^1,2Christoph Burkhardt,²Maria Schönbächler
¹Institute of Geochemistry and Petrology, Clausiusstrasse 25, ETH Zürich, CH-8092 Zürich, Switzerland
²Origins Laboratory, Department of Geophysical Sciences, The University of Chicago, IL 60637, USA

The progressive dissolution of the carbonaceous chondrites Orgueil (CI1), Murchison (CM2) and Allende (CV3) with acids of increasing strength reveals correlated W isotope variations ranging from 3.5 ε182W and 6.5 ε183W in the initial leachate (acetic acid) to –60 ε182W and –40 ε183W in the leachate residue. The observed variations are readily explained by variable mixing of s-process depleted and s-process enriched components. One W s-process carrier is SiC, however, the observed anomaly patterns and mass-balance considerations require at least on additional s-process carrier, possibly a silicate or sulfide. The data reveal well-defined correlations, which provide a test for s-process nucleosynthesis models. The correlations demonstrate that current models need to be revised and highlight the need for more precise W isotope data of SiC grains. Furthermore the correlations provide a mean to disentangle nucleosynthetic and radiogenic contributions to 182W (ε182Wcorrected= ε182Wmeasured –(1.41±0.05) × ε183Wmeasured; ε182Wcorrected= ε182Wmeasured –(–0.12±0.06) × ε184Wmeasured), a prerequisite for the successful application of the Hf-W chronometer to samples with nucleosynthetic anomalies.

The overall magnitude of the W isotope variations decreases in the order CI1>CM2>CV3. This can be interpreted as the progressive thermal destruction of an initially homogeneous mixture of presolar grains by parent-body processing. However, not only the magnitude but also the W anomaly patterns of the three chondrites are different. In particular leach step 2, that employs nitric acid, reveals a s-deficit signature for Murchison, but a s-excess for Orgueil and Allende. This could be the result of redistribution of anomalous W into a new phase by parent-body alteration, or, the fingerprint of dust processing in the solar nebula. Given that the thermal and aqueous alteration of Murchison is between the CI and CV3 chondrites, parent-body processing is probably not the sole cause for creating the different patters. Small-scale nebular redistribution of anomalous W may have played a role as well. Similar nebular processes possibly acted differently on specific carrier phases and elements, resulting in the diverse nucleosynthetic signatures observed in planetary materials today.

Reference
Burkhardt C, Schönbächler M (2015) Intrinsic W nucleosynthetic isotope variations in carbonaceous chondrites: Implications for W nucleosynthesis and nebular vs. parent body processing of presolar materials.
Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.06.012]
Copyright Elsevier

¹Jean Bollard, ¹James N. Connelly, ¹Martin Bizzarro
¹Centre for Star and Planet Formation, Natural History Museum of Denmark, Copenhagen, Denmark

The CB chondrites are metal-rich meteorites with characteristics that sharply distinguish them from other chondrite groups. Their unusual chemical and petrologic features and a young formation age of bulk chondrules dated from the CBa chondrite Gujba are interpreted to reflect a single-stage impact origin. Here, we report high-precision internal isochrons for four individual chondrules of the Gujba chondrite to probe the formation history of CB chondrites and evaluate the concordancy of relevant short-lived radionuclide chronometers. All four chondrules define a brief formation interval with a weighted mean age of 4562.49 ± 0.21 Myr, consistent with its origin from the vapor-melt impact plume generated by colliding planetesimals. Formation in a debris disk mostly devoid of nebular gas and dust sets an upper limit for the solar protoplanetary disk lifetime at 4.8 ± 0.3 Myr. Finally, given the well-behaved Pb-Pb systematics of all four chondrules, a precise formation age and the concordancy of the Mn-Cr, Hf-W, and I-Xe short-lived radionuclide relative chronometers, we propose that Gujba may serve as a suitable time anchor for these systems.

Reference
Bollard J, Connelly JN, Bizzarro M (2015) Pb-Pb dating of individual chondrules from the CBa chondrite Gujba: Assessment of the impact plume formation model. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12461]

Published by arrangement with John Wiley&Sons

¹J. Borovička et al. (>10)*
¹Astronomical Institute of the Czech Academy of Sciences, Ondřejov, Czech Republic

The Križevci H6 meteorite was recovered on the basis of fireball data obtained by the cameras of the Croatian Meteor Network. The fireball, which occurred on February 4, 2011, 23:20:40 UT, was also observed by meteor cameras in Slovenia and by the Autonomous Fireball Observatory in Martinsberg, Austria, which belongs to the European Fireball Network. Here, we present detailed data on fireball trajectory, velocity, deceleration, light curve, and orbit. We also modeled the atmospheric fragmentation of the meteoroid on the basis of the light curve and deceleration. The initial mass of the meteoroid was between 25–100 kg, most probably about 50 kg. Severe fragmentation occurred at heights of approximately 60 and 31 km, under dynamic pressures of 0.1 and 3 MPa, respectively. The peak absolute magnitude of −13.7 was reached during the second severe fragmentation event. The recovered 291 g meteorite was probably the only fragment with a terminal mass exceeding 100 g. The orbit had a low inclination of 0.6 degrees, perihelion distance 0.74 AU, and semimajor axis 1.54 AU. Križevci can be ranked among the 10 best documented meteorite falls.

Reference
Borovička J et al. (2015) The instrumentally recorded fall of the Križevci meteorite, Croatia, February 4, 2011. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12469]

Published by arrangement with John Wiley and Sons

^1,2Mutsumi Komatsu, ¹Timothy J. Fagan, ³Takashi Mikouchi, ⁴Michail I. Petaev, ⁵Michael E. Zolensky
¹Department of Earth Sciences, Waseda University, Tokyo, Japan
²Waseda Institute for Advanced Study, Waseda University, Tokyo, Japan
³Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan
⁴Department of Earth and Planetary Sciences, Harvard University and Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA
⁵NASA Johnson Space Center, Astromaterials Research and Exploration Science (ARES), Houston, Texas, USA

MnO/FeO ratios in olivine from amoeboid olivine aggregates (AOAs) reflect conditions of nebular condensation and can be used in concert with matrix textures to compare metamorphic conditions in carbonaceous chondrites. LIME (low-iron, Mn-enriched) olivine was identified in AOAs from Y-81020 (CO3.05), Kaba (CV~3.1), and in Y-86009 (CV3), Y-86751 (CV3), NWA 1152 (CR/CV3), but was not identified in AOAs from Efremovka (CV3.1–3.4) or Allende (CV>3.6). According to thermodynamic models of nebular condensation, LIME olivine is stable at lower temperatures than Mn-poor olivine and at low oxygen fugacities (dust enrichment <10× solar). Although this set of samples does not represent a single metamorphic sequence, the higher subtypes tend to have AOA olivine with lower Mn/Fe, suggesting that Mn/Fe decreases during parent body metamorphism. Y-81020 has the lowest subtype and most forsteritic AOA olivine (Fo>95) in our study, whereas Efremovka AOAs are slightly Fe-rich (Fo>92). AOA olivines from Kaba are mostly forsteritic, but rare Fe-rich olivine precipitated from an aqueous fluid. A combination of precipitation of Fe-rich olivine and diffusion of Fe into primary olivine grains resulted in iron-rich compositions (Fo97–59) in Allende AOAs. Variations from fine-grained, nonporous matrix toward higher porosity and coarser lath-like matrix olivine can be divided into six stages represented by (1) Y-81020, Efremovka, NWA 1152; (2) Y-86751 lithology B; (3) Y-86009; (4) Kaba; (5) Y-86751 lithology A; (6) Allende. These stages are inferred to represent general degree of metamorphism, although the specific roles of thermally driven grain growth and diffusion versus aqueous dissolution and precipitation remain uncertain.

Reference
Komatsu M, Fagan TJ, Mikouchi T, Petaev MI, Zolensky ME (2015) LIME silicates in amoeboid olivine aggregates in carbonaceous chondrites: Indicator of nebular and asteroidal processes. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12460]

Published by arrangement with John Wiley&Sons

¹Ryuji Okazaki et al. (>10)*
¹Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, 33, Hakozaki, Higashi-ku 812-8581, Fukuoka, Japan
*Find the extensive, full author and affiliation list on the publishers website

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Reference
Okazaki R et al. (2015) Mineralogy and noble gas isotopes of micrometeorites collected from Antarctic snow. Earth, Planets and Space 2015, 67:90
Link to Article [doi:10.1186/s40623-015-0261-8]

¹Hans-Peter Gail, ¹Stephan Henke, ^2,3Mario Trieloff
¹Institut für Theoretische Astrophysik, Zentrum für Astronomie, Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
e-mail: gail@uni-heidelberg.de
²Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany
³Klaus-Tschira-Labor für Kosmochemie, Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany

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Reference
Gail H-P, Henke S, Trieloff M (2015) Thermal evolution and sintering of chondritic planetesimals – II. Improved treatment of the compaction process. Astronomy&Astrophysics 576, A60

Link to Article [http://dx.doi.org/10.1051/0004-6361/201424278]

¹Ernesto Palomba et al. (>10)*
¹INAF-IAPS, Via del Fosso del Cavaliere 100, I-00133 Rome, Italy
*Find the extensive, full author and affiliation list on the publishers website

The discovery of olivine on Vesta’s surface by the VIR imaging spectrometer onboard the Dawn space mission has forced us to reconsider our views of Vestan petrogenetic models. Olivines were expected to be present in the interior of Vesta: in the mantle of a vertically layered body as invoked by the magma ocean models, or at the base (or within) the mantle-crust boundary as proposed by fractionation models. Olivines have been detected by VIR-Dawn in two wide areas near Arruntia and Bellicia, regions located in the northern hemisphere. Interestingly, these olivine-rich terrains are far from the Rheasilvia and the more ancient Veneneia basins, which are expected to have excavated the crust down to reach the mantle. In this work we present our attempts to identify other undetected olivine rich areas on Vesta by using spectral parameters sensitive to olivine such as the Band Area Ratio (BAR) and other specific parameters created for the detection of olivines on Mars (forsterite, fayalite and a generic olivine index). As a preliminary step we calibrated these parameters by means of VIS-IR spectra of different HED meteorite samples: behaviors versus sample grain size and albedo were analyzed and discussed. We selected the BAR and the forsterite index as the best parameters that can be used on Vesta. A cross-correlation analysis has been applied in order to detect olivine signature on the VIR hyperspectral cubes. These detections have then been confirmed by an anti-correlation analysis between the BAR and one of the olivine parameters, independent of the first method applied.
In agreement with the recent discovery, Arruntia and Bellicia were found to be as the most olivine-rich areas, i.e. where the parameter values are strongest. In addition we detected 6 new regions, all but one located in the Vesta north hemisphere. This result confirms again that the old petrogenetic models cannot be straightforwardly applied to Vesta and should be reshaped in the view of these new detections. An alternative and very recent option can be represented by the model according to which surface “eruption” of material from the mantle, including olivine can reach the surface of Vesta.

Reference
Palomba E et al. (2015) Detection of new olivine-rich locations on Vesta. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.06.011]

Copyright Elsevier

¹Wilfred F.M. Röling, ¹Joost W. Aerts, ¹C.H. Lucas Patty, ²Inge Loes ten Kate, ^3,4Pascale Ehrenfreund, ^1,5Susana O.L. Direito
¹Molecular Cell Physiology, Faculty of Earth and Life Sciences, VU University Amsterdam, Amsterdam, the Netherlands.
²Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands.
³Space Policy Institute, George Washington University, Washington, DC, USA.
⁴Leiden Observatory, University of Leiden, Leiden, the Netherlands.
⁵School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK.

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Reference
Röling WFM, Aerts JW, Patty CHL, ten Kate IL, Ehrenfreund P, Direito SOL (2015) The Significance of Microbe-Mineral-Biomarker Interactions in the Detection of Life on Mars and Beyond. Astrobiology 15(6): 492-507.
Link to Article [doi:10.1089/ast.2014.1276]

^1,2A. Pommier,³K. Leinenweber, ⁴M. Tasaka
¹University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA 92093, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
³Department of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287, USA
⁴Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, USA

Electrical conductivity measurements were performed during melting experiments of olivine compacts (dry and hydrous Fo77 and Fo90) at 4 and 6 GPa in order to investigate melt transport properties and quantify the effect of partial melting on electrical properties. Experiments were performed in the multi-anvil apparatus and electrical measurements were conducted using the impedance spectroscopy technique with the two-electrode method. Changes in impedance spectra were used to identify the transition from an electrical response controlled by the solid matrix to an electrical response controlled by the melt phase. This transition occurs slightly above the solidus temperature and lasts until View the MathML sourceTsolidus+75°C (±25). At higher temperature, a significant increase in conductivity (corresponding to an increase in conductivity values by a factor ranging from ∼30 to 100) is observed, consistent with the transition from a tube-dominated network to a structure in which melt films and pools become prominent features. This increase in conductivity corresponds to an abrupt jump for all dry samples and to a smoother increase for the hydrous sample. It is followed by a plateau at higher temperature, suggesting that the electrical response of the investigated samples lacks sensitivity to temperature at an advanced stage of partial melting. Electron microprobe analyses on quenched products indicated an increase in Mg# (molar Mg/(Mg+Fe)Mg/(Mg+Fe)) of olivine during experiments (∼77–93 in the quenched samples with an initial Fo77 composition and ∼92–97 in the quenched samples with an initial Fo90 composition) due to the partitioning of iron to the melt phase. Assuming a respective melt fraction of 0.10 and 0.20 before and after the phase of significant increase in conductivity, in agreement with previous electrical and permeability studies, our results can be reproduced satisfactorily by two-phase electrical models (the Hashin and Shtrikman bounds and the modified brick layer model), and provide a melt conductivity value of 78 (±8) S/m for all Fo77 samples and 45 (±5) S/m for the Fo90 sample. Comparison of our results with electromagnetic sounding data of the deep interior of the Moon supports the hypothesis of the presence of interconnected melt at the base of the lunar mantle. Our results underline that electrical conductivity can be used to investigate in situ melt nucleation and migration in the interior of terrestrial planets.

Reference
Pommier A, Leinenweber K, Tasaka M (2015) Experimental investigation of the electrical behavior of olivine during partial melting under pressure and application to the lunar mantle. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.05.052]

Copyright Elsevier

¹Dwijesh Ray, ¹S. Ghosh, ¹S.V.S. Murty
¹PLANEX, Physical Research Laboratory, Ahmedabad 380009, India

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Ray D, Ghosh D, Murty SVS (2015) Shock-Thermal History of Kavarpura IVA Iron: Evidences from Microtextures and Nickel Profiling. Planetary and Space Science (in Press)
Link to Article [doi:10.1016/j.pss.2015.05.016]