The selenium isotope composition of lunar rocks: Implications for the formation of the Moon and its volatile loss

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.

Formation of chondrules and matrix in Kakangari chondrites

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.

Bulk composition of regolith fines on lunar crater floors: Initial investigation by LRO/Mini-RF

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.

 

Thermal state and solidification regime of the martian core: Insights from the melting behavior of FeNi-S at 20 GPa

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.

Hydrogen in chondrites: Influence of parent body alteration and atmospheric contamination on primordial components

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.

An analytical formulation of isotope fractionation due to self-shielding

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.

Qarabawi’s Camel Charm: Tracing the meteoritic origins of a cultural artifact

1Rhiannon G. Mayne,2Catherine M. Corrigan,2Timothy J. McCoy,3James M. D. Day,2Timothy R. Rose
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13478]
1Oscar E. Monnig Meteorite Collection, Texas Christian University, Fort Worth, Texas, 76109 USA
2Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, 20013‐7012 USA
1Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, 92093‐0244 USA
Published by arrangement with John Wiley & Sons

Qarabawi’s Camel Charm was acquired from Abdullah Qarabawi of the Ababda tribe of eastern Egypt. The charm consists of a chain with four links and an ~6.5 cm diameter flattened disk with the Arabic inscription “Allahu Akbar,” which translates as “God is Greatest.” Belief in the evil eye is prevalent among the Ababda, even to the modern day, and as men identify camels and the cultural objects and activities related to them as one of their most important possessions, charms and amulets are often used to ward off its influence. Nondestructive analyses of the disk and metallographic examination of the distal link reveal a deformed medium octahedral pattern, confirming the meteoritic origin of the Camel Charm. Major, minor, and trace element compositions are consistent with classification as a IIIAB iron. Combined heating to modest temperatures (~600 °C) and cold working were used in the manufacture of the Camel Charm. Although compositionally similar to the Wabar IIIAB irons, chemical differences, the significant distance between Wabar and eastern Egypt, and the lack of established trade routes suggest that the Camel Charm source material was a meteorite unknown as an unworked specimen. This meteorite has been given the name Wadi El Gamal, the name of a National Park in the Ababda homelands.