¹Atencio, D.,¹Cunha, D.,²Ribeiro Moutinho, A.L.,³Zucolotto, M.E.,¹Tosi, A.A.,³Nassif Villaça, C.V.
Brazilian Journal of Geology 50, 2020 Link to Article [DOI: 10.1590/2317-4889202020190085]
¹Universidade de São Paulo, São Paulo (SP), Brazil
²International Meteorite Collector Association, Jacareí (SP), Brazil
³Universidade Federal do Rio de Janeiro, Rio de Janeiro (RJ), Brazil

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¹Maximilian P.Reitze,¹Iris Weber,¹Andreas Morlok,¹Harald Hiesinger,¹Karin E.Bauch,¹Aleksandra N.Stojic,²Jörn Helbert
Earth and Planetary Science Letters 554, 116697 Link to Article [https://doi.org/10.1016/j.epsl.2020.116697]
¹Institut für Planetologie, Westfälische Wilhelms-Universität (WWU) Münster, 48149 Münster, Germany
²Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
Copyright Elsevier

We analyzed plagioclase feldspar samples that were well-characterized in terms of chemical composition as well as degree of Al,Si order in mid-infrared reflection spectra between 7 μm and 14 μm (1429 cm−1 and 714 cm−1). The chemical compositions were derived with an electron microprobe analyzer. To determine the degree of Al,Si order, powder X-ray diffraction methods were applied. For the interpretation of the infrared spectra, we used the wavelength of the Christiansen feature (CF) and the autocorrelation function for a specific wavelength region. The CF shifts from around 7.72 μm (1296 cm−1) in Na-richest samples to 8.10 μm (1234 cm−1) in the Ca-richest sample. Combining the CF position and the autocorrelation-derived value allowed to determine the degree of Al,Si order of the samples based on reflection spectra. The wavelength of the Transparency feature (TF) in the finest analyzed grain size fraction also depends on the chemical composition and the degree of Al,Si order. Our results are helpful for the interpretation of data returned by the MERTIS experiment onboard BepiColombo. The data help to distinguish between space weathering, shock effects, and ordering effects in plagioclase samples.

¹Zachary A. Torrano,^1,2Jemma Davidson,¹Meenakshi Wadhwa
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13587]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, 85287 USA
²Center for Meteorite Studies, Arizona State University, Tempe, Arizona, 85287 USA
Published by arrangement with John Wiley & Sons

The similarities between CV and CK chondrites are so substantial that some studies have argued for a common parent body origin. These similarities also mean that they are susceptible to misclassification as one another. It is, therefore, important to accurately classify CV and CK chondrites to properly compare the properties of the two groups and evaluate the single parent body hypothesis. In this study, we re‐evaluate the current classification of Northwest Africa (NWA) 2900 as a CV3 chondrite. Based on chondrule abundance (~13%), average chondrule diameter (1.11 ± 0.67 mm), iron content in chondrule olivine (Fa1 to Fa35 with a peak near ~Fa31) and matrix olivine (Fa33 to Fa36), magnetite abundance (~4 vol%), elemental abundances in olivine (Cr2O3, Al2O3, TiO2, MnO, NiO, and CaO) and magnetite (Cr2O3, Al2O3, TiO2, and NiO), and comparison with previously reported data for CV and CK chondrites, we propose that the classification of NWA 2900 be changed from CV3 chondrite to CK3 chondrite, with characteristics most similar to those of the CK3.8 subtype. We additionally suggest minor modifications to the compositional criteria used to distinguish between CV and CK chondrites and demonstrate that NWA 2900 extends the range of bulk oxygen isotope compositions of CK chondrites.

¹Matthew S. Huber,¹Elizaveta Kovaleva,²Ulrich Riller
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13582]
¹Department of Geology, University of the Free State, 205 Nelson Mandela Dr, Bloemfontein, 9300 South Africa
²Institut für Geologie, Universität Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany
Published by arrangement with John Wiley & Sons

The Vredefort impact structure, South Africa, is comparable to the Sudbury impact structure, Canada, in size, age, and target rock composition. Both impact structures feature impact melt dikes. The melt sheet of the Sudbury impact (Sudbury Igneous Complex; SIC) is genetically linked to the Sudbury offset dikes in the underlying target rock. At Vredefort, the melt sheet was eroded so that only the granophyre dikes retain compositional melt sheet characteristics. XRF analyses of 43 samples from four granophyre dikes are similar to previous studies, but identify an anomalous mafic phase within one of the dikes. The results from the Vredefort granophyre dikes are compared to the Sudbury offset dikes and shown to follow similar geochemical trends, controlled by crystallization of feldspar and pyroxene. The mafic granophyre phase is compositionally remarkably similar to the offset dike compositions. The program Rhyolite‐MELTS was used to test possible melt sheet compositions. Modeling results are broadly consistent with the overall chemical and mineral composition of the dikes. Modeling is consistent with offset dikes being derived from the basal mafic layer of the SIC, and the granophyre dikes being derived from alkali‐depleted bulk continental crust. For all modeled compositions, crystallization primarily occurred at temperatures between 1150°C and 1000°C. The emplacement of the felsic granophyre dikes from a homogenized crustal melt suggests emplacement within tens of years after the impact event. The presence of the mafic phase in one of the granophyre dikes is explained by its emplacement following some differentiation of the Vredefort melt sheet.

¹C.A.Lorenz,¹A.I.Buikin,^2,3A.A.Shiryaev,¹O.V.Kuznetsova
Geochemistry [Chemie der Erde] (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2020.125686]
¹Vernadsky Institute of Geochemistry and Analytical chemistry RAS, Kosygin St. 19, 119999, Moscow, Russia
²A. N. Frumkin Institute of physical chemistry and electrochemistry RAS, Leninsky pr. 31 korp. 4, Moscow, 119071, Russia
³Institute of geology of ore deposits, Petrography, Mineralogy, and Geochemistry RAS, Staromonetnyi per, 35, 119017 Moscow, Russia
Copyright Elsevier

Aubrites are achondritic meteorites (enstatite pyroxenites) that were formed in highly reduced magmatic environments on a differentiated parent body sharing a common oxygen isotope reservoir with enstatite chondrites (EC), Earth and Moon, and could be considered as a geochemical model of the early proto-Earth. Some pyroxenes of the Pesyanoe aubrite have high abundance of gaseous inclusions, captured during the crystallization of the rocks. Investigation of the inclusions by IR spectroscopy reveals presence of OH− groups and C–H bonds. The former are assigned to protonated point defects in enstatite lattice and the latter to compounds occupying void walls. Molecular water and CO2 were not observed. Volatile components released from the samples of the Pesyanoe enstatite by stepwise crushing and heating are composed of CO2, H2O and a non-condensable phase. Hydrogen isotopic composition of volatiles extracted in form of molecular water in Px-separates varies in the range δD = −61 – −84‰ with mean value of δD = −73 ± 16‰ VSMOW and is within the ranges of ECs and Earth’s mantle. The total abundance of H2 in the pyroxene of Pesyanoe were estimated as at least 0.024 ppm that is too low in comparison with that of enstatite chondrites (≥30 ppm H2) and could indicate nearly complete degassing of the Pesyanoe primitive precursor material during the Pesyanoe parent body accretion or a mantle degassing in igneous differentiation process. In a last case a primitive precursor could have D/H ratio different from that of enstatite chondrites.

^1,2Alexander A. Nemchin et al. (>10)
Geochemistry [Chemie der Erde] (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2020.125683]
¹Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China
²School of Earth and Planetary Sciences, Curtin University, Perth, GPO Box U1987, WA, 6845, Australia
Copyright Elsevier

Since the Apollo 14 mission delivered samples of the Fra Mauro formation, interpreted as ejecta of the Imbrium impact, defining the age of this impact has emerged as one of the critical tasks required for the complete understanding of the asteroid bombardment history of the Moon and, by extension, the inner Solar System. Significant effort dedicated to this task has resulted in a substantial set of ages centered around 3.9 Ga and obtained for the samples from most Apollo landing sites using a variety of chronological methods. However, the available age data are scattered over a range of a few tens of millions of years, which hinders the ability to distinguish between the samples that are truly representative of the Imbrium impact and those formed/reset by other, broadly contemporaneous impact events. This study presents a new set of U-Pb ages obtained for the VHK (very high K) basalt clasts found in the Apollo 14 breccia sample 14305 and phosphates from (i) several fragments of impact-melt breccia extracted from Apollo 14 soil sample 14161, and (ii) two Apollo 15 breccias 15455 and 15445. The new data obtained for the Apollo 14 samples increase the number of independently dated samples from this landing site to ten. These Apollo 14 samples represent the Fra Mauro formation, which is traditionally viewed as Imbrium ejecta, and therefore should record the age of the Imbrium impact. Using the variance of ten ages, we propose an age of 3922 ± 12 Ma for this event. Samples that yield ages within these limits can be considered as possible products of the Imbrium impact, while those that fall significantly outside this range should be treated as representing different impact events. Comparison of this age for Imbrium (determined from Apollo 14 samples) with the ages of another eleven impact-melt breccia samples collected at four other landing sites and a related lunar meteorite suggests that they can be viewed as part of Imbrium ejecta. Comprehensive review of 40Ar/39Ar ages available for impact melt samples from different landing sites and obtained using the step-heating technique, suggests that the majority of the samples that gave robust plateau ages are indistinguishable within uncertainties and altogether yield a weighted average age of 3916 ± 7 Ma (95 % conf., MSWD = 1.1; P = 0.13) and a median average age of 3919 + 14/-12 Ma, both of which agree with the confidence interval obtained using the U-Pb system. These samples, dated by 40Ar/39Ar method, can be also viewed as representing the Imbrium impact. In total 36 out of 41 breccia samples from five landing sites can be interpreted to represent formation of the Imbrium basin, supporting the conclusion that Imbrium material was distributed widely across the near side of the Moon. Establishing temporal limits for the Imbrium impact allows discrimination of ten samples with Rb-Sr and 40Ar/39Ar ages about 50 Ma younger than 3922 ± 12 Ma. This group may represent a separate single impact on the Moon and needs to be investigated further to improve our understanding of lunar impact history.

^1,2Ian A. Crawford,³Katherine H. Joy,¹Jan H. Pasckert,¹Harald Hiesinger
Philosophical Transactions of the Royal Society A 379, 2188. Link to Article [https://doi.org/10.1098/rsta.2019.0562]
¹Department of Earth and Planetary Sciences, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK
²Centre for Planetary Sciences at UCL/Birkbeck, Gower Street, London WC1E 6BT, UK
³Department of Earth and Environmental Sciences, The University of Manchester, Oxford Road, M13 9PL Manchester, UK
⁴Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany

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¹Dai, D.,^1,2Bao, H.,^1,2Liu, S.,^1,3Yin, F.
Yanshi Xuebao/Acta Petrologica Sinica 36, 1850-1856 Link to Article [DOI: 10.18654/1000-0569/2020.06.13]
¹Institute of Geology, Hunan University of Science and Technology, Xiangtan, 411201, China
²Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China
³Hunan Provincial Key Laboratory of Shale Gas Resource Utilization, Xiangtan, 411201, China

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¹Ku, Y.,¹Jacobsen, S.B.
Science Advances 6, eabd0511 Link to Article [DOI: 10.1126/sciadv.abd0511]
¹Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, United States

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¹Korochantseva, E.V.,²Verchovsky, A.B.,¹Buikin, A.I.,¹Lorents, K.A.,¹Korochantsev, A.V.
Geochemistry International 58, 1239-1256 Link to Article [DOI: 10.1134/S0016702920110075]
¹Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, 119991, Russian Federation
²The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom

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