^1,2,3Stephen M. Elardo,⁴Matthieu Laneuville,⁵Francis M. McCubbin,⁶Charles K. Shearer
Nature Geoscience 13, 339–343 Link to Article [DOIhttps://doi.org/10.1038/s41561-020-0559-4]
¹Department of Geological Sciences, University of Florida, Gainesville, FL, USA
²Geophysical Laboratory, Carnegie Institution for Science, Washington, DC, USA
³Department of Physics, Astronomy, and Geosciences, Towson University, Towson, MD, USA
⁴Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, Japan
⁵NASA Johnson Space Center, Houston, TX, USA
⁶Institute of Meteoritics, University of New Mexico, Albuquerque, NM, USA

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^1,2Jessica J. Barnes et al. (>10)
Nature Geoscience 13, 260–264 Link to Article [DOIhttps://doi.org/10.1038/s41561-020-0552-y]
¹NASA Johnson Space Center, Houston, TX, USA
²Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

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^1,2Christopher R. J. Charles,^2,3Phil J. A. McCausland,²Donald W. Davis
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13492]
¹TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada
²Department of Earth Sciences, University of Toronto, 22 Russell St., Toronto, Ontario, M5S 3B1 Canada
³Institute for Earth and Space Exploration, Western University, London, Ontario, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

Lovina, classified as an ungrouped ataxite, is controversial and its identity as a meteorite has been questioned. In this work, we use Pb isotopes on targeted troilite nodules in Lovina as a test of its antiquity and provenance. Although precise ages cannot be obtained, LA‐ICP‐MS offers a rapid, straightforward procedure to establish the source of lead, whether ancient (meteoritic) or modern (terrestrial). For nine pristine, unweathered nodules in Lovina, we find a lead isotopic composition of: ²⁰⁶Pb/²⁰⁸Pb = 0.492 ± 0.003 (2σ, MSWD 0.79; 95%) and ²⁰⁷Pb/²⁰⁶Pb = 0.852 ± 0.003 (2σ, MSWD 1.09; 95%) with no detectable uranium. All lead compositions of the troilite fall in the range expected for modern environmental and mantle lead and are distinctly different from the primordial Canyon Diablo Troilite (CDT) composition of ancient meteoritic troilite. Although the origin of Lovina remains unknown, we conclude that lead in the Lovina troilite is unsupported by U decay and originated from a terrestrial source.

¹Johan Villeneuve,¹Yves Marrocchi,²Emmanuel Jacquet
Earth and Planetary Science Letters 116318 Link to Article [https://doi.org/10.1016/j.epsl.2020.116318]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandœuvre-lès-Nancy, 54501, France
²Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Muséum national d’Histoire naturelle, Sorbonne Université, CNRS, CP52, 57 rue Cuvier, 75005 Paris, France
Copyright Elsevier

We determined the silicon isotopic compositions of silicates (olivine and low-Ca pyroxene) in type I and type II chondrules of the carbonaceous chondrites Allende, Kaba, NWA (Northwest Africa) 5958, and MIL (Miller Range) 07342. Type I chondrule silicates show large, mass-dependent Si isotopic fractionations, with Si values ranging from −7‰ to +2.6‰, whereas the Si values of type II chondrule silicates are close to zero and vary by less than 2‰. When present, Mg-rich relict olivine grains in type II chondrules show larger Si variations than their FeO-rich counterparts. In type I chondrules, low-Ca pyroxenes yield systematically lighter Si values than Mg-rich olivines. Our results show that type I chondrules are complex objects whose Si isotopic compositions derived from their precursors and SiO-rich gas-melt interactions. This corroborates that type I chondrules are nebular products that formed under open-system conditions. Our data also suggest that at least some type II chondrules derived from their type I counterparts. Overall, this demonstrates that recycling was common during the evolution of the protoplanetary disk.

^1,2Hauke Vollstaedt,^1,2Klaus Mezger,^3,2IngoLeya
Earth and Planetary Science Letters 116289 Link to Article [https://doi.org/10.1016/j.epsl.2020.116289]
¹Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland
²Center for Space and Habitability, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
³Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
Copyright Elsevier

The Moon and Earth share similar relative abundances and isotope compositions of refractory lithophile elements, indicating that the Moon formed from a silicate reservoir that is chemically indistinguishable from the Earth’s primitive silicate mantle. In contrast, most volatile elements are depleted in lunar mare basalts compared to Earth’s mantle and differ in their isotope composition. However, the depletion of volatile elements is not a simple function of their condensation temperature, indicating multiple mechanisms that established the lunar volatile element budget. Specifically, the chalcophile elements S, Se and Te are not depleted in lunar basalts compared to their terrestrial counterparts. In this study, the abundances and stable isotope compositions of the volatile and chalcophile element Se measured in three lunar mare basalts and seven soils are used to refine the processes that caused volatile element depletion on the Moon. The Se isotope composition of two lunar mare basalts (Se = 1.08 and 0.8‰) is significantly heavier compared to chondrites (−0.20 ± 0.26‰; 2 s.d.) and terrestrial basalts (0.29 ± 0.24‰; 2 s.d.). The offset in the Se isotope composition is attributed to a volatility controlled loss of Se from the Moon. The lack of chalcophile element depletion in lunar mare basalts is then explained by sulphide segregation in the Earth’s mantle after the Moon forming impact followed by a late veneer of chondritic material to the Earth. Seven lunar soils were found to have chondritic S/Se ratios, but have Se values that are 6 to 13‰ heavier compared to mare basalts. This fractionation is likely the result of coupled and repeating processes of meteoritic material addition and concomitant partial evaporation. Results from numerical modelling indicate that isotope fractionation in lunar soils is due to partial evaporation of FeSe and FeS with evaporative loss of about 20% for both Se and S.

¹Jens Barosch,^2,3,4Denton S.Ebel,^1,5Dominik C.Hezel,²Samuel Alpert,⁶Herbert Palme
Earth and Planetary Science Letters 542, 115286 Link to Article [https://doi.org/10.1016/j.epsl.2020.116286]
¹University of Cologne, Department of Geology and Mineralogy, Zülpicher Str. 49b, 50674 Köln, Germany
²American Museum of Natural History, Department of Earth and Planetary Sciences, NY 10024, New York, USA
³Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA
⁴Graduate School and Graduate Center of the City University of New York, NY, USA
⁵Natural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD, London, UK
⁶Forschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, D-60325, Frankfurt am Main, Germany
Copyright Elsevier

The study of chondritic meteorites and their components allows us to understand processes and conditions in the protoplanetary disk. Chondrites with high and about equal proportions of chondrules and matrix are ideal candidates to not only study the formation conditions of chondrules, but also the relationship between these two major components. An important question is whether these formed in the same or in separate reservoirs in the protoplanetary disk. So far, such studies have been mainly restricted to carbonaceous chondrites. We here expand these studies to the K (Kakangari-like) chondrite grouplet. These have various distinctive properties, but the abundance of major components – chondrules and matrix – is similar to other primitive meteorites. We obtained a comprehensive petrographic and chemical dataset of Kakangari and Lewis Cliff 87232 chondrules and matrix. Chondrules in Kakangari show a large compositional scatter, supporting material addition to chondrules during their formation. Contrary to almost all other chondrite groups, the majority of Kakangari chondrules are not mineralogically zoned. However, Kakangari chondrules were likely initially zoned, but then lost this zonation during chondrule remelting and fragmentation. Average compositions of bulk chondrules, matrix and bulk Kakangari are identical and approximately solar for Mg/Si. This might indicate the formation of chondrules and matrix from a common reservoir and would agree with findings from carbonaceous and Rumuruti chondrites: chondrules and matrix in most chondrite groups were not transported through the protoplanetary disk and then mixed together. Rather, these major components are genetically related to each other and formed in the same reservoir.

^1,2E.Heggy,¹E.M.Palmer,²T.W.Thompson,³B.J.Thomson,⁴G.W.Patterson
Earth and Planetary Science Letters 541, 116274 Link to Article [https://doi.org/10.1016/j.epsl.2020.116274]
¹University of Southern California, Ming Hsieh Department of Electrical and Computer Engineering, 3737 Watt Way, Los Angeles, CA 90089, USA
²Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³Department of Earth and Planetary Sciences, The University of Tennessee Knoxville, 1621 Cumberland Avenue, Knoxville, TN 37996, USA
⁴The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
Copyright Elsevier

Identifying polarimetric radar signatures of ice in smooth regolith fines on the floors of permanently shadowed lunar craters is hindered by uncertainties in their dielectric properties. We address this deficiency through polarimetric radar analysis of surface backscatter to derive the dielectric constant () of smooth, rock-free regolith fines covering brecciated crater floors observed by Mini-RF, which offer ideal locations for unambiguous retrieval of surface from linear polarimetric scattering models and CPR analysis for volatile identification. Specifically, we select fines covering crater fills in north polar and equatorial regions to constrain the range of variability of as a function of latitude and crater diameter, where we hypothesize that the latter is indicative of the excavation depth of these fines. Our observations suggest that there is measurable variability in the dielectric properties of fines on lunar crater floors as a function of crater size and potentially with impact excavation depth, suggesting that small craters <5-km in diameter have ranging from 2.3-to-3, and large ones >5-km have higher values of that range from 3-to-3.8. We find that the most plausible explanation for the observed variability of of regolith fines on crater floors is mineralogical differences, suggesting an increase in metal abundance in the original excavated substrate with depth, i.e., in the uppermost kilometer of the lunar crust. Finally, we suggest that regolith fines on the floors of permanently shadowed craters <5 km in diameter are optimal targets for the unambiguous detection of water-ice enrichment using S-band radar observations.

¹Forrest Gilfoy,¹Jie Li
Earth and Planetary Science Letters 541, 116285 Link to Article [https://doi.org/10.1016/j.epsl.2020.116285]
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, 48109, USA
Copyright Elsevier

A series of multi-anvil experiments have been conducted to define the iron-rich liquidus of the iron-nickel-sulfur (FeNi-S) system at 20 GPa, the estimated pressure of the martian core-mantle boundary (CMB). The liquidus curve of FeNi-S containing about 9 wt.% Ni has a concave up shape, and is as much as 400 K lower than the liquidi previously applied to the martian core with sparse experimental constraints. Unlike existing liquidi of Fe-S and FeNi-S at 23 GPa, which predict a fully molten core for a narrow range of sulfur content between 14 and 15 wt.% S, our results are consistent with a molten state for all proposed core compositions, and establishes a new minimum CMB temperature of 1500 K for 10 wt.% S and 1250 K for 16 wt.% S. Extrapolating our FeNi-S liquidus to high pressures and comparing it to calculated areotherms, we find that three core crystallization regimes are possible. For a martian core with moderate sulfur content (10 to 13 wt.%) or lower, crystallization takes the form of iron snow near the CMB, while for cores with higher sulfur content (15-16 wt.%), solidification occurs near the center of the planet in the form of solid Fe3S. At an intermediate sulfur content of 14 wt.%, Fe3S would precipitate over a broad depth range and may appear fully molten to surface observations.

^1,2Lionel G.Vacher,¹Laurette Piani,¹Thomas Rigaudier,¹Dorian Thomassin,¹Guillaume Florin,¹Maxime Piralla,Yves Marrocchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.007]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy F-54501, France
²Department of Physics, Washington University, St. Louis, St. Louis, MO, USA
Copyright Elsevier

Hydrogen occurs at the near percent level in the most hydrated chondrites (CI and CM) attesting to the presence of water in the asteroid-forming regions. Their H abundances and isotopic signatures are powerful proxies for deciphering the distribution of H in the protoplanetary disk and the origin of Earth’s water. Here, we report H contents and isotopic compositions for a set of carbonaceous and ordinary chondrites, including previously analyzed and new samples analyzed after the powdered samples were degassed under vacuum at 120°C for 48 hours to remove adsorbed atmospheric water. By comparing our results to literature data, we reveal that the H budgets of both H-poor and H-rich carbonaceous chondrites are largely affected by atmospheric moisture, and that their precise quantification requires a specific pre-degassing procedure to correct for terrestrial contamination. Our results show that indigenous H contents of CI carbonaceous chondrites usually considered the most hydrated meteorites might be almost a factor of two lower than those previously reported, with uncontaminated D/H ratios differing significantly from that of Earth’s oceans. Without pre-degassing, the H concentrations of H-poor samples (e.g., CVs chondrites) are also affected by terrestrial contamination. After correction for contamination, it appears that the amount of water in chondrites is not controlled by the matrix modal abundance, suggesting that the different chondritic parent bodies accreted variable amounts of water-ice grains. Our results also imply that (i) thermal metamorphism play an important role in determining the H content of both CV and ordinary chondrites but without affecting drastically their H isotopic composition since no clear D enrichment is observed with the increase of petrographic type and (ii) the D enrichment of ordinary chondrite organics does not result from the loss of isotopically light H₂ induced by metal oxidation but is rather linked to the persistence of a thermally resistant D-rich component.

¹J.R.Lyons
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.001]
¹School of Earth & Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287, United States
Copyright Elsevier

Isotope fractionation due to photochemical self-shielding is believed to be responsible for the enrichment of inner solar system planetary materials in the rare isotopes of carbon, nitrogen, and oxygen relative to the Sun. Self-shielding may also contribute to sulfur isotope mass-independent fractionation in modern atmospheric sulfates, although its role in the early Earth atmosphere has not yet been convincingly established. Here, I present an analytical formulation of isotopic photodissociation rate coefficients that describe self-shielding isotope signatures for 3 and 4-isotope systems broadly representative of O and S isotopes. The analytic equations are derived for idealized molecular spectra, making an analytic formulation tractable. The idealized spectra characterize key features of actual isotopologue spectra, particularly for CO and SO2, but are applicable to many small molecules and their isotopologues. The analytic expressions are convenient for evaluating the magnitude of isotope effects without having to pursue involved numerical solutions. More importantly, the analytic expressions illustrate the origin of particular isotope signatures, such as the previously unexplained large mass-dependent fractionation associated with photodissociation of optically-thick SO2. The formulation presented here elucidates the origin of some of these important isotopic fractionation processes.