whatever the cause, the terrestrial deficit in ⁵⁰V implies that bulk Earth cannot be entirely reconstructed by mixtures of different meteorites.

Reference
Nielsen SG, Prytulak J, Wood BJ and Halliday AN (2014) Vanadium isotopic difference between the silicate Earth and meteorites. Earth and Planetary Science Letters 389:167–175.
[doi:10.1016/j.epsl.2013.12.030]
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David A. Minton^a and Harold F. Levison^b

^aPurdue University Department of Earth, Atmospheric, & Planetary Sciences, 550 Stadium Mall Drive, West Lafayette, IN 47907
^bSouthwest Research Institute and NASA Lunar Science Institute, 1050 Walnut St. Suite 300, Boulder, CO 80302

We develop a model for planetesimal-driven migration (PDM) in the context of rocky planetary embryos in the terrestrial planet region during the runaway and oligarchic growth phases of inner planet formation. We develop this model by first showing that there are five necessary and sufficient criteria that must be simultaneously satisfied in order for a rocky inner solar system embryo to migrate via PDM. To investigate which embryos within a given disk satisfy the five criteria, we have developed a Monte Carlo planetesimal merger code that simulates the growth of embryos from a planetesimal disk with nebular gas. The results of our Monte Carlo planetesimal merger code suggest that, for typical values of the minimum mass solar nebula for the inner solar system, an average of 0.2 embryos capable of PDM emerge over the lifetime of the disk. Many disks in our simulations produce no migration candidates, but some produced as many as 3. The number of embryos that experience PDM in a disk increases with increasing disk mass and decreasing il planetesimal mass, although we were not able to simulate disks where the average initial planetesimal size was smaller than 50 km. For disks 4× more massive than the standard minimum mass solar nebula, we estimate that an average of 1.5 embryos capable of PDM emerge, with some producing as many as 7.

Reference
Minton DA and Levison HF (2014) Planetesimal-driven migration of terrestrial planet embryos. Icarus
[doi:10.1016/j.icarus.2014.01.001]
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