¹Naveen,²S.Sarkar, ¹T.Nirmal Kumar,¹D.Ray, ²S.Bhattacharya, ¹A.D.Shukla, ³H.Moitra, ²A.Dagar, ²P.Chauhan, ⁴K.Sen, ^1,5S.Das
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2018.12.007]
¹Physical Research Laboratory, Ahmedabad, 380 009, India
²Space Applications Centre, Ahmedabad, 380 015, India
³Indian Institute of Technology Kharagpur, Kharagpur, 721302, India
⁴National Centre for Polar and Ocean Research, Goa, 403 804, India
⁵Department of Earth and Environmental Sciences, University of Texas at Arlington, TX, 76019, USA

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^1,2,3Harry Zekollari et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.12.035]
¹Earth System Science, Vrije Universiteit Brussel, Brussels, Belgium
²Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zürich, Zürich, Switzerland
³Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Birmensdorf, Switzerland
Copyright Elsevier

Antarctic blue ice zones, the most productive locations for meteorite recovery on Earth, contain old ice that is easily accessible and available in large quantities. However, the mechanisms behind these meteorite traps remain a topic of ongoing debate. Here, we propose an interdisciplinary approach to improve our understanding of a meteorite trap in Dronning Maud Land (East Antarctica) on the Nansen blue ice field meteorite trap (2600-3100 m above sea level), where more than half of the Asuka meteorites have been collected. Based on 185 surface blue ice samples, one of the largest observed spatial patterns in oxygen isotopic variation to date is found. Relying on meteorites for which the terrestrial ages are determined using 14C and 36Cl, this surface ice is interpreted to date from the Last Interglacial up to the present-day. By combining state-of-the-art satellite derived surface velocities, surface mass balance modelling and ice flow modelling, we estimate that about 75% to 85% of the meteorites found on the ice field were supplied by ice flow after entering the ice sheet in an accumulation area of a few hundred square kilometres located south (upstream) of the ice field. Less than 0.4 new meteorites per year are supplied to the ice field through ice flow, suggesting that the hundreds of meteorites found 25 years after the first visit to this ice field mostly represent meteorites that were previously not found, rather than newly supplied meteorites. By combining these findings, the infall rate of meteorites from space is estimated, which is in line with values from the literature, but situated at the higher end of the range. A comparison of the oxygen isotopic variation of the surface blue ice to that of the European Project for Ice Coring in Antarctica (EPICA), Dronning Maud Land (EDML) ice core (located 750 km to the west, at the same elevation), suggests that the regional changes in topography have been relatively limited since the Last Interglacial, supporting theories of an overall stable East Antarctic Ice Sheet (EAIS) over this time period.

¹Tetsuya Komabayashi, ¹Giacomo Pesce, ²Guillaume Morard,²Daniele Antonangeli,^3,4Ryosuke Sinmyo,⁵Mohamed Mezouar
American Mineralogist 104, 94-99 Link to Article [https://doi.org/10.2138/am-2019-6636]
¹School of GeoSciences and Centre for Science at Extreme Conditions, University of Edinburgh, EH9 3FE, U.K.
²Sorbonne Université, Muséum National d’Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 75005 Paris, France
³Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
⁴Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan.
⁵ Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France
Copyright: The Mineralogical Society of America

The phase transition between a face-centered cubic (fcc) and hexagonal close-packed (hcp) structures in Fe-4wt% Si alloy was examined in an internally resistive heated diamond-anvil cell (DAC) under high-pressure (P) and high-temperature (T) conditions to 71 GPa and 2000 K by in situ synchrotron X-ray diffraction. Complementary laser-heated DAC experiments were performed in Fe-6.5wt% Si. The fcc-hcp phase transition boundaries in the Fe-Si alloys are located at higher temperatures than that in pure Fe, indicating that the addition of Si expands the hcp stability field. The dP/dT slope of the boundary of the entrant fcc phase in Fe-4wt% Si is similar to that of pure Fe, but the two-phases region is observed over a temperature range increasing with pressure, going from 50 K at 15 GPa to 150 K at 40 GPa. The triple point, where the fcc, hcp, and liquid phases coexist in Fe-4wt% Si, is placed at 90–105 GPa and 3300–3600 K with the melting curve same as in Fe is assumed. This supports the idea that the hcp phase is stable at Earth’s inner core conditions. The stable structures of the inner cores of the other terrestrial planets are also discussed based on their P-T conditions relative to the triple point. In view of the reduced P-T conditions of the core of Mercury (well below the triple point), an Fe-Si alloy with a Si content up to 6.5 wt% would likely crystallize an inner core with an fcc structure. Both cores from Venus and Mars are currently believed to be totally molten. Upon secular cooling, Venus is expected to crystallize an inner core with an hcp structure, as the pressures are similar to those of the Earth’s core (far beyond the triple point). Martian inner core will take an hcp or fcc structure depending on the actual Si content and temperature.

¹Yves Marrocchi, ^1,2Romain Euverte, ¹Johan Villeneuve,³ Valentina Batanova,⁴ Benoit Welsch,⁵Ludovic Ferrière,⁶Emmanuel Jacquet
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.12.038]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
²LSCE, CNRS, UMR 8212, Gif-sur-Yvette, 91198, France
³Université Grenoble Alpes, ISTerre, CNRS, UMR 5275, Grenoble, 38000, France
⁴Department of Geology and Geophysics, University of Hawaii-Manoa, 1680 East-West Road, Honolulu, Hawaii 96822, USA
⁵Natural History Museum, Burgring 7, A-1010 Vienna, Austria
⁶IMPMC, CNRS & Muséum national d’Histoire naturelle, UMR 7590, CP52, 57 rue Cuvier, 75005 Paris, France
Copyright Elsevier

We have studied porphyritic olivine-rich chondrules of the carbonaceous chondrite Kaba (CV3) by combined high-resolution X-ray mapping, quantitative electron microprobe analyses, and oxygen isotopic analyses via secondary ion mass spectrometry. These chondrules contain smaller inner-chondrule olivine grains characterized by low refractory element (Ca, Al, Ti) contents, and larger outer-chondrule olivine crystals that are enriched in refractory elements and show complex Ti and Al oscillatory zonings. Our O isotopic survey revealed that many of the inner-chondrule olivines ¹⁶O-richer than the relatively isotopically uniform outer-chondrule olivines. Inner-chondrule olivine crystals—only a minority of which may be derived from earlier generations of chondrules—are likely mostly inherited from nebular condensates similar to AOAs, as they share similar isotopic and chemical features and are thus interpreted as relict grains. Still, being ¹⁶O-poorer than most AOAs, they may have experienced significant exchange with a ¹⁶O-poor reservoir prior to chondrule formation (even if to a lesser degree than relicts in CM2 and ungrouped C2 chondrites). Subsequent incomplete melting of the relict grains produced Ca-Al-Ti-rich melts that engulfed the remaining relict olivine grains. The complex Ti and Al zoning patterns in outer chondrule (host) olivines, in particular the systematic dilution near the margin, seem to reflect gas-melt interactions (with e.g. SiO (g), Mg (g)) which also buffered the O isotopic composition of chondrule hosts. Together, these results demonstrate that important episodes of recycling of nebular condensates occurred in the solar protoplanetary disk.

¹Ai-Cheng Zhang et al. (>10)
American Mineralogist 104, 150-157 Link to Article [https://doi.org/10.2138/am-2019-6597]
¹State Key Laboratory for Mineral Deposits Research and School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
Copyright: The Mineralogical Society of America

Crystallization is one of the most fundamental processes for both solid inorganic and organic materials in nature. The classical crystallization model mainly involves the monomer-by-monomer addition of simple chemical species. Recently, nanoparticle attachment has been realized as an important mechanism of crystallization in comparatively low-temperature aqueous natural and synthetic systems. However, no evidence of crystallization by particle attachment has been reported in petrologically important melts. In this study, we described spherical (Mg,Fe)-oxides with a protrusion surface in a shock-induced melt pocket from the martian meteorite Northwest Africa 7755. Transmission electron microscopic observations demonstrate that the (Mg,Fe)-oxides are structure-coherent intergrowth of ferropericlase and magnesioferrite. The magnesioferrite is mainly present adjacent to the interface between (Mg,Fe)-oxides spherules and surrounding silicate glass, but not in direct contact with the silicate glass. Thermodynamic and kinetic considerations suggest that development of the spherical (Mg,Fe)-oxides can be best interpreted with crystallization by particle attachment and subsequent Ostwald ripening. This indicates that crystallization by particle attachment can also take place in high-temperature melts and has potential implications for understanding the nucleation and growth of early-stage crystals in high-temperature melts, such as chondrules in the solar nebula, erupted volcanic melts, and probably even intrusive magmas.

^1,2Wolf Uwe Reimold, ³Alvaro Penteado Crósta, ^2,4Maximilian Hasch, ^2,5Astrid Kowitz, ¹Natalia Hauser,⁶Joana Paula Sanchez, ⁷Luiz Sergio Amarante Simões, ^2,3Grace Juliana de Oliveira, ²Patrice T. Zaag
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13233]
¹Geochronology Laboratory, Institute of Geosciences, University of Brasilia, , Brasilia, DF, CEP 70910 900 Brazil
²Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, , 10115 Berlin, Germany
³ Institute, State University of Campinas, UNICAMP, , Campinas, SP, Brazil
⁴ of Geosciences, University of the Witwatersrand, , Johannesburg, 2050 South Africa
⁵Bundesanstalt für Materialforschung und –prüfung (BAM), , 12205 Berlin, Germany
⁶ of Science and Technology, Goiás Federal University, , Goiânia, GO, Brazil
⁷Department of Petrology and Metallogeny, São Paulo State University, , Rio Claro, SP, Brazil
Published by arrangement with John Wiley & Sons

Cerro do Jarau is a conspicuous, circular morpho‐structural feature in Rio Grande do Sul State (Brazil), with a central elevated core in the otherwise flat “Pampas” terrain typical for the border regions between Brazil and Uruguay. The structure has a diameter of approximately 13.5 km. It is centered at 30^o12′S and 56^o32′W and was formed on basaltic flows of the Cretaceous Serra Geral Formation, which is part of the Paraná‐Etendeka Large Igneous Province (LIP), and in sandstones of the Botucatu and Guará formations. The structure was first spotted on aerial photographs in the 1960s. Ever since, its origin has been debated, sometimes in terms of an endogenous (igneous) origin, sometimes as the result of an exogenous (meteorite impact) event. In recent years, a number of studies have been conducted in order to investigate its nature and origin. Although the results have indicated a possible impact origin, no conclusive evidence could be produced. The interpretation of an impact origin was mostly based on the morphological characteristics of the structure; geophysical data; as well as the occurrence of different breccia types; extensive deformation/silicification of the rocks within the structure, in particular the sandstones; and also on the widespread occurrence of low‐pressure deformation features, including some planar fractures (PFs). A detailed optical microscopic analysis of samples collected during a number of field campaigns since 2007 resulted in the disclosure of a large number of quartz grains from sandstone and monomict arenite breccia from the central part of the structure with PFs and feather features (FFs), as well as a number of quartz grains exhibiting planar deformation features (PDFs). While most of these latter grains only carry a single set of PDFs, we have observed several with two sets, and one grain with three sets of PDFs. Consequently, we here propose Cerro do Jarau as the seventh confirmed impact structure in Brazil. Cerro do Jarau, together with Vargeão Dome (Santa Catalina state) and Vista Alegre (Paraná State), is one of very few impact structures on Earth formed in basaltic rocks.

¹Francesca E.De Meo,²David Polishook,³Benoît Carry, ⁴Brian J.Burt, ^5,6Henry H.Hsieh,¹Richard P.Binzel, ⁴A.Moskovitz, ⁷Thomas H.Burbine
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.12.016]
¹Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 USA
²Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 0076100, Israel
³Observatoire de la Cte d’Azur, Boulevard de l’Observatoire, 06304 Nice Cedex 4, France
⁴Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
⁵Planetary Science Institute, 1700 E. Ft. Lowell Road, Suite 106, Tucson, AZ 85719, USA
⁶Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 10617, Taiwan
⁷Department of Astronomy, Mount Holyoke College, South Hadley, MA 01075, USA
Copyright Elsevier

Differentiated asteroids are rare in the main asteroid belt despite evidence for  ∼ 100 distinct differentiated bodies in the meteorite record. We have sought to understand why so few main-belt asteroids differentiated and where those differentiated bodies or fragments reside. Using the Sloan Digital Sky Survey (SDSS) to search for a needle in a haystack we identify spectral A-type asteroid candidates, olivine-dominated asteroids that may represent mantle material of differentiated bodies. We have performed a near-infrared spectral survey with SpeX on the NASA IRTF and FIRE on the Magellan Telescope.
We report results from having doubled the number of known A-type asteroids. We deduce a new estimate for the overall abundance and distribution of this class of olivine-dominated asteroids. We find A-type asteroids account for less than 0.16% of all main-belt objects larger than 2 km and estimate there are a total of  ∼ 600 A-type asteroids above that size. They are found rather evenly distributed throughout the main belt, are even detected at the distance of the Cybele region, and have no statistically significant concentration in any asteroid family. We conclude the most likely implication is the few fragments of olivine-dominated material in the main belt did not form locally, but instead were implanted as collisional fragments of bodies that formed elsewhere.

^1,2,3Jessica J.Barnes, ¹Ian A.Franchi, ²Francis M.McCubbin,^1,4Mahesh Anand
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.12.032]
¹School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
²ARES, NASA Johnson Space Center, Houston, Texas 77058, USA
³Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA
⁴Department of Earth Sciences, Natural History Museum, London, UK
Copyright Elsevier

The isotopes of chlorine (³⁷Cl and ³⁵Cl) are highly fractionated in lunar samples compared to most other Solar System materials. Recently, the chlorine isotope signatures of lunar rocks have been attributed to large-scale degassing processes that occurred during the existence of a magma ocean. In this study we investigated how well a suite of lunar basalts, most of which have not previously been analyzed, conform to previous models. The Cl isotope compositions (δ³⁷Cl (‰) = [(³⁷Cl/³⁵Cl_sample/³⁷Cl/³⁵Cl_SMOC)-1]×1000, where SMOC refers to standard mean ocean chloride) recorded range from ∼+7 to +14 ‰ (Apollo 15), +10 to +19 ‰ (Apollo 12), +9 to +15 ‰ (70017), +4 to +8 ‰ (MIL 05035), and +15 to +22 ‰ (Kalahari 009). The Cl isotopic data from the present study support the mixing trends previously reported by Boyce et al., 2015, Barnes et al., 2016, as the Cl isotopic composition of apatites are positively correlated with bulk-rock incompatible trace element abundances in the low-Ti basalts, inclusive of low-Ti and KREEP basalts. This trend has been interpreted as evidence that incompatible trace elements, including Cl, were concentrated in the urKREEP residual liquid of the lunar magma ocean, rather than the mantle cumulates, and that urKREEP Cl had a highly fractionated isotopic composition. The source regions for the basalts were thus created by variable mixing between the mantle (Cl-poor and relatively unfractionated) and urKREEP. The high-Ti basalts show much more variability in measured Cl isotope ratios and scatter around the trend formed by the low-Ti basalts. Most of the data for lunar meteorites also fits the mixing of volatiles in their sources, but Kalahari 009, which is highly depleted in incompatible trace elements, contains apatites with heavily fractionated Cl isotopic compositions. Given that Kalahari 009 is one of the oldest lunar basalts and ought to have been derived from very early-formed mantle cumulates, a heavy Cl isotopic signature is likely not related to its mantle source, but more likely to magmatic or secondary alteration processes, perhaps via impact-driven vapor metasomatism of the lunar crust.