V. Piétu¹, S. Guilloteau^2,3, E. Di Folco^2,3, A. Dutrey^2,3 and Y. Boehler⁴

¹IRAM, 300 rue de la piscine, 38406 Saint Martin d’Hères France
²Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
³CNRS, LAB, UMR 5804, 33270 Floirac, France
⁴Centro de Radioastronomìa y Astrofìsica, UNAM, Apartado Postal 3-72, 58089 Morelia, Michoacàn, Mexico

Context. Most Class II sources (of nearby star-forming regions) are surrounded by disks with weak millimeter continuum emission. These “faint” disks may hold clues to the disk dissipation mechanism. However, the physical properties of protoplanetary disks have been directly constrained by imaging only the brightest sources.
Aims. We attempt to determine the characteristics of such faint disks around classical T Tauri stars and to explore the link between disk faintness and the proposed disk dispersal mechanisms (accretion, viscous spreading, photo-evaporation, planetary system formation).
Methods. We performed high angular resolution (0.3′′) imaging of a small sample of disks (9 sources) with low 1.3 mm continuum flux (mostly <30 mJy) with the IRAM Plateau de Bure interferometer and simultaneously searched for ¹³CO (or CO) J = 2−1 line emission. Using a simple parametric disk model, we determined characteristic sizes for the disks in dust and gas, and we constrained surface densities in the central 50 AU.
Results. All disks are much smaller than the bright disks imaged so far, both in continuum and ¹³CO lines (5 detections). In continuum, half of the disks are very small, with characteristic radii less than 10 AU, but still have high surface density values. Small sizes appear to be the main cause of the low disk luminosity. Direct evidence for grain growth is found for the three disks that are sufficiently resolved. Low continuum opacity is attested in only two systems, but we cannot firmly distinguish between a low gas surface density and a lower dust emissivity resulting from grain growth. Finally, we report a tentative discovery of a ~20 AU radius cavity in DS Tau, which with the (unresolved) “transition” disk of CX Tau, brings the proportion of “transitional” disks to a similar value to that of brighter sources. The existence of cavities cannot by itself explain their observed low mm flux.
Conclusions. This study highlights a category of very compact dust disks that still exhibit high surface densities, which may represent up to 25% of the whole disk population. While its origin is unclear with the current data alone, it may be related to the compact planetary systems found by the Kepler mission.

Reference
V. Piétu V, Guilloteau S, Di Folco E, Dutrey A and Boehler Y (2014) Faint disks around classical T Tauri stars: Small but dense enough to form planets Astronomy & Astrophysics 564:A95.
[doi:10.1051/0004-6361/201322388]
Reproduced with permission © ESO
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Maria C. Valdes^a, Manuel Moreira^b, Julien Foriel^a and Frédéric Moynier^a,b

^aDepartment of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University in St. Louis, United States
^bInstitut de Physique du Globe de Paris, Université Paris Diderot, 1 rue Jussieu, 75005 Paris, France

Calcium is the fifth most abundant element in the Earth and in chondrites and is a pure lithophile element which does not partition into planetary cores. Therefore, the calcium isotopic composition of the mantle represents the bulk Earth and calcium isotopes have the potential to reveal genetic links between Earth and meteorites. However, whether calcium exhibits significant mass-dependent variations among Earth and the various chondrite groups, and the magnitude of these variations, is still contentious. Here we have developed a new method to analyze calcium isotope ratios with high precision using multiple-collector inductively-coupled-plasma mass-spectrometry. The method has been applied to a range of terrestrial and meteoritic samples. We find that the Earth, the Moon, and the aubrite parent body are indistinguishable from enstatite, ordinary, and CO chondritic meteorites. Therefore, enstatite chondrites cannot be excluded as components of Earth’s building blocks based on calcium isotopes, as has been proposed previously. In contrast, CI, CV, CM and CR carbonaceous chondrites are largely enriched in lighter calcium isotopes compared to Earth, and, overall, exhibit a wide range in calcium isotopic composition. Calcium is the only major element, along with oxygen, for which isotopic variations are observed among carbonaceous chondrite groups. These calcium isotope variations cannot be attributed to volatility effects, and it is difficult to ascribe them to the abundance of isotopically light refractory inclusions. The calcium isotope data presented in this study suggest that both ordinary and enstatite chondrites are representative of the bulk of the refractory materials that formed Earth. On the basis of calcium isotopes, carbonaceous chondrites (with the exception of CO) are not representative of the fraction of condensable material that accreted to form the terrestrial planets and can be excluded as unique contenders for the building blocks of Earth; however, on the basis of other isotopic systems, CO chondrites can be excluded as well.

Reference
Valdes MC, Moreira M, Foriel J and Moynier F (2014) The nature of Earth’s building blocks as revealed by calcium isotopes. Earth and Planetary Science Letters 394:135.
[doi:10.1016/j.epsl.2014.02.052]
Copyright Elsevier

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Indhu Varatharajan, Neeraj Srivastava and Sripada V.S. Murty

PLANEX, Physical Research Laboratory, Ahmedabad 380009, India

A comparative assessment of the mineralogy of young basalts (∼1.2 Ga to ∼2.8 Ga) from the western nearside, Moscoviense basin, and the Orientale basin of the Moon has been made using Level 2 Moon Mineralogy Mapper (M³) data from the Chandrayaan-1 mission. Spectral data characteristics of the individual units have been generated from fresh small craters to minimize the complications due to space weathering. Representative spectra for individual units and the derived spectral parameters (Band centers and Integrated Band Depth Ratio) have been used to study composition of these young basalts. A modified approach ofGaffey et al. (2002) (for olivine-pyroxene mixtures) and the methodology of Adams (1974) (for interpreting pyroxene type) have been used to improve our understanding of the spectral behavior of these basalts. Most of the young basalts of Oceanus Procellarum are characterized by abundant olivines and they show complex volcanic history. Vast exposures of olivine concentrated units having higher abundance of olivine content than high-Ca pyroxenes are emplaced in the northern Oceanus Procellarum region. Mostly, they show distinct stratigraphic gradation with the immediately underlying units of relatively lower olivine content. The Moscoviense unit shows signatures of Fe-rich glasses along with clinopyroxenes. The basalts of Orientale basin are typically devoid of olivine and are rich in high-Ca pyroxene. Thus, mineralogy of these mare basalts which erupted during the late stage volcanism vary across the Moon’s surface; however, broader observations reveal apparently higher FeO content in the younger basalts of western nearside and Orientale region.

Reference
Varatharajan I, Srivastava N and Murty SVS (2014) Mineralogy of young lunar mare basalts: Assessment of temporal and spatial heterogeneity using M3 data from Chandrayaan-1. Icarus
[doi:10.1016/j.icarus.2014.03.045]
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Edward R.D. Scott^a, Tatiana V. Krot^a, Joseph I. Goldstein^b and Shigeru Wakita^a,c

^aHawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, 96822, USA
^bDept. of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 01003, USA
^cCenter for Computational Astrophysics, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

We have studied cloudy taenite, metallographic cooling rates, and shock effects in 30 H3-6 chondrites to elucidate the thermal and early impact history of the H chondrite parent body. We focused on H chondrites with old Ar-Ar ages (>4.4 Gyr) and unshocked and mildly shocked H chondrites, as strongly shocked chondrites with such old ages are very rare. Cooling rates for most H chondrites at 500 °C are 10-50 °C/Myr and do not decrease systematically with increasing petrologic type as predicted by the onion-shell model in which types 3 to 5 are arranged in concentric layers around a type 6 core. Some type 4 chondrites cooled slower than some type 6 chondrites and type 3 chondrites did not cool faster than other types, contrary to the onion-shell model. Cloudy taenite particle sizes, which range from 40 to 120 nm, are inversely correlated with metallographic cooling rates and show that the latter were not compromised by shock heating. The three H4 chondrites that were used to develop the onion-shell model, Ste. Marguerite, Beaver Creek, and Forest Vale, cooled through 500 °C at ⩾5000 °C/Myr. Our thermal modeling shows that these rates are 50 higher than could be achieved in a body that was heated by ²⁶Al and cooled without disturbance by impact. Published Ar-Ar ages do not decrease systematically with increasing petrologic type but do correlate inversely with cloudy taenite particle size suggesting that impact mixing decreased during metamorphism. Metal and silicate compositions in regolith breccias show that impacts mixed material after metamorphism without causing significant heating. Impacts during metamorphism created Portales Valley and two other H6 chondrites with large metallic veins, excavated the fast-cooled H4 chondrites around 3-4 Myr after accretion, and mixed petrologic types. Metallographic data do not require catastrophic disruption by impact during cooling.

Reference
Scott ERD, Krota TV, Goldstein JI and Shigeru Wakita S (in press) Thermal and Impact History of the H Chondrite Parent Asteroid during Metamorphism: Constraints from Metallic Fe-Ni. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.03.038]
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Chi Ma¹ and Alexander N. Krot²

¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
²Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mänoa, Honolulu, Hawai’i 96822, U.S.A.

Hutcheonite (IMA 2013-029), Ca₃Ti₂(SiAl₂)O₁₂, is a new garnet mineral that occurs with monticellite, grossular, and wadalite in secondary alteration areas along some cracks between primary melilite, spinel, and Ti,Al-diopside in a Type B1 Fractionation and Unidentified Nuclear effects (FUN) Ca-Al-rich inclusion (CAI) Egg-3 from the Allende CV (Vigarano type) carbonaceous chondrite. The mean chemical composition of type hutcheonite by electron probe microanalysis is (wt%) CaO 34.6, TiO₂ 25.3, SiO₂ 20.9, Al₂O₃ 15.7, MgO 2.1, FeO 0.7, V₂O₃ 0.5, total 99.8, giving rise to an empirical formula of Ca_2.99(Ti⁴⁺_1.53Mg_0.25Al_0.17Fe²⁺_0.05V³⁺_0.03)(Si_1.68Al_1.32)O₁₂. The end-member formula is Ca₃Ti₂(SiAl₂)O₁₂. Hutcheonite has the Ia3̄d garnet structure with a = 11.843 Å, V = 1661.06 ų, and Z = 8, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 3.86 g/cm³. Hutcheonite is a new secondary phase in Allende, apparently formed by iron-alkali-halogen metasomatic alteration of the primary CAI phases like melilite, perovskite, and Ti,Al-diopside on the CV chondrite parent asteroid. Formation of the secondary Ti-rich minerals like hutcheonite during the metasomatic alteration of the Allende CAIs suggests some mobility of Ti during the alteration. The mineral name is in honor of Ian D. Hutcheon, a cosmochemist at Lawrence Livermore National Laboratory, California, U.S.A.

Reference
Ma C and Krot AN (2014) Hutcheonite, Ca3Ti2(SiAl2)O12, a new garnet mineral from the Allende meteorite: An alteration phase in a Ca-Al-rich inclusion. American Mineralogist 99:667.
[doi:10.2138/am.2014.4761]
Copyright: The Mineralogical Society of America

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Sabrina Ferrari^1,3, Fabrizio Nestola¹, Matteo Massironi^1,2, Alessandro Maturilli³, Jörn Helbert³, Matteo Alvaro^1,4, M. Chiara Domeneghetti⁵ and Federico Zorzi¹

¹Department of Geosciences, University of Padua, Via G. Gradenigo 6, 35131 Padova, Italy
²Astronomical Observatory of Padua, INAF, Vicolo Osservatorio 5, 35122 Padova, Italy
³Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin-Adlershof, Germany
⁴IRSPS, G. D’Annunzio University, Via Pindaro 42, 65127 Pescara, Italy
⁵Department of Earth and Environmental Sciences, University of Pavia, Via Ferrata 1, 27100 Pavia, Italy

This work was carried out within the framework of the European Space Agency and Japanese Aerospace Exploration Agency BepiColombo space mission to Mercury and intends to provide valid tools for the interpretation of spectra acquired by the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) on board of BepiColombo.
Two C2/c augitic pyroxenes, with different Mg/Fe ratios and constant Ca contents, were investigated by in situ high-temperature thermal infrared spectroscopy and in situ high-temperature single-crystal X-ray diffraction up to temperatures of about 750 and 770 K, respectively.
The emissivity spectra of the two samples show similar band center shifts of the main three bands toward lower wavenumbers with increasing temperature. In detail, with increasing temperature bands 1 and 2 of both samples show a much stronger shift with respect to band 3, which remains almost unchanged. Our results indicate that the center positions of bands 1 and 2 are strong functions of the temperature, whereas the center position of band 3 is a strong function of the Mg# [with Mg# = Mg/(Mg + Fe²⁺) atomic ratio].
The analysis of the thermal behavior gives similar thermal expansion volume coefficients, α_V, for the Mg-rich and Fe-rich samples, with α_V = 2.72(8) and 2.72(7) × 10^-5 K^-1, respectively, using the Berman (1988) equation. This correspondence totally explains the band center shifts similarity between the two samples.
Our data suggest that MERTIS spectra will be able to provide indications ofC2/c augitic pyroxene Mg# and will allow a correct interpretation that is independent on the spectra acquisition temperature.

Reference
Ferrari S, Nestola F, Massironi M, Maturilli A, Helbert J, Alvaro M, Domeneghetti MC and Zorzi F (2014) In-situ high-temperature emissivity spectra and thermal expansion of C2/c pyroxenes: Implications for the surface of Mercury. American Mineralogist 99:786.
[doi:10.2138/am.2014.4698]
Copyright: The Mineralogical Society of America

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Heather A. Viles

School of Geography and the Environment, University of Oxford, Oxford OX1 3QY, UK

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

Reference
Viles HA (2014) Solar system: Cracking up on asteroids. Nature 508:190.
[doi:10.1038/nature13222]

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Marco Delbo¹ et al.*
*Find the extensive, full author and affiliation list on the publishers website.

^aLaboratoire Lagrange, UNS-CNRS, Observatoire de la Côte d’Azur, Boulevard de l’Observatoire-CS 34229, 06304 Nice Cedex 4, France

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

Reference
Delbo M (2014) Thermal fatigue as the origin of regolith on small asteroids. Nature 508:233.
[doi:10.1038/nature13153]

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Justin Filiberto¹, Juliane Gross², Jarek Trela^1,† and Eric C. Ferré¹

¹Department of Geology, Southern Illinois University, 1259 Lincoln Drive, MC 4324, Carbondale, Illinois 62901, U.S.A.
²Department of Earth and Planetary Sciences, The American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, U.S.A.
^†Virginia Tech, Department of Geosciences, 4044 Derring Hall (0420), Blacksburg, Virginia 24061, U.S.A.

Meteorite Northwest Africa (NWA) 6963 was classified as a basaltic shergottite based on mineralogy, but here we show that it is a gabbroic rock with a quartz-alkali feldspar intergrowth that represents a late-stage granitic melt. NWA 6963 contains clinopyroxene and maskelynite grains up to 5 mm in length, with minor ferroan olivine, spinel, ilmenite, merrillite, apatite, Fe-sulfides, and high-Si glass. NWA 6963 also contains areas of quartz and alkali-feldspar intergrowths up to ~1 mm in size. Based on mineral abundances and textural analysis, we suggest that NWA 6963 is an intrusive rock similar to a terrestrial gabbro. Infiltration of the martian crust by young gabbroic bodies would suggest that estimates of crustal composition, density, and thickness based on the surface chemistry alone would be problematic and the martian crust may be even more heterogenous than is seen from orbit alone. Investigations of crater walls, where intrusive crustal rocks would be exposed, are needed to discover the launch sites of the shergottites and the full heterogeneity of the martian crust.

Reference
Filiberto J, Gross J, Trela J and Ferré EC (2014) Gabbroic Shergottite Northwest Africa 6963: An intrusive sample of Mars‡. American Mineralogist 99:601.
[doi:10.2138/am.2014.4638]
Copyright: The Mineralogical Society of America

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Chi Ma*, John R. Beckett and George R. Rossman

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.

During a nanomineralogy investigation of the Allende meteorite with analytical scanning electron microscopy, two new minerals were discovered; both occur as micro- to nano-crystals in an ultrarefractory inclusion, ACM-1. They are allendeite, Sc₄Zr₃O₁₂, a new Sc- and Zr-rich oxide; and hexamolybdenum (Mo,Ru,Fe,Ir,Os), a Mo-dominant alloy. Allendeite is trigonal, R3̄, a = 9.396, c = 8.720, V = 666.7 ų, and Z = 3, with a calculated density of 4.84 g/cm³ via the previously described structure and our observed chemistry. Hexamolybdenum is hexagonal, P6₃/mmc, a = 2.7506, c = 4.4318 Å, V = 29.04 ų, and Z = 2, with a calculated density of 11.90 g/cm³ via the known structure and our observed chemistry. Allendeite is named after the Allende meteorite. The name hexamolybdenum refers to the symmetry (primitive hexagonal) and composition (Mo-rich). The two minerals reflect conditions during early stages of the formation of the Solar System. Allendeite may have been an important ultrarefractory carrier phase linking Zr-,Sc-oxides to the more common Sc-,Zr-enriched pyroxenes in Ca-Al-rich inclusions. Hexamolybdenum is part of a continuum of high-temperature alloys in meteorites supplying a link between Os- and/or Ru-rich and Fe-rich meteoritic alloys. It may be a derivative of the former and a precursor of the latter.

Reference
Ma C, Beckett JR and Rossman GR (2014) Allendeite (Sc4Zr3O12) and hexamolybdenum (Mo,Ru,Fe), two new minerals from an ultrarefractory inclusion from the Allende meteorite. American Mineralogist 99:654.
[doi:10.2138/am.2014.4667]
Copyright: The Mineralogical Society of America

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