^1,2Yuto Kidokoro, ²Koichiro Umemoto, ²Kei Hirose, ³Yasuo Ohishi
American Mineralogist 102, 2230-2234 Link to Article [DOI
https://doi.org/10.2138/am-2017-6033]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
²Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8550, Japan
³Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan
Copyright: The Mineralogical Society of America

We have investigated the phase transition in SiC between the zinc-blende and rock-salt structures at high pressure and temperature in a laser-heated diamond-anvil cell. Results demonstrate that the transition occurs at 74 GPa and 2100 K with a 21% density increase, reflecting the coordination number rising from four to six. In addition, our ab initio calculations show that the boundary has a negative Clapeyron slope of −4.0 MPa/K at 2000 K. The experimentally determined phase boundary is located between those predicted by GGA and B3LYP functional. This transition may take place inside carbon-rich extrasolar planets, forming a boundary with a large density jump. Since SiC is rigid and highly thermally conductive, thermal convection in an SiC-dominant layer is not likely to occur. Nevertheless, the convection may be possible if planet interiors include both silicon carbide and silicate, and in this case the phase transition could affect the style of thermal convection.

^1,2Carsten Münker, ²Raúl O. C. Fonseca, ³Toni Schulz
Nature Geoscience 10, 822-826 Link to Article [doi:10.1038/ngeo3048]
¹Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Strasse 49b, 50674 Köln, Germany
²Steinmann Institut, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, German
³Department für Lithosphärenforschung, Universität Wien, Althanstrasse 14, 1090 Wien, Austria

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¹M. I. F. Barth,¹D. Harries,¹F. Langenhorst,²P. Hoppe
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12992]
¹Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Jena, Germany
²Max-Planck-Institut für Chemie, Mainz, Germany
Published by arrangement with John Wiley & Sons

The ungrouped carbonaceous chondrite Acfer 094 is among the least altered samples of the early solar system. We have studied concentric sulfide–oxide aggregates from this meteorite by transmission electron microscopy (TEM) and nanoscale secondary ion mass spectrometry (NanoSIMS). The main minerals present are magnetite, pentlandite, and pyrrhotite/troilite. The outer parts of the aggregates include μm-sized olivine and pyroxenes with variable Mg/Fe ratios. One aggregate contains taenite (56.7 wt% Ni) within its central part that is surrounded by pentlandite and magnetite. We conclude that both phases have formed by oxidation and sulfidization of metal and, based on the metal and sulfide Fe/Ni ratio, a (sulfide)-formation temperature of 400–550 °C can be constrained. This temperature is higher than any temperature that could be expected to have occurred on the Acfer 094 parent body, and also textural evidence indicates that the aggregates formed before parent-body accretion. We therefore conclude that the formation of the sulfide–oxide aggregates occurred most likely in the solar nebular at highly variable H2O and H2S fugacities. Oxygen-isotopic compositions of magnetite within these assemblages show that they are indistinguishable from the meteorite’s matrix (δ17OSMOW ≈ 4 ± 8‰, δ18OSMOW ≈ 10 ± 6‰, and ∆17OSMOW ≈ −1 ± 5‰). An enrichment of 17,18O relative to chondrules of Acfer 094 suggests a link between the formation of the sulfide–oxide aggregates and the preaccretionary processing of matrix grains in a volatile-enriched nebular environment.

^1,2Naoya Imae,³Yoshihiro Nakamuta
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12996]
¹Antarctic Meteorite Research Center, National Institute of Polar Research, Tachikawa, Tokyo, Japan
²Department of Polar Science, School of Multidisciplinary Sciences, SOKENDAI (The Graduate University for Advanced Studies), Tachikawa, Tokyo, Japan
³Kyushu University Museum, Kyushu University, Fukuoka, Japan
Published by arrangement with John Wiley & Sons

Using the in-plane rotation of polished thin section, the X-ray diffraction patterns exhibiting a high degree of randomness similar to powder pattern were obtained for 10 CO3 chondrites, which distinguished 130 reflections of olivine in the chondrules from that in the matrix, and showed systematic differences among subtypes based on the full width at half maximum intensity of two olivine 130 peaks. A lower petrologic subtype is characterized by sharp and strong peaks for forsteritic olivines in type I chondrules and by a weak and broad peak for ferroan matrices, and the higher petrologic subtypes are characterized by sharp and strong peaks for recrystallized matrices and a weakened or absent peak of magnesian olivines. The systematic change in the split peak of olivine 130 was linked with the mean diffusion length of Mg-Fe in olivine phenocrysts in type I chondrules. Fe-Ni diffusion in metals was considered to estimate the peak temperature of CO3.0, near the surface on the parent body. The peak metamorphic temperatures were estimated to be ~600–910 K using the onion-shell model when the cooling time was 106–108 yr on the parent body. A weak peak for ferroan olivine of CO3.0 suggests the amorphous silicate in matrices. The modal abundance of the amorphous Fe-silicate for subtype 3.0 (15% for Allan Hills [ALH] 77307 and 9% for Yamato [Y]-81020) was also evaluated from the deviation in trend of the relative peak ratios of the Fe-rich (≥Fa25) and Mg-rich (<Fa25) olivines for subtypes. The existence of martensites was suggested for ALH 77307. Amorphous silicate in matrices is a more resistant primordial component that produced the CO3 chondrites than martensite.

¹Chizu Kato, ^1,2Frédéric Moynier
Earth and Planetary Science Letters 479, 330-339 Link to Article [https://doi.org/10.1016/j.epsl.2017.09.028]
¹Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, CNRS UMR 7154, Paris, France
²Institut Universitaire de France, 75005, Paris, France
Copyright Elsevier

The abundance of moderately volatile elements, such as Zn and Ga, show variable depletion relative to CI between the Earth and primitive meteorite (chondrites) parent bodies. Furthermore, the first solar system solids, the calcium–aluminum-rich inclusions (CAIs), are surprisingly rich in volatile element considering that they formed under high temperatures. Here, we report the Ga elemental and isotopic composition of a wide variety of chondrites along with five individual CAIs to understand the origin of the volatile elements and to further characterize the enrichment of the volatile elements in high temperature condensates. The δ71Ga (permil deviation of the 71Ga/69Ga ratio from the Ga IPGP standard) of carbonaceous chondrites decreases in the order of CI>CM>CO>CV and is inversely correlated with the Al/Ga ratio. This implies that the Ga budget of the carbonaceous chondrites parent bodies were inherited from a two component mixing of a volatile rich reservoir enriched in heavy isotope of Ga and a volatile poor reservoir enriched in light isotope of Ga. Calcium–aluminum-rich inclusions are enriched in Ga and Zn compared to the bulk meteorite and are both highly isotopically fractionated with δ71Ga down to −3.56‰ and δ66Zn down to −0.74‰. The large enrichment in the light isotopes of Ga and Zn in the CAIs implies that the moderately volatile elements were introduced in the CAIs during condensation in the solar nebula as opposed to secondary processing in the meteorite parent body and supports a change in gas composition in which CAIs were formed.

¹Jan Leitner,¹Peter Hoppe,²Jutta Zipfel
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12994]
¹Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
²Sektion Meteoritenforschung, Forschungsinstitut und Naturmuseum Senckenberg, Frankfurt, Germany
Published by arrangement with John Wiley & Sons

We report on the investigation of presolar grain inventories of hydrated lithic clasts in three metal-rich carbonaceous chondrites from the CR clan, Acfer 182 (CH3), Isheyevo (CH3/CBb3), and Lewis Cliff (LEW) 85332 (C3-un), as well as the carbon- and nitrogen-isotopic compositions of the fine-grained clast material. Eleven presolar silicate grains as well as nine presolar silicon carbide (SiC) grains were identified in the clasts. Presolar silicate abundances range from 4 to 22 parts per million (ppm), significantly lower than in pristine meteorites and interplanetary dust particles (IDP), and comparable to recent findings for CM2s and CR2 interchondrule matrix. SiC concentrations lie between 9 and 23 ppm, and are comparable to the values for CI, CM, and CR chondrites. The results of our investigation suggest similar alteration pathways for the clast material, the interchondrule matrix of the CR2 chondrites, and the fine-grained fraction of CM2 chondrites. Fine-grained matter of all three meteorites contains moderate to high 15N-enrichments (~50‰ ≤ δ15N ≤ ~1600‰) compared to the terrestrial value, indicating the presence of primitive organic material. We observed no correlation between 15N-enrichments and presolar dust concentrations in the clasts. This is in contrast to the findings from a suite of primitive IDPs, which display in several cases enhanced bulk 15N/14N ratios and high presolar grain abundances of several hundred or even thousand ppm. The bulk 15N/14N ratios of the clasts are comparable to the range for primitive IDPs, suggesting a nitrogen carrier less susceptible to destruction by aqueous alteration than silicate stardust.

^1,2,3Surya S. Rout,^1,2,4Philipp R. Heck,⁵Dieter Isheim,^1,2,4Thomas Stephan,⁶Nestor J. Zaluzec,⁷Dean J. Miller,^1,2,4,8Andrew M. Davis,⁴David N. Seidman
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12988]
¹Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, Illinois, USA
²Chicago Center for Cosmochemistry, Chicago, Illinois, USA
³Physikalisches Institut, Space Research and Planetary Sciences, Universität Bern, Bern, Switzerland
⁴Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
⁵Department of Material Science & Engineering, Northwestern University Center for Atom-Probe Tomography, Northwestern University, Evanston, Illinois, USA
⁶Photon Science Division, Argonne National Laboratory, Argonne, Illinois, USA
⁷Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, USA
⁸Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, USA
Published by arrangement with John Wiley & Sons

We report the first combined atom-probe tomography (APT) and transmission electron microscopy (TEM) study of a kamacite–tetrataenite (K–T) interface region within an iron meteorite, Bristol (IVA). Ten APT nanotips were prepared from the K–T interface with focused ion beam scanning electron microscopy (FIB-SEM) and then studied using TEM followed by APT. Near the K-T interface, we found 3.8 ± 0.5 wt% Ni in kamacite and 53.4 ± 0.5 wt% Ni in tetrataenite. High-Ni precipitate regions of the cloudy zone (CZ) have 50.4 ± 0.8 wt% Ni. A region near the CZ and martensite interface has <10 nm sized Ni-rich precipitates with 38.4 ± 0.7 wt% Ni present within a low-Ni matrix having 25.5 ± 0.6 wt% Ni. We found that Cu is predominantly concentrated in tetrataenite, whereas Co, P, and Cr are concentrated in kamacite. Phosphorus is preferentially concentrated along the K-T interface. This study is the first precise measurement of the phase composition at high spatial resolution and in 3-D of the K-T interface region in a IVA iron meteorite and furthers our knowledge of the phase composition changes in a fast-cooled iron meteorite below 400 °C. We demonstrate that APT in conjunction with TEM is a useful approach to study the major, minor, and trace elemental composition of nanoscale features within fast-cooled iron meteorites.

¹William S.Cassata
Earth and Planetary Science Letters 479, 322-329 Link to Article [https://doi.org/10.1016/j.epsl.2017.09.034]
¹Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore CA, 94550, USA
Copyright Elsevier

The evolution of Mars’ atmosphere to its currently thin state incapable of supporting liquid water remains poorly understood and has important implications for Martian climate history. Martian meteorites contain trapped atmospheric gases that can be used to constrain both the timing and effectiveness of atmospheric escape processes. In this paper, measurements of xenon isotopes in two ancient Martian meteorites, ALH 84001 and NWA 7034, are reported. The data indicate an early episode of atmospheric escape that mass fractionated xenon isotopes culminated within a few hundred million years of planetary formation, and little change to the atmospheric xenon isotopic composition has occurred since this time. In contrast, on Earth atmospheric xenon fractionation continued for at least two billion years (Pujol et al., 2011). Such differences in atmospheric Xe fractionation between the two planets suggest that climate conditions on Mars may have differed significantly from those on Archean Earth. For example, the hydrogen escape flux may not have exceeded the threshold required for xenon escape on Mars after 4.2–4.3 Ga, which indicates that Mars may have been significantly drier than Earth after this time.

¹H.Genda, ¹R.Brasser, ^2,3S.J.Mojzsis
Earth and Planetary Science Letters 480, 25-32 Link to Article [https://doi.org/10.1016/j.epsl.2017.09.041]
¹Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
²Department of Geological Sciences, University of Colorado, UCB 399, 2200 Colorado Avenue, Boulder, CO 80309-0399, USA
³Institute for Geological and Geochemical Research, Research Center for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 45 Budaörsi Street, H-1112 Budapest, Hungary
Copyright Elsevier

Overabundances in highly siderophile elements (HSEs) of Earth’s mantle can be explained by conveyance from a singular, immense (D∼3000km) “Late Veneer” impactor of chondritic composition, subsequent to lunar formation and terrestrial core-closure. Such rocky objects of approximately lunar mass (∼0.01 M⊕) ought to be differentiated, such that nearly all of their HSE payload is sequestered into iron cores. Here, we analyze the mechanical and chemical fate of the core of such a Late Veneer impactor, and trace how its HSEs are suspended – and thus pollute – the mantle. For the statistically most-likely oblique collision (∼45°), the impactor’s core elongates and thereafter disintegrates into a metallic hail of small particles (∼10 m). Some strike the orbiting Moon as sesquinary impactors, but most re-accrete to Earth as secondaries with further fragmentation. We show that a single oblique impactor provides an adequate amount of HSEs to the primordial terrestrial silicate reservoirs via oxidation of (<m-sized) metal particles with a hydrous, pre-impact, early Hadean Earth.