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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