^1,2Ai-Cheng Zhang, ³Chi Ma, ⁴Naoya Sakamoto, ¹Ru-Cheng Wang,⁵Wei-Biao Hsu, ^2,4Hisayoshi Yurimoto
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
²Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
⁴Isotope Imaging Laboratory, Creative Research Institution Sousei, Hokkaido University, Sapporo 001-0021, Japan
⁵Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China

Titanium-rich minerals are common in Ca-Al-rich inclusions from primitive chondrites. They are important not only for testing the condensation models for a gas with a solar composition, but also for constraining the redox conditions of the early solar nebula. In this study, we report the detailed mineralogical features and its oxygen isotope compositions of a Ti-Sc-rich ultrarefractory inclusion A0031 from a CH3 chondrite Sayh al Uhaymir 290. The A0031 inclusion has a compact and layered texture with the interior consisting of panguite, Sc-rich anosovite, Ti-rich davisite, and anorthite. A few hexaferrum, perovskite, and spinel crystals are present as inclusions in these minerals. Outside of Ti-rich davisite are a layer of Al-Ti-rich diopside and two grains of enstatite. This texture strongly suggests that A0031 has a condensation origin. Panguite is its third occurrence in nature and similar in composition to the type panguite from the Allende meteorite. Sc-rich anosovite in A0031 has a chemical formula of (Ti4+,Ti3+,Mg,Sc,Al)3O5 with the pseudobrookite structure. This is the second report of Ti3O5 in nature, but is the first description of anosovite formed in the solar nebula as an ultrarefractory phase. The discovery of Sc-rich anosovite in A0031 reveals the stability of Ti3O5 in the early solar nebula and supports the prediction of previous equilibrium condensation calculations. The panguite, Sc-rich anosovite, and Ti-rich davisite in A0031 show a large variation in Ti3+/Titot. The primitive nature of A0031 implies that the variations in Ti3+/Titot among different Ti-rich minerals are primary features. We propose that the distribution of Ti3+ and Ti4+ could be controlled mainly by their various competition abilities of incorporating into these Ti-Sc-Al-rich minerals. Similarity of Ti3+/Titot value between Ti-rich davisite from A0031 and those in other carbonaceous chondrites indicates that most refractory inclusions might have formed in highly reducing nebular settings. The 16O-depleted isotope compositions of A0031 confirm the existence of diverse oxygen reservoirs for CH Ca-Al-rich inclusions in the solar nebula.

Reference
Zhang A-C, Ma C, Sakamoto N, Wang R-C, Hsu W-B, Yurimoto H (2015) Mineralogical anatomy and implications of a Ti-Sc-rich ultrarefractory inclusion from Sayh al Uhaymir 290 CH3 chondrite. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.052]

Copyright Elsevier

¹Jean-Philippe Combe et al. (>10)*
¹Bear Fight Institute, 22 Fiddler’s Road, P.O. Box 667, Winthrop, WA 98862, USA
*We currently do not have a copyright agreement with this publisher and cannot display the abstract here

The surface composition of the northern regions of Vesta, observed by the Dawn spacecraft, offers the possibility to test several hypotheses related to impact-related processes. We used mostly imaging spectrometry in the visible and near infrared to assess the distribution of mafic lithologies, hydrated components and albedo properties, and use the link with howardite, eucrite and diogenite meteorites (HEDs) to investigate the origin of those materials. We established that Rheasilvia ejecta reached part of the northern regions, and have a diogenitic-rich composition characteristic of the lower crust. Investigations of the antipodes of the two major impact basins (Rheasilvia and Veneneia) did not reveal any correlation between geographic location, geological features and the surface composition. The northern wall of Mamilia crater, which is one of the freshest craters above 22°N, contains relatively pure eucritic-rich, diogenitic-rich and dark, hydrated materials, which are representative of the rest of the northern regions (and most of Vesta), with the exception of an olivine-like component found in Bellicia crater by Ammannito et al. (2013, Nature Volume 504, Issue 7478, pp. 122-125). We determined that similar types of materials are found in various proportions over a large region, including Bellicia, Arruntia and Pomponia craters, and their origin does not seem to be related to Rheasilvia ejecta. These materials are hydrated, which could indicate an exogenous origin, and not as dark as expected for carbonaceous chondrites, which likely compose the majority of dark hydrated materials on Vesta. Spectral mixture analysis reveals that mixtures of pyroxenes (hypersthene, pigeonite and diopside) could offer an alternative interpretation to olivine in this area.

Reference
Combe J-P et al. (2015) Composition of the northern regions of Vesta analyzed by the Dawn Mission. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.04.026]

Copyright Elsevier

¹Sherry K. Fieber-Beyer, ¹Michael J. Gaffey
¹Department of Space Studies, University Stop 9008, University of North Dakota, 58202

The research is an integrated effort beginning with telescopic observations and extending through detailed mineralogical characterizations to provide constraints on the composition and meteorite affinities of a subset of fourteen asteroids in/near the 3:1 Kirkwood Gap. Eight asteroids were identified as having either one or two absorption features, while six were deemed featureless. The compositional analysis of asteroids (355) Gabriella and (1447) Utra reveal Fs and Fa values which are consistent with values for the L-type ordinary chondrites (Fs19-22 and Fa22-26). The location of these two bodies with respect to each other and to the previously identified L-chondrite parent body asteroid (1722) Goffin, suggests a small L-chondrite genetic family. These results support the model that the L-chondrites come from an asteroid family rather than from a single object. Asteroids (1368) Numidia, (1587) Kahrstadt, (1854) Skortsov, (2497) Kulikovskij, and (5676) Voltaire were analyzed and determined to have “basaltic” silicate mineralogies similar to those of the HED (howardite – eucrite – diogenite) meteorite group. In particular, we found that the compositions of (1368), (1587) and (1854) are consistent with olivine-orthopyroxenitic diogenites, while (2497) and (5676)’s compositions are consistent with harzburgitic diogenites. The Band I and Band II absorption feature depths are much shallower than seen in diogenite spectra, typically ∼70% depth ( Burbine et al. 2000). The nature of the weak features seen in the asteroid spectra when compared to measured band depths of in situ diogenite samples indicate an additional mechanism(s) acting to weaken the features, most likely space weathering. The aforementioned five asteroids are plausible sources for the olivine-orthopyroxenitic diogenites and harzburgitic diogenites, and very well may be fragments of Vesta. Asteroid (46) Hestia is an interesting object whose surface minerals may be consistent with a CR2 chondrite; however, the unique spectrum deserves further study in the future. Featureless asteroids (248) Lameia, (1960) Guisan, (3345) Tarkovskij and (6212) 1993 MS1 surface materials are likely organic assemblages consistent with the Type 1 or 2 carbonaceous chondrite meteorite class; however specific terrestrial meteorite analog could not be identified. The spectra of asteroids (3228) Pire and (3999) Aristarchus are consistent with each other and have been assigned to the Eulalia by Walsh et al. (2013). Spectrally they are similar to (495) in terms of blue-slope and albedo ( Fieber-Beyer et al. 2012), thus increasing our confidence the three bodies are truly related dynamically and genetically. By extrapolation and due to their location adjacent to the 3:1 Kirkwood Gap, (3228) and (3999) are plausible sources of the CV3OXB carbonaceous chondrites.

Reference
Fieber-Beyer SK, Gaffey MJ (2015) Near-infrared Spectroscopy of 3:1 Kirkwood Gap Asteroids III. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.04.034]

Copyright Elsevier

^1,2,3Nan Liu et al. (>10)*
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, 60637, USA
²Chicago Center for Cosmochemistry, Chicago, IL 60637, USA
³Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
*Find the extensive, full author and affiliation list on the publishers website

We present strontium, barium, carbon, and silicon isotopic compositions of 61 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing both strontium and barium contamination. For the first time, by using correlated 88Sr/86Sr and 138Ba/136Ba ratios in mainstream SiC grains, we are able to resolve the effect of 13C concentration from that of 13C-pocket mass on s-process nucleosynthesis, which points toward the existence of large 13C pockets with low 13C concentrations in asymptotic giant branch stars. The presence of such large 13C pockets with a variety of relatively low 13C concentrations seems to require multiple mixing processes in parent asymptotic giant branch stars of mainstream SiC grains.

Reference
Liu N et al. (2015) Correlated Strontium and Barium Isotopic Compositions of Acid-cleaned Single Mainstream Silicon Carbides from Murchison. Astrophysical Journal 803,12
Link to Article [doi:10.1088/0004-637X/803/1/12]

¹Tommaso Di Rocco,¹Andreas Pack
¹Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Georg-August-Universität, Goldschmidtstraße 1, D-37077 Göttingen, Germany

We have conducted time and ƒO2-dependent oxygen isotope exchange experiments between chondrule analogue melts and H2O in the phase. The aim of our study is to address the question whether the oxygen isotope composition of chondrules is the result of exchange with the ambient nebular gas or has been inherited from the precursor material. The silicate melt-H2O vapor exchange experiments were carried out in a vertical gas-mixing furnace using the metal loop technique at 1500°C. The duration ranged from 5 to 1440 minutes and ƒO2 was set between IW-3.8 and IW-1.3 using the H2O/H2 buffer. Our experiments show that 50% exchange between H2O gas and silicate melt occurs in ∼4 hours at ƒO2 = IW-3.8 and in ∼1 hour at ƒO2 = IW-1.3. At solar nebula conditions, significant exchange occurs only if chondrule-melting times were several hours.

Reference
Di Rocco T, Pack A (2015) Triple oxygen isotope exchange between chondrule melt and water vapor: an experimental study. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.038]

Copyright Elsevier

The most popular papers on Cosmochemistry Papers in April were:

1-Touboul M, Puchtel IS, Walker RJ (2015) Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon. Nature (in Press) Link to Article [doi:10.1038/nature14355]

2-Martín-Torres FJ et al. (2015) Transient liquid water and water activity at Gale crater on Mars. Nature Geoscience (in Press) Link to Article [doi:10.1038/ngeo2412]

3-Kraus RG, Root S, Lemke RW, Stewart ST, Jacobsen SB, Mattsson TR (2015) Impact vaporization of planetesimal cores in the late stages of planet Formation. Nature Geoscience 8, 269–272 Link to Article [doi:10.1038/ngeo2369]

4-Kruijer TS, Kleine T, Fischer-Gödde M, Sprung S (2015) Lunar tungsten isotopic evidence for the late veneer. Nature (in Press) Link to Article [doi:10.1038/nature14360]

5-Mastrobuono-Battisti A, Perets HB, Raymond SN (2015) A primordial origin for the compositional similarity between the Earth and the Moon. Nature 520, 212–215 Link to Article [doi:10.1038/nature14333]

¹Megan Bruck Syal, ²Peter H. Schultz ³Miriam A. Riner
¹Lawrence Livermore National Laboratory, PO Box 808 L-405, Livermore, California 94551, USA
²Department of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook St Box 1846, Providence Rhode Island 02912, USA
³Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson Arizona 85719, USA Miriam A. Riner

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

Reference
Bruck Syal M,Schultz PH, Riner MA (2015) Darkening of Mercury’s surface by cometary carbon.
Nature Geoscience 8, 352–356
Link to Article [doi:10.1038/ngeo2397]

¹Jinhai Zhang et al. (>10)*
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
*Find the extensive, full author and affiliation list on the publishers website

We report the surface exploration by the lunar rover Yutu that landed on the young lava flow in the northeastern part of the Mare Imbrium, which is the largest basin on the nearside of the Moon and is filled with several basalt units estimated to date from 3.5 to 2.0 Ga. The onboard lunar penetrating radar conducted a 114-m-long profile, which measured a thickness of ∼5 m of the lunar regolith layer and detected three underlying basalt units at depths of 195, 215, and 345 m. The radar measurements suggest underestimation of the global lunar regolith thickness by other methods and reveal a vast volume of the last volcano eruption. The in situ spectral reflectance and elemental analysis of the lunar soil at the landing site suggest that the young basalt could be derived from an ilmenite-rich mantle reservoir and then assimilated by 10–20% of the last residual melt of the lunar magma ocean.

Reference
Zhang J et al. (2015) Volcanic history of the Imbrium basin: A close-up view from the lunar rover Yutu. Proceedings of the National Academy of Sciences 112 (17) 5342-5347
Link to Article [doi:10.1073/pnas.1503082112]

¹Chi Ma, ²Oliver Tschauner, ¹John R. Beckett, ³Yang Liu, ¹George R. Rossman, ⁴Kirill Zhuravlev, ⁴Vitali Prakapenka, ⁵Przemyslaw Dera, ⁶Lawrence A. Taylor

¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
²High Pressure Science and Engineering Center and Department of Geoscience, University of Nevada, Las Vegas, NV 89154, USA
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁴GSECARS, University of Chicago, Argonne National Laboratory, Argonne, IL 60439, USA
⁵e Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA
⁶Planetary Geosciences Institute, Department of Earth and Planetary Science, University of Tennessee, Knoxville, TN 37996, USA

Tissintite is a new vacancy-rich, high-pressure clinopyroxene, with a composition essentially equivalent to plagioclase. It was discovered in maskelynite (shocked plagioclase) and is commonly observed included within, or in contact with, shock-melt pockets in the Tissint meteorite, a depleted olivine-phyric shergottite fall from Mars. The simple composition of tissintite (An58–69) and its precursor plagioclase (An59–69) together with the limited occurrence, both spatially (only in maskelynite less than ∼25 μm of a shock melt pocket) and in terms of bulk composition, make tissintite a “goldilocks” phase. It formed during a shock event severe enough to allow nucleation and growth of vacancy-rich clinopyroxene from a melt of not too calcic and not too sodic plagioclase composition that was neither too hot nor too cold. With experimental calibration, these limitations on occurrence can be used to place strong constraints on the thermal history of a shock event. The kinetics for nucleation and growth of tissintite are probably slower for more-sodic plagioclase precursors, so tissintite is most likely to occur in depleted olivine-phyric shergottites like Tissint and other highly shocked meteorites and lunar and terrestrial rocks that consistently contained calcic plagioclase precursors in the appropriate compositional range for a shock of given intensity.
Tissintite, (Ca0.45Na0.31□0.24)(Al0.97Fe0.03Mg0.01)(Si1.80Al0.20)O6(Ca0.45Na0.31□0.24)(Al0.97Fe0.03Mg0.01)(Si1.80Al0.20)O6, is a C2/cC2/c clinopyroxene, containing 42–60 mol% of the Ca-Eskola component, by far the highest known. The cell parameters are a=9.21a=9.21 (17) Å, b=9.09b=9.09 (4) Å, c=5.20c=5.20 (2) Å, β=109.6β=109.6 (9)°, V=410V=410 (8) Å3, Z=4Z=4. The density is 3.32 g/cm3 and we estimate a cell volume for the Ca-Eskola end-member pyroxene of 411±13 Å3411±13 Å3, which is consistent with a previous estimate and, therefore, supports the importance of this component in clinopyroxenes from ultra-high pressure metamorphic rocks from the Earth’s upper mantle. At least in C2/cC2/c clinopyroxenes as sodic as tissintite, the a- and b-cell parameters as a function of vacancy concentration intersect at ∼0.3 vacancies pfu, much lower than the Ca-Eskola end-member (0.5), an inversion of anisotropy suggesting an elastic instability that drives clinopyroxene toward a disordered trigonal structure closely related to that of wadeite; it may mark the boundary beyond which the breakdown of vacancy-rich clinopyroxene to a wadeite-structured phase + stishovite becomes stable, although this was not observed in Tissint.

Reference
Ma C, Tschauner O, Beckett JR, Liu Y, Rossman GR, Zhuravlev K, Prakapenka V, Dera P, Taylor LA (2015) Tissintite, (Ca, Na, □)AlSi2O6, a highly-defective, shock-induced, high-pressure clinopyroxene in the Tissint martian Meteorite. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.03.057]

Copyright Elsevier

^1,2,3Han, J., ¹Brearley, A. J.
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
²USRA Lunar and Planetary Institute, Houston, Texas, USA
³NASA Johnson Space Center, Houston, Texas, USA

The microstructures and compositions of olivine and refractory components in six amoeboid olivine aggregates (AOAs) in the Allan Hills A77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique with transmission electron microscopy. In the AOAs, refractory components (perovskite, melilite, spinel, anorthite, and Al-Ti-bearing diopside) provide evidence of a high degree of textural and compositional heterogeneity, suggesting that these phases have formed by disequilibrium gas–solid condensation at high temperatures under highly dynamic conditions. We infer different possible reactions of early-condensed solid minerals (perovskite and spinel) with a nebular gas, forming diopside with wide ranges of Al and Ti contents and/or anorthite. The progressive, incomplete consumption of spinel in these reactions may have resulted in the Cr enrichment in the remaining, unreacted spinel in the AOAs. In contrast to the refractory components, olivines in the AOAs have equilibrated textures with 120° triple junctions, indicating that the AOAs were subjected to high-temperature annealing after agglomeration of olivine and refractory components. Because the AOAs consist of fine-grained olivine grains with numerous pores, the annealing is constrained by experimental data to have occurred for a short duration of the order of a few hours to tens of hours depending on the annealing temperature. In comparison, the effects of annealing on the refractory components are minimal, probably due to pinning of grain boundaries in the multiphase assemblages that inhibited grain growth.

Reference
Han J, Brearley AJ (2015) Microstructural evidence for complex formation histories of amoeboid olivine aggregates from the ALHA77307 CO3.0 chondrite. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12439]

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