¹Luca Bindi et al. (>10)*
¹Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Florence, Italy
*Find the extensive, full author and affiliation list on the publishers website

Decagonite is the second natural quasicrystal, after icosahedrite (Al63Cu24Fe13), and the first to exhibit the crystallographically forbidden decagonal symmetry. It was found as rare fragments up to ~60 μm across in one of the grains (labeled number 126) of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The meteoritic grain contains evidence of a heterogeneous distribution of pressures and temperatures that occurred during impact shock, in which some portions of the meteorite reached at least 5 GPa and 1200 °C. Decagonite is associated with Al-bearing trevorite, diopside, forsterite, ahrensite, clinoenstatite, nepheline, coesite, pentlandite, Cu-bearing troilite, icosahedrite, khatyrkite, taenite, Al-bearing taenite, and steinhardtite. Given the exceedingly small size of decagonite, it was not possible to determine most of the physical properties for the mineral. A mean of seven electron microprobe analyses (obtained from three different fragments) gave the formula Al70.2(3)Ni24.5(4)Fe5.3(2), on the basis of 100 atoms. A combined TEM and single-crystal X-ray diffraction study revealed the unmistakable signature of a decagonal quasicrystal: a pattern of sharp peaks arranged in straight lines with 10-fold symmetry together with periodic patterns taken perpendicular to the 10-fold direction. For quasicrystals, by definition, the structure is not reducible to a single three-dimensional unit cell, so neither cell parameters nor Z can be given. The likely space group is P105/mmc, as is the case for synthetic Al71Ni24Fe5. The five strongest powder-diffraction lines [d in Å (I/I0)] are: 2.024 (100), 3.765 (50), 2.051 (45), 3.405 (40), 1.9799 (40). The new mineral has been approved by the IMA-NMNC Commission (IMA2015-017) and named decagonite for the 10-fold symmetry of its structure. The finding of a second natural quasicrystal informs the longstanding debate about the stability and robustness of quasicrystals among condensed matter physicists and demonstrates that mineralogy can continue to surprise us and have a strong impact on other disciplines.

Reference
Bindi L. et al. (2015) Decagonite, Al71Ni24Fe5, a quasicrystal with decagonal symmetry from the Khatyrka CV3 carbonaceous chondrite. American Mineralogist 100, 2340-2343
Link to Article [doi:10.2138/am-2015-5423]
Copyright: The Mineralogical Society of America

¹T. Michikami, ²A. Hagermann, ³T. Kadokawa, ¹A. Yoshida, ³A. Shimada, ⁴S. Hasegawa, ³A.Tsuchiyama
¹Faculty of Engineering, Kinki University, Hiroshima Campus, 1 Takaya Umenobe, Higashi-Hiroshima, Hiroshima 739-2116, Japan
²Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
³Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kiashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8052, Japan
⁴Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 252-8510, Japan

Laboratory impact experiments have found that impact fragments tend to be elongated. Their shapes, as defined by axes a, b and c, these being the maximum dimensions of the fragment in three mutually orthogonal planes (a ⩾ b ⩾ c), are distributed around mean values of the axial ratios b/a ∼0.7 and c/a ∼0.5. This corresponds to a: b: c in the simple proportion 2: √2: 1. The shape distributions of some boulders on asteroid Eros, the small- and fast-rotating asteroids (diameter < 200 m and rotation period < 1 h), and asteroids in young families, are similar to those of laboratory fragments created in catastrophic disruptions. Catastrophic disruption is, however, a process that is different from impact cratering. In order to systematically investigate the shapes of fragments in the range from impact cratering to catastrophic disruption, impact experiments for basalt targets 5 to 15 cm in size were performed. A total of 28 impact experiments were carried out by firing a spherical nylon projectile (diameter 7.14 mm) perpendicularly into the target surface at velocities of 1.60 to 7.13 km/s. More than 12,700 fragments with b ⩾ 4 mm generated in the impact experiments were measured. We found that the mean value of c/a in each impact decreases with decreasing impact energy per unit target mass. For instance, the mean value of c/a in an impact cratering event is nearly 0.2, which is considerably smaller than c/a in a catastrophic disruption (∼0.5). The data presented here can provide important evidence to interpret the shapes of asteroids and boulders on asteroid surfaces, and can constrain current interpretations of asteroid formation. As an example, by applying our experimental results to the boulder shapes on asteroid Itokawa’s surface, we can infer that Itokawa’s parent body must have experienced a catastrophic disruption.

Reference
Michikami T, Hagermann A, Kadokawa T, Yoshida A, Shimada A, Hasegawa S, Tsuchiyama A (2015) Fragment shapes in impact experiments ranging from cratering to catastrophic disruption. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.09.038]

Copyright Elsevier

¹Yogita Kadlag, ¹Harry Becker
¹Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany

The 187Re-187Os systematics, abundances of highly siderophile elements (HSE: Re, platinum group elements and Au), Te, Se and S as well as major and minor elements were determined in separated components of two unequilibrated L chondrites QUE 97008 (L3.05) and Ceniceros (L3.7). The 187Re-187Os systematics are disturbed in the components of both meteorites, most likely due to open system behaviour of Re during terrestrial weathering of QUE 97008 and alteration on the L chondrite parent body as indicated by an internal errorchron generated for components of Ceniceros. The HSE abundance patterns suggest that the bulk rock abundances were mainly controlled by two different end members. Non-magnetic fractions display lower Re/Os and HSE/Ir than CI chondrites. Chondrules, metal-troilite spherules and fine magnetic fractions, are depleted in refractory HSE and show higher Rh/Ir, Pd/Ir and Au/Ir than in CI chondrites. The different HSE compositions indicate the presence of unequilibrated alloys and loss of refractory HSE- rich carrier phases from the precursors of some L chondrite components. Gold is decoupled from other HSE in magnetic fractions and shows chalcophile affinities with a grain size dependent variation similar to S and Se, presumably inherited from preaccretionary processes. Tellurium is depleted in all components compared to other analysed siderophile elements, and its abundance was most likely controlled by fractional condensation and different geochemical affinities. The volatility dependent depletion of Te requires different physical and chemical conditions than typical for the canonical condensation sequence as represented by carbonaceous chondrites. Tellurium also shows variable geochemical behaviour, siderophile in Ceniceros, predominantly chalcophile in QUE 97008. These differences may have been inherited from element partitioning during chondrule formation. Selenium and S on the other hand are almost unfractionated from each other and only show complementary S/Se in a few components, presumably due to the effects of volatility or metal-silicate partitioning during chondrule formation. Terrestrial weathering had negligible effects on the S, Se and Te systematics.

Reference
Kadlag Y, Becker H (2015) 187Re-187Os Systematics, Highly Siderophile Element, S-Se-Te Abundances in the Components of Unequilibrated L Chondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.09.026]
Copyright Elsevier

¹J. P. Grotzinger et al. (>10)*
¹Division of Geologic and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
*Find the extensive, full author and affiliation list on the publishers Website

The landforms of northern Gale crater on Mars expose thick sequences of sedimentary rocks. Based on images obtained by the Curiosity rover, we interpret these outcrops as evidence for past fluvial, deltaic, and lacustrine environments. Degradation of the crater wall and rim probably supplied these sediments, which advanced inward from the wall, infilling both the crater and an internal lake basin to a thickness of at least 75 meters. This intracrater lake system probably existed intermittently for thousands to millions of years, implying a relatively wet climate that supplied moisture to the crater rim and transported sediment via streams into the lake basin. The deposits in Gale crater were then exhumed, probably by wind-driven erosion, creating Aeolis Mons (Mount Sharp).

Reference
Grotzinger JP et al. (2015) Deposition, exhumation, and paleoclimate of an ancient lake deposit, Gale crater, Mars. Science 350, 6257
Link to Article [DOI: 10.1126/science.aac7575]
Reprinted with permission from AAAS

^1,2James Badro, ³John P. Brodholt, ^1,2Hélène Piet, ¹Julien Siebert, ⁴Frederick J. Ryerson
¹Institut de Physique du Globe de Paris, Sorbonne Paris Cité, UMR CNRS 7154, 75005 Paris, France
²Earth and Planetary Science Laboratory, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;
³Department of Earth Sciences, University College London, London WC1E 6BT, United Kingdom;
⁴Lawrence Livermore National Laboratory, Livermore, CA 94550

We combine, for the first time to our knowledge, two approaches to study Earth’s core composition: a geochemical approach based on trace element depletion in the mantle and a geophysical approach based on a seismically lighter and faster (than pure iron−nickel) core. The joint approach allows making strong statements; first of all, as opposed to the current belief, Earth must have accreted material that is more oxidized than the present-day mantle, similar to that of planetesimals such as 4-Vesta, and got reduced to its present state during core formation. Secondly, core light-element concentrations in those conditions are 2.7% to 5% oxygen alongside 2% to 3.6% silicon; the oxygen concentrations in the core are higher than previously thought, and, conversely, silicon concentrations are lower than previous estimates.

Reference
Badro J, Brodholt JP, Piet H, Siebert J, Ryerson FR (2015) Core formation and core composition from coupled geochemical and geophysical constraints. Proceedings of the National Academy of Sciences 112, 12310-12314
Link to Article [doi:10.1073/pnas.1505672112]

^1,2Annemarie E. Pickersgill, ¹Roberta L. Flemming,^1,3Gordon R. Osinski
¹Department of Earth Sciences and Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario, Canada
²Department of Physics and Astronomy, The University of Western Ontario, London, Ontario, Canada
³School of Geographical & Earth Sciences, University of Glasgow, Lilybank Gardens, Glasgow, UK

Studies of shock metamorphism of feldspar typically rely on qualitative petrographic observations, which, while providing invaluable information, can be difficult to interpret. Shocked feldspars, therefore, are now being studied in greater detail by various groups using a variety of modern techniques. We apply in situ micro-X-ray diffraction (μXRD) to shocked lunar and terrestrial plagioclase feldspar to contribute to the development of a quantitative scale of shock deformation for the feldspar group. Andesine and labradorite from the Mistastin Lake impact structure, Labrador, Canada, and anorthite from Earth’s Moon, returned during the Apollo program, were examined using optical petrography and assigned to subgroups of the optical shock level classification system of Stöffler (1971). Two-dimensional μXRD patterns from the same samples revealed increased peak broadening in the chi dimension (χ), due to strain-related mosaicity, with increased optical signs of deformation. Measurement of the full width at half maximum along χ (FWHMχ) of these peaks provides a quantitative way to measure strain-related mosaicity in plagioclase feldspar as a proxy for shock level.

Reference
Pickersgill AE, Flemming RL, Osinski GR (2015) Toward quantification of strain-related mosaicity in shocked lunar and terrestrial plagioclase by in situ micro-X-ray diffraction. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12514]
Published by arrangement with John Wiley & Sons

^1,2Jennifer Buz,^1,3Benjamin P. Weiss,^3,4Sonia M. Tikoo,^3,4David L. Shuster,⁵Jérôme Gattacceca,¹Timothy L. Grove
¹Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
³Department of Earth and Planetary Science, University of California, Berkeley, CA, USA
⁴Berkeley Geochronology Center, Berkeley, CA, USA
⁵CNRS, Aix-Marseille University, Aix-en-Provence, France

Recent paleomagnetic studies of Apollo samples have established that a core dynamo existed on the Moon from at least 4.2 to 3.56 billion years ago (Ga). Because there is no lunar dynamo today, a longstanding mystery has been the origin of magnetization in very young lunar samples [<~200 million years old (Ma)]. Possible sources of this magnetization include transient fields generated by meteoroid impacts, remanent fields from nearby rocks magnetized during an earlier dynamo epoch, a weak late dynamo, and spontaneous remanence formed in a near-zero field. To further understand the source of the magnetization in young lunar samples, we conducted paleomagnetic, petrographic, and 40Ar/39Ar geochronometry analyses on a young impact melt glass rind from the exterior of ~3.35 Ga mare basalt 12017. Cosmic ray track densities and our 40Ar/39Ar and cosmogenic 38Ar analyses constrain the glass formation age to be 10 μT) core dynamo field nor impact-generated fields.

Reference
Buz J, Weiss BP, Tikoo SM, Shuster DL, Gattacceca J, Grove TL (2015) Magnetism of a Very Young Lunar Glass. Journal of Geophysical Research (Planets)
Link to Article [DOI: 10.1002/2015JE004878]
Published by arrangement with John Wiley & Sons

¹Jerrod Lessel,¹Keith Putirka
¹Department of Earth and Environmental Sciences, California State University, 2576 East San Ramon Avenue, Mail Stop ST24, Fresno, California 93740, U.S.A.

Tests show that terrestrial mineral+liquid geothermobarometers are not well equipped for use on martian rocks, which tend to have much higher FeO and lower Al2O3. Here, we present new calibrations of thermometers and barometers using experimental data on martian samples from the literature. These new models recover P-T conditions with a greater accuracy compared to models calibrated using terrestrial compositions. We applied these new calibrations to primitive martian mantle-derived melts Yamato 980459 (Y98) and Northwest Africa (NWA) 6234 and several surface basalts (Gusev). Our new models yield similar P-T conditions for NWA and Y98 compositions of 1.4–1.7 GPa and 1500–1550 °C, which are close to estimates by most prior studies. Our models yield somewhat lower P estimates compared to Lee et al. (2009), apparently because our Si-activity model (from Beattie 1993) includes an Al2O3-correction (where lower Al2O3, as in martian samples, leads to lower P estimates). For Gusev basalt compositions, our new models yield P-T estimates of 1.0–1.3 GPa and 1340–1390 °C; furthermore, we also obtain P = 1.03 GPa and T = 1340 °C, for a Gusev composition from Monders et al. (2007), which comes very close to the Monders et al. (2007) estimate for multiple saturation, of 1.0 GPa and 1325 °C, derived from phase saturation relationships. Given the different ages of these meteorites, with Gusev at 3.65 Ga (Greeley et al. 2005) and Y98 at 4.3 Ga (Bouvier et al. 2005, 2008, 2009; Werner et al. 2014), their thermal contrasts may represent secular cooling of Mars. We estimate a mantle potential temperature difference of ~200 °C, with mantle potential temperatures of 1450 ±50 °C for Gusev and 1650 ±50 °C for Y98; this implies a cooling rate of 300 °C/Ga. This would appear to be a much more rapid rate of cooling compared to Earth, as may be expected by Mars’ higher surface/volume ratio.

Reference
Lessel J, Putirka K (2015) New thermobarometers for martian igneous rocks, and some implications for secular cooling on Mars. American Mineralogist 100, 2163-2171
Link to Article [doi:10.2138/am-2015-4732]
Copyright: The Mineralogical Society of America

¹Neva A. Fowler-Gerace, ^1,2Kimberly T. Tait
¹Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada
²Department of Natural History, Mineralogy, Royal Ontario Museum, 100 Queens Park, Toronto, Ontario M5S 2C6, Canada

Phosphoran olivine (1–7 wt% P2O5) is a metastable phase known from fewer than a dozen meteoritic or terrestrial occurrences. We have thoroughly examined phosphoran olivine in the Springwater pallasite to characterize its distribution, textural relationships, and geochemistry. Phosphoran olivine is abundant in Springwater as randomly distributed millimeter-scale partial overgrowths on the P-free olivine crystals. Geochemical analyses support the substitution mechanism of P into the tetrahedral Si site with octahedral site vacancies for charge balance; observed trace element variations, on the other hand, are not related to P substitution. Element mapping reveals fine-scale oscillatory P zoning in unusual serrate patterns, indicating rapid crystal nucleation from a melt as proposed by Boesenberg and Hewins (2010) and a subsequently variable rate of crystallization. The timing of phosphoran olivine formation in Springwater is constrained to after the period of macroscopic olivine rounding but before the cooling of the metal matrix; because the phosphoran overgrowths overprint specific host grain boundary modifications, we suggest that the episode of extremely rapid cooling necessary to crystallize and preserve this rare phase may have been triggered by an additional impact to the parent body.

Reference
Fowler-Gerace NA, Tait KT (2015) Phosphoran olivine overgrowths: Implications for multiple impacts to the Main Group pallasite parent Body. American Mineralogist 100, 2043-2052
Link to Article [doi:10.2138/am-2015-5344]
Copyright: The Mineralogical Society of America

¹James J. Papike, ¹ Paul V. Burger, ¹Aaron S. Bell, ¹Charles K. Shearer, ²Loan Le, ³John Jones
¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
²JSC Engineering, Technology and Science (JETS), NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
³NASA Johnson Space Center, Houston, Texas 77058, U.S.A.

Spinel is a very important rock-forming mineral that is found in basalts from Earth, Mars, the Earth’s Moon, and basaltic meteorites. Spinel can be used as a sensitive indicator of petrologic and geochemical processes that occur in its host rock. This paper highlights the role of increasing fO2 (from IW-1 to FMQ+2) in converting a >90% normal spinel to an ~25% magnetite (inverse) spinel, the trajectory of DVspinel/melt as it relates to the ratio of V3+/V4+ in the melt, and the crystal chemical attributes of the spinel that control the intrinsic compatibility of both V3+ and V4+. This work examines the nuances of the V partitioning and provides a crystal chemical basis for understanding Fe3+, Cr, and V substitution into the octahedral sites of spinel. Understanding this interplay is critical for using spinels as both indicators of planetary parentage and reconstructing the redox history of magmatic systems on the terrestrial planets. Three potential examples for this use are provided. In addition, this work helps explain the ubiquitous miscibility gap between spinels with changing ülvospinel contents.

Reference
Papike JJ, Burger PV, Bell AS, Shearer CK, Le L, Jones J (2015) Normal to inverse transition in martian spinel: Understanding the interplay between chromium, vanadium, and iron valence state partitioning through a crystal-chemical lens. American Mineralogist 100, 2018-2025
Link to Article [doi:10.2138/am-2015-5208]
Copyright: The Mineralogical Society of America