^1,2Péter Németh,^2,3Laurence A.J. Garvie
American Mineralogist 105, 276–281 Link to Article [https://doi.org/10.2138/am-2020-7305]
¹Institute of Materials and Environmental Chemistry, Research Center for Natural Sciences, Hungarian Academy of Sciences, H-1117 Budapest, Magyar Tudósok Körútja 2, Hungary
²School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-6004, U.S.A.
³Center for Meteorite Studies, Arizona State University, Tempe, Arizona 85287-6004, U.S.A.
Copyright: The Mineralogical Society of America

Shock caused by impacts can convert carbonaceous material to diamond. During this transition, new materials can form that depend on the structure of the starting carbonaceous materials and the shock conditions. Here we report the discovery of cage-like nanostructured carbonaceous materials, including carbon nano-onions and bucky-diamonds, formed through extraterrestrial impacts in the Gujba (CB_a) meteorite. The nano-onions are fullerene-type materials and range from 5 to 20 nm; the majority shows a graphitic core-shell structure, and some are characterized by fully curved, onion-like graphitic shells. The core is either filled with carbonaceous material or empty. We show the first, natural, 4 nm sized bucky-diamond, which is a type of carbon nano-onion consisting of multilayer graphitic shells surrounding a diamond core. We propose that the nano-onions formed during shock metamorphism, either the shock or the release wave, of the pre-existing primitive carbonaceous material that included nanodiamonds, poorly ordered graphitic material, and amorphous carbonaceous nanospheres. Bucky-diamonds could have formed either through the high-pressure transformation of nano-onions, or as an intermediate material in the high-temperature transformation of nanodiamond to nano-onion. Impact processing of planetary materials was and is a common process in our solar system, and by extension, throughout extrasolar planetary bodies. Together with our previous discovery of interstratified graphite-diamond in Gujba, our new findings extend the range of nano-structured carbonaceous materials formed in nature. Shock-formed nano-onions and bucky-diamonds are fullerene-type structures, and as such they could contribute to the astronomical 217.5 nm absorption feature.

¹Maree McGregor,²Erin L.Walton,³Christopher R.M.McFarlane,¹John G.Spray
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.052]
¹Planetary and Space Science Centre, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
²Department of Physical Sciences, MacEwan University, Edmonton, AB, T5J 4S2, Canada
³Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
Copyright Elsevier

In situ U-Pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry (LA-ICP-MS) on shocked and thermally metamorphosed apatite, titanite, and zircon grains from the Steen River impact structure, Canada. The dated relict mineral phases occur within impact melt-bearing breccias that underwent post-deposition sintering at 450°C > T < 850°C. Apatite yields a refined lower intercept age of 141 ± 4 Ma, which we interpret as the best estimate for the Steen River impact event. Titanite was only partially reset; yielding a lower intercept age of 113 ± 41 Ma. Zircon yields a lower intercept impact age of 120 ± 14 Ma, which is considered a minimum best-estimate impact age. The most reset zircon ages that control this lower intercept are complicated by combinations of common-Pb incorporation and evidence for recent Pb loss associated with granularized and radiation-damaged domains. All three phases preserve Paleoproterozoic crystalline basement ages, with a single concordant ²⁰⁶Pb/²³⁸U age of 1914 ± 39 Ma from apatite, an upper intercept age of 1882 ± 11 Ma from zircon, and an upper intercept age of 1842 ± 9 Ma from titanite. For apatite, the degree of isotopic resetting is largely thermally controlled, with the extent of reset closely associated with textural setting (degree of grain armouring, melt proximity and sample temperature) and, to a lesser extent, by shock/thermally generated microstructures (i.e., planar fractures, micro-vesicles, and recrystallized domains). While titanite records an impact age that falls within error of apatite and zircon, the majority of grains experienced only partial isotopic resetting, which we attribute to incomplete Pb loss associated with rapid cooling and post-depositional sintering of the breccia matrix below the titanite closure temperature (∼800°C). In zircon, ancient (impact) Pb-loss was facilitated along defect-related, fast-diffusion pathways within pre-impact metamict domains, shock defects, and via recrystallization. These same domains were also subject to recent (post-impact) Pb loss and common Pb contamination, significantly compromising the reliability of zircon ages. The distribution of U-Pb ratios in apatite and titanite is unlike those obtained in crystalline targets, a feature we interpret to be characteristic of impact structures developed in mixed (sedimentary – crystalline) targets, such as Steen River. In this case disaggregated melt systems create thermal regimes distinct from those derived from predominantly igneous/metamorphic targets. With an age of 141 ± 4 Ma, Steen River joins the Dellen (Sweden), Gosses Bluff (Australia), Mjølnir (Barents Sea), and Morokweng (South Africa) impact structures in being formed at, or close to, the Jurassic-Cretaceous boundary.

¹Eleanor C.McIntosh,¹James M.D.Day,²Yang Liu,¹Courtney Jiskoot
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.051]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Copyright Elsevier

Impactors striking the Moon since the formation of its crust have left an indelible imprint on the lunar surface, in the guise of craters and associated impact rocks. The lunar crust has low intrinsic abundances of the highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au), at greater than 3000 times lower than in chondrite meteorites. Consequently, during impact, bolides with chondritic or differentiated iron-rich compositions should impart elevated HSE signatures to the lunar crust. Here we examine glassy lunar impact melt coats (IMC) from the outside rims of Apollo 16 cataclastic anorthosites (60015, 65325) and breccias (65035), as well as both fragments and powders of Antarctic anorthositic regolith breccia (ARB) meteorites (Miller Range 090034/36/70/75 and MacAlpine Hills 88105) for their petrography, mineral chemistry and bulk-rock compositions. The HSE concentrations for IMC range from ∼0.001 to 0.1 × CI chondrite, with measured ¹⁸⁷Os/¹⁸⁸Os between 0.1189 and 0.1366. Anorthositic regolith breccia meteorites, which have components with 2.6-4.1 Ga ages, have similar HSE concentrations to IMC, but typically have lower ¹⁸⁷Os/¹⁸⁸Os (0.1164-0.1284). These latter Os ratios are generally less radiogenic than those measured in ∼3.8-3.9 Ga Apollo impact melt breccias. The Apollo 16 IMC are not well-dated, but their KREEPy trace-element signatures and associated ages of 3.7 to 3.8 Ga for Apollo 16 glasses might imply, at least in part, an origin from the Imbrium or Serenitatis basin-forming impacts. Within the IMC, metal-schreibersite-troilite assemblages record significant inter-element HSE fractionation which is also reflected in bulk HSE patterns for both IMC and ARB meteorites. Variations in relative and absolute HSE compositions directly reflect the control of metal and sulfide segregation within and between impact melt and breccia lithologies. Collectively, IMC and ARB meteorites exhibit approximately 50% of the variation in Ru/Ir and ¹⁸⁷Os/¹⁸⁸Os observed in lunar impact melt breccias. These results imply that significant variations in inter-element compositions can occur during impact brecciation and melting and so some impact melt rock HSE compositions may not record the compositions of impactors that struck the Moon with fidelity. Nonetheless, the generally low ¹⁸⁷Re/¹⁸⁸Os of lunar impact melt rocks means that osmium isotope ratios provide evidence for impact composition, and a change from ordinary to carbonaceous-like impactors either with time – or location – striking the Moon.

¹Hidetomo HONGU,¹Akira YOSHIASA,¹Tsubasa TOBASE ,²Maki OKUBE,²Kazumasa SUGIYAMA,³Tsutomu SATO
Journal of Mineralogical and Petrological Sciences 114, 224-230 Link to Article [DOI https://doi.org/10.2465/jmps.180927]
¹Graduate School of Science and Technology, Kumamoto University
²Institute for Materials Research, Tohoku University
³Graduate School of Engineering, Hokkaido University

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¹Masami KANZAKI
Journal of Mineralogical and Petrological Sciences 114, 214-218 Link to Article [DOI https://doi.org/10.2465/jmps.190414]
¹Institute for Planetary Materials, Okayama University

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^1,2Emily C. First,^1,3Tanis C. Leonhardi,¹Julia E. Hammer
Contributions to Mineralogy and Petrology 175, 13 Link to Article [DOI
https://doi.org/10.1007/s00410-019-1638-7]
¹Department of Geology and GeophysicsUniversity of Hawai‘i at Mānoa, Honolulu,USA
²Department of Earth, Environmental and Planetary Sciences Brown University, Providence,USA
³Department of Earth and Planetary Science University of California, Berkeley,Berkeley,USA

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¹Martin Ferus et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113670]
¹J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Dolejškova 3, CZ18223 Prague 8, Czech Republic
Copyright Elsevier

The main objective of this study is to provide data on the bulk elemental composition, mineralogy and the possible origin of the Porangaba meteorite, whose fall was observed at 17:35 UT on 9 January 2015 on several sites of the state of São Paulo in Brazil. The surface of the meteorite was mapped by Scanning Electron Microscopy (SEM) and optical microscopy. The mineralogy and the bulk elemental composition of the meteorite were studied using Energy-Dispersive and Wavelength-Dispersive X-ray Spectroscopy (EDS/WDS) together with Electron BackScatter Diffraction (EBSD). The bulk elemental composition was also independently analysed by Atomic Absorption Spectrometry (AAS), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Laser Ablation ICP MS (LA ICP-MS) and Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS). Based on the available visual camera records of the Porangaba meteorite fall, its orbit was tentatively calculated, and possible candidates for the source bodies in the Solar system were proposed. We also present a laboratory simulation of a Porangaba-like (L4 Ordinary Chondrite) meteor emission spectra. These can be used as benchmark spectra for the identification of meteor rock types through their comparison with meteor spectra recorded by high-speed video-cameras equipped with simple grating spectrographs.

^1,2Sheng Gou,^1,3Zongyu Yue,^1,2,3Kaichang Di,¹Wenhui Wan,¹Zhaoqin Liu,^1,2Bin Liu,¹Man Peng,¹Yexin Wang,⁴Zhiping He,⁴Rui Xu
Earth and Planetary Science Letters 535, 116117 Link to Article [https://doi.org/10.1016/j.epsl.2020.116117]
¹State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100101, China
²State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
³CAS Center for Excellence in Comparative Planetology, Hefei 230026, China
⁴dKey Laboratory of Space Active Opto-Electronics Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
Copyright Elsevier

Space weathering introduces confounding effects on visible and near infrared reflectance spectra of airless bodies, which considerably darkens the reflectance, reddens the continuum slope and suppresses absorption features. It’s mainly attributed to the gradual formation and accumulation of submicroscopic metallic iron (SMFe) on regolith grains. In situ spectral measurements from Chang’e-4 rover provide a unique opportunity to investigate the space weathering effects on the intact lunar farside regolith. SMFe abundance at the landing site, which is 0.32±0.06 wt.%, is retrieved from in situ measured reflectance spectra by using Hapke model. The derived Is/FeO maturity index (82±15) indicates the Finsen crater ejecta-sourced regolith is mature, which is consistent with the geologic background that it had experienced about 3.7 Ga space weathering.

¹K.K.Larsen,¹D.Wielandt,¹M.Schiller,^1,2A.N.Krot,¹M.Bizzarro
Earth and Planetary Science Letters 535, 116088 Link to Article [https://doi.org/10.1016/j.epsl.2020.116088]
¹Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Copenhagen DK-1350, Denmark
²Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, HI 96822, USA
Copyright Elsevier

Refractory inclusions [Ca-Al-rich Inclusions (CAIs) and Amoeboid Olivine Aggregates (AOAs)] in primitive meteorites are the oldest Solar System solids. They formed in the hot inner protoplanetary disk and, as such, provide insights into the earliest disk dynamics and physicochemical processing of the dust and gas that accreted to form the Sun and its planetary system. Using the short-lived 26Al to 26Mg decay system, we show that bulk refractory inclusions in CV (Vigarano-type) and CR (Renazzo-type) carbonaceous chondrites captured at least two distinct 26Al-rich (26Al/27Al ratios of ∼5 × 10−5) populations of refractory inclusions characterized by different initial 26Mg/24Mg isotope compositions (μ26Mg*0). Another 26Al-poor CAI records an even larger μ26Mg*0 deficit. This suggests that formation of refractory inclusions was punctuated and recurrent, possibly associated with episodic outbursts from the accreting proto-Sun lasting as short as <8000 yr. Our results support a model in which refractory inclusions formed close to the hot proto-Sun and were subsequently redistributed to the outer disk, beyond the orbit of Jupiter, plausibly via stellar outflows with progressively decreasing transport efficiency. We show that the magnesium isotope signatures in refractory inclusions mirrors the presolar grain record, demonstrating a mutual exclusivity between 26Al enrichments and large nucleosynthetic Mg isotope effects. This suggests that refractory inclusions formed by incomplete thermal processing of presolar dust, thereby inheriting a diluted signature of their isotope systematics. As such, they record snapshots in the progressive sublimation of isotopically anomalous presolar carriers through selective thermal processing of young dust components from the proto-Solar molecular cloud. We infer that 26Al-rich refractory inclusions incorporated 26Al-rich dust which formed <5 Myr prior to our Sun, whereas 26Al-poor inclusions (such as FUN- and PLAC-type CAIs) incorporated >10 Myr old dust.

^1,2,3C.Cartier,⁴O.Namur,⁵ L.R.Nittler,⁵S.Z.Weider,⁶E.Crapster-Pregont,⁶A.Vorburger,⁶E.A.Franck,¹B.Charlier
Earth and Planetary Science Letters 534, 116108 Link to Article [https://doi.org/10.1016/j.epsl.2020.116108]
¹Département de Géologie, Université de Liège, 4000, Sart Tilman, Belgium
²Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, 63038, France
³CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
⁴Department of Earth and Environmental Sciences, KU Leuven, Leuven, 3001, Belgium
⁵Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC 20015, USA
⁶Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
Copyright Elsevier

In this study we investigate the likeliness of the existence of an iron sulfide layer (FeS matte) at the core-mantle boundary (CMB) of Mercury by comparing new chemical surface data obtained by the X-ray Spectrometer onboard the MESSENGER spacecraft with geochemical models supported by high-pressure experiments under reducing conditions. We present a new data set consisting of 233 Ti/Si measurements, which combined with Al/Si data show that Mercury’s surface has a slightly subchondritic Ti/Al ratio of 0.035 ± 0.008. Multiphase equilibria experiments show that at the conditions of Mercury’s core formation, Ti is chalcophile but not siderophile, making Ti a useful tracer of sulfide melt formation. We parameterize and use our partitioning data in a model to calculate the relative depletion of Ti in the bulk silicate fraction of Mercury as a function of a putative FeS layer thickness. By comparing the model results and surface elemental data we show that Mercury most likely does not have a FeS layer, and in case it would have one, it would only be a few kilometers thick (<13km). We also show that Mercury’s metallic Fe(Si) core cannot contain more than ∼1.5 wt.% sulfur and that the formation of this core under reducing conditions is responsible for the slightly subchondritic Ti/Al ratio of Mercury’s surface.