¹Corentin Le Guillou,²Hitesh G. Changela,²Adrian J. Brearley
¹Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum, Germany
²Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA

The CR chondrites carry one of the most pristine records of the solar nebula materials that accreted to form planetesimals. They have experienced very variable degrees of aqueous alteration, ranging from incipient alteration in their matrices to the complete hydration of all of their components. In order to constrain their chemical alteration pathways and the conditions of alteration, we have investigated the mineralogy and Fe oxidation state of silicates in the matrices of 8 CR chondrites, from type 3 to type 1. Fe-L edge X-ray Absorption Near Edge Structure (XANES) was performed on matrix FIB sections using synchrotron-based scanning transmission X-ray microscopy (STXM). The Fe3+/∑FeFe3+/∑Fe ratio of submicron silicate particles was obtained and coordinated with TEM observations.
In all the least altered CR chondrites (QUE 99177, EET 87770, EET 92042, LAP 02342, GRA 95229 and Renazzo), we find that the matrices consist of abundant submicron Fe-rich hydrated amorphous silicate grains, mixed with nanometer-sized phyllosilicates. The Fe3+/∑FeFe3+/∑Fe ratios of both amorphous and nanocrystalline regions are very high with values ranging from 68 to 78%. In the most altered samples (Al Rais and GRO 95577), fine-grained phyllosilicates also have a high Fe3+/∑FeFe3+/∑Fe ratio (around 70%), whereas the coarse, micrometer-sized phyllosilicates are less oxidized (down to 55%) and have a lower iron content.
These observations suggest the following sequence: submicron Fe2+-amorphous silicate particles were the building blocks of CR matrices; after accretion they were quickly hydrated and oxidized, leading to a metastable, amorphous gel-like phase. Nucleation and growth of crystalline phyllosilicates was kinetically-limited in most type 3 and 2 CRs, but increased as alteration became more extensive in Al Rais and GRO 95577. The decreasing Fe3+/∑FeFe3+/∑Fe ratio is interpreted as a result of the transfer of Fe3+ from silicates to oxides during growth, while aqueous alteration progressed (higher temperature, longer duration, change of fluid composition). In a fully closed system, equilibrium thermodynamics suggest that the water to rock ratios, typically assumed to be low (<1) for chondrites, should primarily control the iron valency of the silicates and predict a lower Fe3+/∑FeFe3+/∑Fe ratio. Such a high Fe3+/∑FeFe3+/∑Fe value could be accounted for, however, if the system was partially open, at least with respect to H2 (and other gases as well). Rapid degassing of the fluid would have favored more oxidizing fluid conditions. Recently proposed scenarios involving some degree of water D/H increase through Rayleigh isotopic fractionation are supported by these results.

Reflectance
Le Guillou C, Changela HG, Brearley AJ (2015) Widespread oxidized and hydrated amorphous silicates in CR chondrites matrices: Implications for alteration conditions and H2 degassing of asteroids. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.02.031]

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^1,2,3,4Johan Villeneuve, ^1,5,6Guy Libourel,¹Camille Soulié
¹CRPG-Université de Lorraine, CNRS, UMR 7358, 15 Rue Notre-Dame des Pauvres, BP20, 545401 Vandoeuvre-lès-Nancy, France
²Université d’Orléans, ISTO, UMR 7327, 45071 Orléans, France
³CNRS/INSU, ISTO, UMR 7327, 45071 Orléans, France
⁴BRGM, BP 36009, 45060 Orléans, France
⁵Ecole Nationale Supérieure de Géologie – Université de Lorraine, Rue du Doyen Marcel Roubault, BP40, 54501, Vandoeuvre-lès-Nancy, France
⁶Geoazur, OCA, Université de Nice – Sophia Antipolis, CNRS/IRD, 250 rue Albert Einstein, Sophia Antipolis, 06560 Valbonne, France

In unequilibrated chondrites, the ferromagnesian silicates in chondrules exhibit wide ranges of mg# = Mg/(Mg + Fe), allowing to sub-divide porphyritic chondrules into either type I (mg# > 0.9) or type II (mg# < 0.9). Although both chondrule types formed under oxidizing conditions relative to the canonical solar nebula, it is generally inferred that type II chondrules formed in more oxidizing conditions than type I. In order to check whether this redox difference was established during chondrule formation, or reflects differences in their precursors, we have undertaken a set of experiments aimed at heating type I olivine-rich (A) chondrule proxy, i.e. forsterite + Fe metal + Ca-Mg-Si-Al glass mixtures, under oxidizing conditions. We show that high temperature (isothermal) oxidation of type IA-like assemblages is a very efficient and rapid process (e.g., few tens of minutes) to form textures similar to type IIA chondrules. Due to the rapid dissolution of Fe metal blebs, a FeO increase in the melt and in combination with the dissolution of magnesian olivine allows the melt to reach ferroan olivine saturation. Crystallization of ferroan olivine occurs either as new crystal in the mesostasis or as overgrowths on the remaining unresorbed forsterite grains (relicts). Interruption of this process at any time before its completion by rapid cooling allows to reproduce the whole range of textures and chemical diversity observed in type A chondrules, i.e., from type I to type II.

Several implications on chondrule formation processes can be inferred from the presented experiments. Type I chondrules or fragments of type I chondrules are very likely the main precursor material involved in the formation of most type II chondrules. Formation of porphyritic olivine type II chondrules is very likely the result of processes generating crystal growth by chemical disequilibrium at high temperature rather than processes generating crystallization only by cooling rates. This questions the reliability of chondrule thermal history (e.g., cooling rate values) hitherto inferred for producing porphyritic textures from dynamical cooling rate experiments only. Type A chondrule formation can be a very fast process. After periods of sub-isothermal heating or slow cooling (< 50 K/h) as short as several tens of minutes and no longer than few hundreds of minutes at 1500 -1800°C, type A chondrules terminates their formation by a fast cooling (> 103-104 K/h) in order to preserve their glassy mesostasis. Such inferred thermal history being at odds with nebular shock models, we thus advocate that impacts on planetesimals causing rapid melting and vaporization may provide the high density and highly volatile-enriched gaseous environments required to form chondrules. In this scenario, chondrules and their diversity should result from various degrees of interaction of the ejected fragments with the impact vapor plume; the most oxidizing conditions recorded in type IIA chondrules being very likely the closest to those imposed by the impact vapor plume.

Reference
Villeneuve J, Libourel G, Souliéa C (2015) Relationships between type I and type II chondrules: Implications on chondrule formation processes. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.03.033]

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¹G.J. Consolmagno,^2,3G.J. Golabek,⁴D. Turrini,⁵M. Jutzi,⁶S. Sirono,⁷V. Svetsov, ⁸K. Tsiganis
¹Specola Vaticana, V-00120, Vatican City State
²Institute of Geophysics, ETH Zurich, Sonneggstrasse 5, CH-8092 Zürich, Switzerland
³Bayerisches Geoinstitut, University of Bayreuth, D-95440 Bayreuth, Germany
⁴Istituto di Astrofisica e Planetologia Spaziali INAF-IAPS, Via Fosso del Cavaliere 100, 00133 Rome, Italy
⁵Physics Institute, Space Research and Planetary Sciences, Center for Space and Habitability, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
⁶Earth and Environmental Sciences, Nagoya University, Tikusa-ku, Furo-cho, Nagoya 464-8601 Japan
⁷Institute for Dynamics of Geospheres, Leninskiy Prospekt 38-1, Moscow, Russia
⁸Unit of Mechanics, Section of Astrophysics, Astronomy & Mechanics, Department of Physics, Aristotle University of Thessaloniki, GR 54 124 Thessaloniki, Greece

It is difficult to find a Vesta model of iron core, pyroxene and olivine-rich mantle, and HED crust that can match the joint constraints of (a) Vesta’s density and core size as reported by the Dawn spacecraft team; (b) the chemical trends of the HED meteorites, including the depletion of sodium, the FeO abundance, and the trace element enrichments; and (c) the absence of exposed mantle material on Vesta’s surface, among Vestoid asteroids, or in our collection of basaltic meteorites. These conclusions are based entirely on mass-balance and density arguments, independent of any particular formation scenario for the HED meteorites themselves. We suggest that Vesta either formed from source material with non-chondritic composition or underwent after its formation a radical physical alteration, possibly caused by collisional processes, that affected its global composition and interior structure.

Reference
Consolmagno GJ, Golabek GJ, Turrini D, Jutzi M, Sirono S, Svetsov V, Tsiganis K (2015) Is Vesta an Intact and Pristine Protoplanet? Icarus (in Press)
Link im Article [doi:10.1016/j.icarus.2015.03.029]

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¹Alexander Hubbard
¹Department of Astrophysics, American Museum of Natural History, New York, NY 10024-5192, USA

About 4-5% of chondrules are compound: two separate chondrules stuck together. This is commonly believed to be the result of the two component chondrules having collided shortly after forming, while still molten. This allows high velocity impacts to result in sticking. However, at T ∼1100 K, the temperature below which chondrules collide as solids (and hence usually bounce), coalescence times for droplets of appropriate composition are measured in tens of seconds. Even at 1025 K, at which temperature theory predicts that the chondrules must have collided extremely slowly to have stuck together, the coalescence time scale is still less than an hour. These coalescence time scales are too short for the collision of molten chondrules to explain the observed frequency of compound chondrules. We suggest instead a scenario where chondrules stuck together in slow collisions while fully solid; and the resulting chondrule pair was subsequently briefly heated to a temperature in the range of 900-1025 K. In that temperature window the coalescence time is finite but long, covering a span of hours to a decade. This is particularly interesting because those temperatures are precisely the critical window for thermally ionized MRI activity, so compound chondrules provide a possible probe into that vital regime.

Reference
Hubbard A (2015) Compound Chondrules fused Cold. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.02.030]

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¹Scott L. Murchie et al (>10)*
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
*Find the extensive, full author and affiliation list on the publishers website

A principal data product from MESSENGER’s primary orbital mission at Mercury is a global multispectral map in eight visible to near-infrared colors, at an average pixel scale of 1 km, acquired by the Mercury Dual Imaging System. The constituent images have been calibrated, photometrically corrected to a standard geometry, and map projected. Global analysis reveals no spectral units not seen during MESSENGER’s Mercury flybys and supports previous conclusions that most spectral variation is related to changes in spectral slope and reflectance between spectral end-member high-reflectance red plains (HRP) and low-reflectance material (LRM). Comparison of color properties of plains units mapped on the basis of morphology shows that the two largest unambiguously volcanic smooth plains deposits (the interior plains of Caloris and the northern plains) are close to HRP end members and have average color properties distinct from those of most other smooth plains and intercrater plains. In contrast, smaller deposits of smooth plains are nearly indistinguishable from intercrater plains on the basis of their range of color properties, consistent with the interpretation that intercrater plains are older equivalents of smooth plains. LRM having nearly the same reflectance is exposed in crater and basin ejecta of all ages, suggesting impact excavation from depth of material that is intrinsically dark or darkens very rapidly, rather than the product of gradual darkening of exposed material purely by space weathering. A global search reveals no definitive absorptions attributable to Fe2+-containing silicates or to sulfides over regions 20 km or more in horizontal extent, consistent with results from MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer. The only absorption-like feature identified is broad upward curvature of the spectrum centered near 600 nm wavelength. The feature is strongest in freshly exposed LRM and weak or absent in older exposures of LRM. We modeled spectra of LRM as intimate mixtures of HRP with candidate low-reflectance phases having a similar 600-nm spectral feature, under the assumption that the grain size is 1 μm or larger. Sulfides measured to date in the laboratory and coarse-grained iron are both too bright to produce LRM from HRP. Ilmenite is sufficiently dark but would require Ti abundances too high to be consistent with MESSENGER X-Ray Spectrometer measurements. Three phases or mixtures of phases that could be responsible for the low reflectance of LRM are consistent with our analyses. Graphite, in amounts consistent with upper limits from the Gamma-Ray Spectrometer, may be consistent with geochemical models of Mercury’s differentiation calling for a graphite-enriched primary flotation crust from an early magma ocean and impact mixing of that early crust before or during the late heavy bombardment (LHB) into material underlying the volcanic plains. The grain size of preexisting iron or iron sulfide could have been altered to a mix of nanophase and microphase grains by shock during those impacts, lowering reflectance. Alternatively, iron-bearing phases and carbon in a late-accreting carbonaceous veneer may have been stirred into the lower crust or upper mantle. Decoupling of variations in color from abundances of major elements probably results from the very low content and variation of Fe2+ in crustal silicates, such that reflectance is controlled instead by minor opaque phases and the extent of space weathering.

Reference
Murchie SL et al. (2015) Orbital Multispectral Mapping of Mercury with the MESSENGER Mercury Dual Imaging System: Evidence for the Origins of Plains Units and Low-Reflectance Material. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.03.027]

Copyright Elsevier

^1,2Melissa A. Morris, ³Laurence A. J. Garvie, ²L. Paul Knauth
¹State University of New York, Cortland, NY 13045, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
³Center for Meteorite Studies, Arizona State University, Tempe, AZ 85287, USA

Many aspects of planet formation are controlled by the amount of gas remaining in the natal protoplanetary disks (PPDs). Infrared observations show that PPDs undergo a transition stage at several megayears, during which gas densities are reduced. Our Solar System would have experienced such a stage. However, there is currently no data that provides insight into this crucial time in our PPD’s evolution. We show that the Isheyevo meteorite contains the first definitive evidence for a transition disk stage in our Solar System. Isheyevo belongs to a class of metal-rich meteorites whose components have been dated at almost 5 Myr after formation of Ca, Al-rich inclusions, and exhibits unique sedimentary layers that imply formation through gentle sedimentation. We show that such layering can occur via the gentle sweep-up of material found in the impact plume resulting from the collision of two planetesimals. Such sweep-up requires gas densities consistent with observed transition disks (10−12–10−11 g cm−3). As such, Isheyevo presents the first evidence of our own transition disk and provides new constraints on the evolution of our solar nebula.

Reference
Morris MA, Garvie LAJ, Knauth LP (2015) New Insight into the Solar System’s Transition Disk Phase Provided by the Metal-rich Carbonaceous Chondrite Isheyevo. Astrophysical Journal 801 L22.
Link to Article [doi:10.1088/2041-8205/801/2/L22]

¹Daniel Ozdín et al. (>10)*
¹Department of Mineralogy and Petrology, Faculty of Natural Sciences, Comenius University, Bratislava, Slovak Republic
*Find the extensive, full author and affiliation list on the publishers website

The Košice meteorite was observed to fall on 28 February 2010 at 23:25 UT near the city of Košice in eastern Slovakia and its mineralogy, petrology, and geochemistry are described. The characteristic features of the meteorite fragments are fan-like, mosaic, lamellar, and granular chondrules, which were up to 1.2 mm in diameter. The fusion crust has a black-gray color with a thickness up to 0.6 mm. The matrix of the meteorite is formed mainly by forsterite (Fo80.6); diopside; enstatite (Fs16.7); albite; troilite; Fe-Ni metals such as iron and taenite; and some augite, chlorapatite, merrillite, chromite, and tetrataenite. Plagioclase-like glass was also identified. Relative uniform chemical composition of basic silicates, partially brecciated textures, as well as skeletal taenite crystals into troilite veinlets suggest monomict breccia formed at conditions of rapid cooling. The Košice meteorite is classified as ordinary chondrite of the H5 type which has been slightly weathered, and only short veinlets of Fe hydroxides are present. The textural relationships indicate an S3 degree of shock metamorphism and W0 weathering grade. Some fragments of the meteorite Košice are formed by monomict breccia of the petrological type H5. On the basis of REE content, we suggest the Košice chondrite is probably from the same parent body as H5 chondrite Morávka from Czech Republic. Electron-microprobe analysis (EMPA) with focused and defocused electron beam, whole-rock analysis (WRA), inductively coupled plasma mass and optical emission spectroscopy (ICP MS, ICP OES), and calibration-free laser induced breakdown spectroscopy (CF-LIBS) were used to characterize the Košice fragments. The results provide further evidence that whole-rock analysis gives the most accurate analyses, but this method is completely destructive. Two other proposed methods are partially destructive (EMPA) or nondestructive (CF-LIBS), but only major and minor elements can be evaluated due to the significantly lower sample consumption.

Reference
Ozdín D et al. (2015) Mineralogy, petrography, geochemistry, and classification of the Košice Meteorite. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12405]

Published by arrangement with John Wiley&Sons

The most popular papers in Cosmochemistry Papers in March were:

1-Krzesińska A, Gattacceca J, Friedrich JM and Rochette P (2015) Impact-related noncoaxial deformation in the Pułtusk H chondrite inferred from petrofabric analysis. Meteoritics & Planetary Sciences (in Press) Link to Article [doi:10.1111/maps.12429]

2-Van Hoesel A, Hoek WZ, Pennock GM, Kaiser K, Plümper O, Jankowski M, Hamers MF, Schlaak N, Küster M, Andronikov AV and Drury MR (2015) A search for shocked quartz grains in the Allerød-Younger Dryas boundary layer. Meteoritics & Planetary Sciences (in Press) Link to Article [doi:10.1111/maps.12435]

3-Charnoz S, Aleon J, Chaumard N, Baillie K, Taillifet E (2015) Growth of calcium-aluminum-rich inclusions by coagulation and fragmentation in a turbulent protoplanetary disk: observations and simulations. Icarus (in Press) Link to Article [doi:10.1016/j.icarus.2015.01.023]

4-Peplowski PN et al. (2015) Geochemical terranes of Mercury’s northern hemisphere as revealed by MESSENGER neutron measurements. Icarus (in Press) Link to Article [doi:10.1016/j.icarus.2015.02.002]

5-Hopkins MD, Mojzsis SJ (2015) A protracted timeline for lunar bombardment from mineral chemistry, Ti thermometry and U–Pb geochronology of Apollo 14 melt breccia zircons. Contributions to Mineralogy and Petrology 169:30 Link to Article [DOI 10.1007/s00410-015-1123-x]

5-Greenberger RN, Mustard JF, Cloutis EA, Pratt LM, Sauer PE, Mann P, Turner K, Dyar MD, Bish DL (2015) Serpentinization, iron oxidation, and aqueous conditions in an ophiolite: Implications for hydrogen production and habitability on Mars. Earth and Planetary Science Letters 416, 21–34 Link to Article [doi:10.1016/j.epsl.2015.02.002]

¹Stein B. Jacobsen
¹Gang Yu
¹Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA

Heterogeneities in terrestrial samples for 182W/183W and 142Nd/144Nd are only preserved in Hadean and Archean rocks while heterogeneities in 129Xe/130Xe and 136Xe/130Xe persist to very young mantle-derived rocks. In contrast, meteorites from Mars show that the Martian mantle preserves heterogeneities in 182W/183W and 142Nd/144Nd up to the present. As a consequence of the probable “deep magma ocean” core formation process, we assume that the Earth and Mars both had a very early two-mantle-reservoir structure with different initial extinct nuclide isotopic compositions (different 182W/183W, 142Nd/144Nd, 129Xe/130Xe, 136Xe/130Xe ratios). Based on this assumption, we developed a simple stochastic model to trace the evolution of a mantle with two initially distinct layers for the extinct isotopes and its development into a heterogeneous mantle by convective mixing and stretching of these two layers. Using the extinct isotope system 182Hf-182W, we find that the mantles of Earth and Mars exhibit substantially different mixing or stirring rates. This is consistent with Mars having cooled faster than the Earth due to its smaller size, resulting in less efficient mantle mixing for Mars. Moreover, the mantle stirring rate obtained for Earth using 182Hf-182W is consistent with the mantle stirring rate of ~500 Myr constrained by the long-lived isotope system, 87Rb-87Sr and 147Sm-143Nd. The apparent absence of 182W/183W isotopic heterogeneity in modern terrestrial rocks is attributed to very active mantle stirring which reduced the 182W/183W isotopic heterogeneity to a relatively small scale (~83 m for a mantle stirring rate of 500 Myr) compared to the common sampling scale of terrestrial basalts (~30 or 100 km). Our results also support the “deep magma ocean” core formation model as being applicable to both Mars and Earth.

Reference
Jacobsen SB, Yu G (2015) Extinct isotope heterogeneities in the mantles of Earth and Mars: Implications for mantle stirring rates. Meteoritics&Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12426]

Published by arrangement with John Wiley & Sons

¹J.A. Barrat,²O. Rouxel,³K. Wang,^4,5F. Moynier,^6,7A. Yamaguchi,⁸A. Bischoff,⁹J. Langlade
¹Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, CNRS UMR 6538, Place Nicolas Copernic, 29280 Plouzané, France
²IFREMER, centre de Brest, 29280 Plouzané, France
³Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
⁴Institut de Physique du Globe de Paris, Institut Universitaire de France, Université Paris Diderot, Sorbonne Paris Cité, 1 rue Jussieu, 75238 Paris Cedex 05, France
⁵Institut Universitaire de France, Paris, France
⁶National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
⁷Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tachikawa, Tokyo 190-8518, Japan
⁸Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
⁹CNRS UMS 3113, I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France

Ureilite meteorites are achondrites that are debris of the mantle of a now disrupted differentiated asteroid rich in carbon. They provide a unique opportunity to study the differentiation processes of such a body. We analyzed the iron isotopic compositions of 30 samples from the Ureilite Parent Body (UPB) including 29 unbrecciated ureilites and one ureilitic trachyandesite (ALM-A) which is at present the sole large crustal sample of the UPB. The δ56Fe of the whole rocks fall within a restricted range, from 0.01 to 0.11‰, with an average of +0.056±0.008‰+0.056±0.008‰, which is significantly higher than that of chondrites. We show that this difference can be ascribed to the segregation of S-rich metallic melts at low degrees of melting at a temperature close to the Fe–FeS eutectic, and certainly before the onset of the melting of the silicates (View the MathML source<1100°C), in agreement with the marked S depletions, and the siderophile element abundances of the ureilites. These results point to an efficient segregation of S-rich metallic melts during the differentiation of small terrestrial bodies.

Reference
Barrat JA, Rouxel O,Wang K, Moynier F,Yamaguchi A,Bischoff A, Langlade J (2015) Early stages of core segregation recorded by Fe isotopes in an asteroidal mantle. Earth and Planetary Science Letters, 419, 93–100
Link to Article [http://dx.doi.org/10.1016/j.epsl.2015.03.026]

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