¹Michael D. Higgins, ¹Pierre-Etienne M.C. Martin, ¹Sciences Appliquées
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.047]
¹Université du Québec à Chicoutimi, Québec G7H 2B1, CANADA
Copyright Elsevier

Abee is an enstatite chondrite breccia dominantly composed of kamacite, enstatite, silica, plagioclase, troilite and niningerite. Clasts are up to 220 mm long and vary in shape from angular to rounded. Some clasts are zoned with kamacite-enriched rims that follow the edge of the clast. Spatial compositional variations were examined in a small block to find out more about the petrological processes that produced this rock, particularly the relationship between the clasts, the matrix and the cores/rims of the zoned clasts. Compositional maps produced using a focussed-beam XRF were segmented into clasts and matrix, and rims and cores where possible. Compositions of most clasts, matrix and rim/cores define a simple, linear trend on simple variation diagrams. If it is assumed that all components were derived from an original homogeneous composition then the variation can be explained either by addition of kamacite or by loss of all other phases. Within this overall compositional variation the kamacite content generally increases as follows: matrix < large homogeneous clasts ≈ zoned clast cores < small homogeneous clasts ≈ zoned clast rims. Production of diversity by addition of kamacite to clasts and rim seems to require a complex history as the source cannot have been the current matrix. It is also difficult to produce the observed chemical variations and zoning by partial melting. However, differentiation by removal of all non-metallic phases may result from repeated impacts: Shock waves would deform kamacite whilst fracturing all other phases. The broken grains would then migrate towards the surface of the clasts where they would spall off into the matrix. This process would also lead to the observed rounding of some clasts. We propose that this shock-differentiation process be called ‘smithing’, as it resembles the ancient process of iron refining.

¹Malcolm J. Rutherford, ¹James W. Head, ¹Alberto E. Saal, ²Erik Hauri, ³Lionel Wilson
American Mineralogist 102, 2045-2053 Link to Article [DOI
https://doi.org/10.2138/am-2017-5994]
¹Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, U.S.A.
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, U.S.A.
³Lancaster Environment Center, Lancaster University, Lancaster LA1 4YQ, U.K.
Copyright: The Mineralogical Society of America

A model for the origin, ascent, and eruption of the lunar A17 orange glass magma has been constructed using petrological constraints from gas solubility experiments and from analyses of the lunar sample 74220 to better determine the nature and origin of this unique explosive eruption. Three stages of the eruption have been identified. Stage 1 of the eruption model extends from ~550 km, the A17 orange glass magma source region based on phase equilibria studies, to 50 km depth in the Moon. Stage 2 extends from ~50 km to 500 m, where a C-O-H-S gas phase formed and grew in volume based on melt inclusion analyses and measurements. The volume of the gas phase at 500 m depth below the surface is calculated to be 7 to 15 vol% of the magma (closed-system) using the minimum and maximum estimates of CO, H2O, and S loss from the melt. In Stage 3, depths shallower than ~450 m, the rising magma exsolved an additional 800–900 ppm H2O and 300 ppm S, increasing the moles in the gas by a factor of 3 to 4. The closed-system gas phase is calculated to reach ~70 vol% at ~130 m depth, enough to fragment the magma and form pyroclastic beads. However, fragmentation (bead formation) is interpreted to have occurred at depths ranging from 600 to 300 m below the lunar surface based on the pressure necessary to explain the C content of the orange glass beads. The gas volume (70%) required to fragment the ascending magma at this depth is a factor of ~5 greater than the volume determined for closed-system degassing of an orange glass magma at 500 m, strongly implying that the gas was produced by open-system degassing as the magma ascended from greater depths.

Formation of the dike carrying the magma up from the ~550 km deep source is considered to occur by a crack propagation mechanism (Wilson and Head 2003, 2017). The rapid dike-propagation process facilitates gas collection by open-system degassing in the upper part of the dike. This is necessary to achieve the gas volumes required for magma fragmentation at 600 m depths, and the magma-ascent velocities to explain the wide areal distribution of the bead deposit. The explosive nature of the picritic orange glass eruption, and the homogeneity of the bead compositions, are consistent with this gas-assisted eruption scenario, as is the evidence of a Fe-metal forming reduction event during Stage 2 followed by a Stage 3 oxidation event in the ascending magma.

¹Jean-Alix Barrat, ¹Pierre Sansjofre, ^2,3Akira Yamaguchi, ⁴Richard C. Greenwood, ⁵Philippe Gillet
Earth and Planetary Science Letters 478, 143-149 Link to Article [https://doi.org/10.1016/j.epsl.2017.08.039]
¹Laboratoire Geosciences Océan (UMR CNRS 6538), Université de Bretagne Occidentale et Institut Universitaire Européen de la Mer, Place Nicolas Copernic, 29280 Plouzané, France
²National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
³Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tachikawa, Tokyo 190-8518, Japan
⁴Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
⁵Earth and Planetary Science Laboratory (EPSL), Ecole Polytechnique Fédérale de Lausanne, Institute of Condensed Matter Physics, Station 3, CH-1015 Lausanne, Switzerland
Copyright Elsevier

We analyzed the C isotopic compositions of 32 unbrecciated ureilites, which represent mantle debris from a now disrupted, C-rich, differentiated body. The δ13C values of their C fractions range from −8.48 to +0.11‰. The correlations obtained between δ13C, δ18O and Δ17O values and the compositions of the olivine cores, indicate that the ureilite parent body (UPB) accreted from two reservoirs displaying distinct O and C isotopic compositions. The range of Fe/Mg ratios shown by its mantle was not the result of melting processes involving reduction with C (“smelting”), but was chiefly inherited from the mixing of these two components. Because smelting reactions are pressure-dependent, this result has strong implications for the size of the UPB, and points to a large parent body, at least 690 km in diameter. It demonstrates that C-rich primitive matter distinct from that represented by carbonaceous chondrites was present in some areas of the early inner Solar System, and could have contributed to the growth of the terrestrial planets. We speculate that differentiated, C-rich bodies, or debris produced by their disruption, were an additional source of volatiles during the later accretion stages of the rocky planets, including Earth.

¹F.Zambon et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.09.021]
¹INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, I-00133 Rome, Italy
Copyright Elsevier

Quadrangle Ac-H-10 ‘Rongo’ (Lat 22°S to 22°N, Lon 288°E-360°E) shows a fairly homogeneous topography, with the presence of notable elevations such as Ahuna Mons, Liberalia Mons, and part of Samhain and Uhola Catenae. The deepest areas correspond to the Rongo crater region, the areas between Samhain and Uhola catenae, and the region of the quadrangle south of Ahuna Mons. A substantial variability in the 2.7-µm band depth distribution is observed across the Rongo quadrangle, indicating an east-west gradient in the abundance of Mg-phyllosilicates. The NH4-phyllosilicates distribution appears quite homogeneous, except some localized regions, such as crater Haulani’s ejecta, the flanks of Ahuna Mons, and crater Begbalel. The two band depths at 2.7 and 3.1 µm display an overall low correlation, suggesting a variable degree of mixing between Mg-phyllosilicates and NH4-phyllosilicates. At the local scale, mineralogical phases other than phyllosilicates are observed. Quadrangle Rongo includes sodium carbonate-rich regions, such as the flanks of Ahuna Mons, a crater Xevioso located in the southern edge of Liberalia Mons, and crater Begbalel, which often display a reduction in both the 2.7- and 3.1-µm band depths, associated with an increased band depth at ∼4 µm, related to the presence of Na-rich carbonate phases. This suggests recent hydrothermal activity in this area, due to several episodes of cryovolcanism, or impacts that unveiled a peculiar composition in the shallow subsurface. Alternatively, the crust in this region might show a variable degree of compactness, such that the formation of Na-carbonates is favored only in specific locations (De Sanctis et al., 2016; Ruesch et al., 2016; Zambon et al., 2017). From a geological standpoint, quadrangle Ac-H-10 Rongo shows a correlation between its two main geologic units (Platz et al., 2017) and the distribution of Mg-phyllosilicates, suggesting a link between geology and mineralogy in this area.

^1,2M.R.M. Izawa, ¹E.A. Cloutis, ¹T. Rhind, ³S.A. Mertzman, ¹Jordan Poitras, ¹Daniel M. Applin, ¹P. Mann
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.09.005]
¹Department of Geography, Univeristy of Winnipeg, Winnipeg MB R3B 2E9 Canada
²Institute for Planetary Materials, Okayama University – Misasa, 827 Yamada, Misasa, Tottori, Japan
³Department of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania, USA 17604-2615
Copyright Elsevier

The utility of spectral reflectance for identification of the main end-member garnets: almandine (Fe2+3Al2Si3O12), andradite (Ca3Fe3+2Si3O12), grossular (Ca3Al2Si3O12), pyrope (Mg3Al2Si3O12), spessartine (Mn2+3Al2Si3O12), and uvarovite (Ca3Cr3+2Si3O12) was studied using a suite of 60 garnet samples. Compositional and structural data for the samples, along with previous studies, were used to elucidate the mechanisms that control their spectral reflectance properties. Various cation substitutions result in different spectral properties that can be determine the presence of various optically-active cations and help differentiate between garnet types. It was found that different wavelength regions are sensitive to different compositional and structural properties of garnets. Crystal-field absorptions involving Fe2+ and/or Fe3+ are responsible for the majority of spectral features in the garnet minerals examined here. There can also be spectral features associated with other cations and mechanisms, such as Fe2+-Fe3+ and Fe2+-Ti4+ intervalence charge transfers. The visible wavelength region is useful for identifying the presence of various cations, in particular, Fe (and its oxidation state), Ti4+, Mn2+, and Cr3+. In the case of andradite, spessartine and uvarovite, the visible region absorption bands are characteristic of these garnets in the sense that they are associated with the major cation that distinguishes each: [6]Fe3+ for andradite, [8]Mn2+ for spessartine, and [6]Cr3+ for uvarovite. For grossular, the presence of small amounts of Fe3+ leads to absorption bands near 0.370 and 0.435 µm. These bands are also seen in pyrope-almandine spectra, which also commonly have additional absorption bands, due to the presence of Fe2+. The common presence of Fe2+ in the dodecahedral site of natural garnets gives rise to three Fe2+ spin-allowed absorption bands in the 1.3, 1.7, and 2.3 µm regions, providing a strong spectral fingerprint for all Fe2+-bearing garnets studied here. Garnets containing Mn2+ have additional visible (∼0.41 µm) spectral features due to [8]Mn2+. Garnets containing Cr3+, exhibits two strong absorption bands near ∼0.7 µm due to spin-forbidden [6]Cr3+ transitions, as well as [6]Cr3+ spin-allowed features near 0.4-0.41 µm and 0.56-0.62 µm, and [6]Cr3+ spin-allowed transitions between 0.41 and 0.68 µm. Common silicate garnet spectra, in summary, are distinct from many other rock-forming silicates and can be spectrally distinct from one garnet species to another. Iron dominates the spectral properties of garnets, and the crystallographic site

¹A.Longobardo et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.09.022]
¹INAF-IAPS, via Fosso del Cavaliere 100, I-00133 Rome, Italy
Copyright Elsevier

We present an analysis of the areal distribution of spectral parameters derived from the VIR imaging spectrometer on board NASA/Dawn spacecraft. Specifically we studied the Occator quadrangle of Ceres, which is bounded by latitudes 22°S to 22°N and longitudes 214°E to 288°E, as part of the overall study of Ceres’ surface composition reported in this special publication. The spectral parameters used are the photometrically corrected reflectance at 1.2 µm, the infrared spectral slope (1.1–1.9 µm), and depths of the absorption bands at 2.7 µm and 3.1 µm that are ascribed to hydrated and ammoniated materials, respectively.

We find an overall correlation between 2.7 µm and 3.1 µm band depths, in agreement with Ceres global behavior, and band depths are shallower and the spectral slope is flatter for younger craters, probably due to physical properties of regolith such as grain size. Spectral variations correlated with the tali geological unit also suggest differences in physical properties. The deepest band, indicating enrichment of ammoniated phyllosilicates, are associated with ejecta generated by impacts that occurred in southern quadrangles.

The most peculiar region of this quadrangle is the Occator crater (20°N 240°E). The internal crater area contains two faculae, which are the brightest areas on Ceres due to exposure of sodium carbonates, and by two types of ejecta, dark and bright, with different spectral properties, probably due to different formation, evolution or age.

^1,2Zachary D.Sharp
Chemical Geology 448, 137-150 Link to Article [https://doi.org/10.1016/j.chemgeo.2016.11.018]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, United States
²Center for Stable Isotopes, University of New Mexico, Albuquerque, NM 87131, United States

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

¹Frank Gyngard, ¹Sachiko Amari, ¹Ernst Zinner, ¹Kuljeet Kaur Marhas2
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.031]
¹Laboratory for Space Sciences and the Department of Physics, Washington University, One Brookings Drive, St. Louis, MO 63130, USA
Copyright Elsevier

We report correlated Si, and Ti isotopic compositions and elemental concentrations of 238 presolar SiC grains from the Murchison CM2 meteorite. Combined with measurements of the C and N isotopic compositions of these 238 grains, 220 were determined to be of type mainstream, 10 type AB, 4 type Y and 4 type Z. SiC grains of diameter ≳2.5µm, to ensure enough material to attempt Ti measurements, were randomly chosen without any other prejudice. The Ti isotopic compositions of the majority of the grains are characterized by enrichments in 46Ti, 47Ti, 49Ti, and 50Ti relative to 48Ti, and show linear isotopic correlations indicative of galactic chemical evolution and neutron capture of the grains parent stars. The variability in the observed Ti signal as a function of depth in most of the grains indicates the presence of distinct subgrains, likely TiC that have been previously observed in TEM studies. Vandium-51 concentrations correlate with those of Ti, indicating V substitutes for Ti in the TiC matrix in many of the grains. No isotopic anomalies in 52Cr/53Cr ratios were observed, and Cr concentrations did not correlate with those of either Ti or V.

¹C. Ashley Norris, ¹Bernard J. Wood
Nature 549, 507-510 Link to Article [doi:10.1038/nature23645]
¹Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

¹Remco C. Hin,¹Christopher D. Coath,²Philip J. Carter, ³Francis Nimmo, Yi-Jen Lai,^1,4Philip A. E. Pogge von Strandmann, ¹Matthias Willbold, ²Zoë M. Leinhardt, ¹Michael J. Walter, ¹Tim Elliott
Nature 549, 511-515 Link to Article [doi:10.1038/nature23899]
¹School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK
²School of Physics, University of Bristol, H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK
³Department of Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, California 95064, USA
⁴London Geochemistry and Isotope Centre, Department of Earth Sciences, University College London, and Department of Earth and Planetary Sciences, Birkbeck, University of London, Gower Street, London WC1E 6BT, UK

We currently do not have a copyright agreement with this publisher and cannot display the abstract here