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

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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]

Published by arrangement with John Wiley & Sons

¹Zack Gainsforth et al. (>10)*
¹Space Sciences Laboratory, University of California at Berkeley, Berkeley, California, USA
*Find the extensive, full author and affiliation list on the publishers website

Using chemical and petrologic evidence and modeling, we deduce that two chondrule-like particles named Iris and Callie, from Stardust cometary track C2052,12,74, formed in an environment very similar to that seen for type II chondrules in meteorites. Iris was heated near liquidus, equilibrated, and cooled at ≤100 °C h-1 and within ≈2 log units of the IW buffer with a high partial pressure of Na such as would be present with dust enrichments of ≈103. There was no detectable metamorphic, nebular, or aqueous alteration. In previous work, Ogliore et al. (2012) reported that Iris formed late, >3 Myr after CAIs, assuming 26Al was homogenously distributed, and was rich in heavy oxygen. Iris may be similar to assemblages found only in interplanetary dust particles and Stardust cometary samples called Kool particles. Callie is chemically and isotopically very similar, but not identical to Iris.

Reference
Gainsforth Z et al. (2015) Constraints on the formation environment of two chondrule-like igneous particles from comet 81P/Wild 2. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12445]

Published by arrangemént with John Wiley & Sons

^1,2Masaaki Miyahara, ^1,3Eiji Ohtani, ⁴Ahmed El Goresy, ⁵Yangting Lin, ⁵Lu Feng, ⁵Jian-Chao Zhang, ⁶Philippe Gillet, ⁷Toshiro Nagase, ¹Jun Muto, ⁸Masahiko Nishijima
¹Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
²Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan
³V. S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Science, 630090 Novosibirsk, Russia
⁴Bayerisches Geoinstitut, Universität Bayreuth, D-95440, Bayreuth, Germany
⁵Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Science, Beijing 100029, China
⁶Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL) Station 1, 1015 Lausanne, Switzerland
⁷Center for Academic Resources and Archives, Tohoku University, Sendai 980-8578, Japan
⁸Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

The Almahata Sitta MS-170 ureilite (a piece of a breccia originating from the asteroid, 2008 TC3) consists mainly of olivine, with many diamond and graphite grains existing between the olivine grains. The occurrences of the diamonds are unique; i.e., i) some diamonds exhibit sub-euhedral habits and ii) some diamonds have large grain-size (up to about 40 μm). Several diamonds are segmented into many fragments by fractures. Individual fragments have similar crystallographic orientation, which implies that the adjacent diamond segments were originally a single crystal. Large diamond assemblages occur besides such individual diamond grains. In one of the largest assemblages (almost about 100 m in size) has also the same crystallographic orientation. They can be regarded as the pieces of a previously unique single diamond, which provides evidence for large single-crystals diamond in meteorites. Almahata Sitta MS-170 is a meteorite fragment from the 2008 TC3 asteroid that underwent less shock than other ureilitic meteorites. It is unlikely that such large diamonds were formed from graphite through a shock-induced phase transformation during planetesimal collision, despite this idea being now widely accepted as the diamond formation mechanism of ureilites. Fine-scale heterogeneous distribution of impurities (hydrogen, nitrogen, and oxygen) exists in single crystal diamonds, indicative of sluggish growth. This distribution is reminiscent of sector zoning growth. Its grain size, the shock features of MS-170, and the C- and N- isotopic composition signatures allow us to revive classical and but not widely accepted models for diamond formation in ureilites; i.e., a diamond formed from partially melted magma or a C–O–H fluid in the deep interior of the ureilite parent-body or, alternatively, through a chemical vapor deposition (CVD) process in the solar nebula. Considering present mineralogical and isotopic features, the former scenario is more favorable.

Reference
Miyahara M, Ohtani E, El Goresy A, Lin Y, Feng L, Zhang J-C, Gillet P, Nagase T, Muto J, Nishijima M (2015)
Unique large diamonds in a ureilite from Almahata Sitta 2008 TC3 Asteroid. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.035]

Copyright Elsevier

¹Ryan C. Ogliore, ¹Kazuhide Nagashima, ¹Gary R. Huss, ²Andrew J. Westphal, ²Zack Gainsforth, ²Anna L. Butterworth
¹Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
²Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA 94720, USA

Individual particles from comet 81P/Wild 2 collected by NASA’s Stardust mission vary in size from small sub-μm fragments found in the walls of the aerogel tracks, to large fragments up to tens of μm in size found towards the termini of tracks. The comet, in an orbit beyond Neptune since its formation, retains an intact a record of early-Solar-System processes that was compromised in asteroidal samples by heating and aqueous alteration. We measured the O isotopic composition of seven Stardust fragments larger than ∼2 μm extracted from five different Stardust aerogel tracks, and 63 particles smaller than ∼2 μ m from the wall of a Stardust track. The larger particles show a relatively narrow range of O isotopic compositions that is consistent with 16O16O-poor phases commonly seen in meteorites. Many of the larger Stardust fragments studied so far have chondrule-like mineralogy which is consistent with formation in the inner Solar System. The fine-grained material shows a very broad range of O isotopic compositions (-70-70‰< Δ17OΔ17O<+60<+60‰) suggesting that Wild 2 fines are either primitive outer-nebula dust or a very diverse sampling of inner Solar System compositional reservoirs that accreted along with a large number of inner-Solar-System rocks to form comet Wild 2.

Reference
Ogliore RC, Nagashima K, Huss GR, Westphal AJ, Gainsforth Z, Butterworth AL (2015) Oxygen Isotopic Composition of coarse- and fine-grained material from Comet 81P/Wild 2. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.028]

Copyright Elsevier

^1,2Alex Ruzicka, ¹Richard Hugo, ^1,2Melinda Hutson
¹Portland State University, Department of Geology, 1721 SW Broadway, Portland, OR, U.S.A
²Cascadia Meteorite Laboratory, Portland State University, 1721 SW Broadway, Portland, OR, U.S.A

We show that olivine microstructures in seven metamorphosed ordinary chondrites of different groups studied with optical and transmission electron microscopy can be used to evaluate the post-deformation cooling setting of the meteorites, and to discriminate between collisions affecting cold and warm parent bodies. The L6 chondrites Park (shock stage S1), Bruderheim (S4), Leedey (S4), and Morrow County (S5) were affected by variable shock deformation followed by relatively rapid cooling, and probably cooled as fragments liberated by impact in near-surface settings. In contrast, Kernouvé (H6 S1), Portales Valley (H6/7 S1), and MIL 99301 (LL6 S1) appear to have cooled slowly after shock, probably by deep burial in warm materials. In these chondrites, post-deformation annealing lowered apparent optical strain levels in olivine. Additionally, Kernouvé, Morrow County, Park, MIL 99301, and possibly Portales Valley, show evidence for having been deformed at an elevated temperature (⩾800-1000 °C). The high temperatures for Morrow County can be explained by dynamic heating during intense shock, but Kernouvé, Park, and MIL 99301 were probably shocked while the H, L and LL parent bodies were warm, during early, endogenically-driven thermal metamorphism. Thus, whereas the S4 and S5 chondrites experienced purely shock-induced heating and cooling, all the S1 chondrites examined show evidence for static heating consistent with either syn-metamorphic shock (Kernouvé, MIL 99301, Park), post-deformation burial in warm materials (Kernouvé, MIL 99301, Portales Valley), or both. The results show the pitfalls in relying on optical shock classification alone to infer an absence of shock and to construct cooling stratigraphy models for parent bodies. Moreover, they provide support for the idea that “secondary” metamorphic and “tertiary” shock processes overlapped in time shortly after the accretion of chondritic planetesimals, and that impacts into warm asteroidal bodies were common.

Reference
Ruzicka A, Hugo R, Hutson M (2015) Deformation and thermal histories of ordinary chondrites: Evidence for post-deformation annealing and syn-metamorphic shock. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.030]

Copyright Elsevier

^1,2Tomáš Magna, ³James M.D. Day, ^1,4Klaus Mezger, ^5,6Manuela A. Fehr, ⁷Ralf Dohmen, ⁸Hasnaa Chennaoui Aoudjehane, ⁹Carl B. Agee
¹Institut für Mineralogie, Universität Münster, Corrensstr. 24, D-48149 Münster, Germany
²Czech Geological Survey, Klárov 3, CZ-118 21 Prague 1, Czech Republic
³Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA
⁴Institut für Geologie, Universität Bern, Baltzerstr. 1+3, CH-3012 Bern, Switzerland
⁵CEPSAR, Department of Environment, Earth & Ecosystems, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
⁶Institut für Geochemie und Petrologie, ETH Zürich, Clausiusstr. 25, CH-8092 Zürich, Switzerland
⁷Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany
⁸Faculty of Sciences, Hassan II University, BP 5366 Maârif, Casablanca, Morocco
⁹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA

Lithium abundances and isotope compositions are reported for a suite of martian meteorites that span the range of petrological and geochemical types recognized to date for Mars. Samples include twenty-one bulk-rock enriched, intermediate and depleted shergottites, six nakhlites, two chassignites, the orthopyroxenite Allan Hills (ALH) 84001 and the polymict breccia Northwest Africa (NWA) 7034. Shergottites unaffected by terrestrial weathering exhibit a range in δ7Li from 2.1 to 6.2‰, similar to that reported for pristine terrestrial peridotites and unaltered mid-ocean ridge and ocean island basalts. Two chassignites have δ7Li values (4.0‰) intermediate to the shergottite range, and combined, these meteorites provide the most robust current constraints on δ7Li of the martian mantle. The polymict breccia NWA 7034 has the lowest δ7Li (−0.2‰) of all terrestrially unaltered martian meteorites measured to date and may represent an isotopically light surface end-member.
The new data for NWA 7034 imply that martian crustal surface materials had both a lighter Li isotope composition and elevated Li abundance compared with their associated mantle. These findings are supported by Li data for olivine-phyric shergotitte NWA 1068, a black glass phase isolated from the Tissint meteorite fall, and some nakhlites, which all show evidence for assimilation of a low-δ7Li crustal component. The range in δ7Li for nakhlites (1.8 to 5.2‰), and co-variations with chlorine abundance, suggests crustal contamination by Cl-rich brines. The differences in Li isotope composition and abundance between the martian mantle and estimated crust are not as large as the fractionations observed for terrestrial continental crust and mantle, suggesting a difference in the styles of alteration and weathering between water-dominated processes on Earth versus possibly Cl–S-rich brines on Mars. Using high-MgO shergottites (>14 wt.% MgO) it is possible to estimate the δ7Li of Bulk Silicate Mars (BSM) to be 4.2 ± 0.9‰ (2σ). This value is at the higher end of estimates for the Bulk Silicate Earth (BSE; 3.5 ± 1.0‰, 2σ), but overlaps within uncertainty.

Reference
Magna T, Day JMD, Mezger K, Fehr MA, Dohmen R, Aoudjehane CH, Agee CB (2015) Lithium isotope constraints on crust–mantle interactions and surface processes on Mars. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.029]

Copyright Elsevier

^1,2Lev J. Spivak-Birndorf et al. (>10*)
¹Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
²Department of Geological Sciences, Indiana University, Bloomington, Indiana, USA
*Find the extensive, full author and affiliation list on the publishers website

Bunburra Rockhole is a unique basaltic achondrite that has many mineralogical and petrographic characteristics in common with the noncumulate eucrites, but differs in its oxygen isotope composition. Here, we report a study of the mineralogy, petrology, geochemistry, and chronology of Bunburra Rockhole to better understand the petrogenesis of this meteorite and compare it to the eucrites. The geochemistry of bulk samples and of pyroxene, plagioclase, and Ca-phosphate in Bunburra Rockhole is similar to that of typical noncumulate eucrites. Chronological data for Bunburra Rockhole indicate early formation, followed by slow cooling and perhaps multiple subsequent heating events, which is also similar to some noncumulate eucrites. The 26Al-26Mg extinct radionuclide chronometer was reset in Bunburra Rockhole after the complete decay of 26Al, but a slight excess in the radiogenic 26Mg in a bulk sample allows the determination of a model 26Al-26Mg age that suggests formation of the parent melt for this meteorite from its source magma within the first ~3 Ma of the beginning of the solar system. The 207Pb-206Pb absolute chronometer is also disturbed in Bunburra Rockhole minerals, but a whole-rock isochron provides a re-equilibration age of ~4.1 Ga, most likely caused by impact heating. The mineralogy, geochemistry, and chronology of Bunburra Rockhole demonstrate the similarities of this achondrite to the eucrites, and suggest that it formed from a parent melt with a composition similar to that for noncumulate eucrites and subsequently experienced a thermal history and evolution comparable to that of eucritic basalts. This implies the formation of multiple differentiated parent bodies in the early solar system that had nearly identical bulk elemental compositions and petrogenetic histories, but different oxygen isotope compositions inherited from the solar nebula.

Reference
Spivak-Birndorf LJ et al. (2015) Geochemistry and chronology of the Bunburra Rockhole ungrouped achondrite. Meteoritics and Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12443]

Published by arrangement with John Wiley & Sons