¹L.Allibert,¹S.Charnoz,^1,4J.Siebert,²S.A.Jacobson,³S.N.Raymond
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114412]
¹Institut de Physique du Globe de Paris, Université de Paris, 1 Rue Jussieu, Paris, France
²Michigan State University, Earth and Environmental Sciences, 288 Farm Ln, East Lansing, MI 48824, USA
³Laboratoire d’Astrophysique de Bordeaux, Allée Geoffroy St Hilaire, Bordeaux, France
⁴Institut Universitaire de France, France
Copyright Elsevier

Dynamical scenarios of terrestrial planets formation involve strong perturbations of the inner part of the solar system by the giant-planets, leading to enhanced impact velocities and subsequent collisional erosion. We quantitatively estimate the effect of collisional erosion on the resulting composition of Earth, and estimate how it may provide information on the dynamical context of its formation. The composition of the Bulk Silicate Earth (BSE, Earth’s primitive mantle) for refractory and lithophile elements (RLE) should be strictly chondritic as these elements are not affected by volatile loss nor by core formation. However, an excess in 142Nd compared to the 144Nd has been emphasized in terrestrial samples compared to most measurements in chondrites. In that case, the Samarium/Neodymium (Sm/Nd) ratio could be roughly 6% higher in the BSE than in chondrites, as suggested from the 146Sm/142Nd isotope system (Boyet and Carlson, 2005). This proposed chemical offset could be the consequence of preferential collisional erosion of the crust during the late stages of Earth’s accretion, leaving a BSE enriched in Sm due to its lower incompatibility compared to Nd (O’Neill and Palme, 2008; Boujibar et al., 2015; Bonsor et al., 2015; Carter et al., 2015, 2018). However, if the present 142Nd of the BSE arises from nucleosynthetic heterogeneities within the protoplanetary disk (Burkhardt et al., 2016; Bouvier and Boyet, 2016; Boyet et al., 2018), then the BSE has no excess in Sm compared to Nd and this hypothesis precludes any significant loss of relatively Nd-enriched component early in the Solar System. Here, we simulate and quantify the erosion of Earth’s crust in the context of Solar System formation scenarios, including the classical model and Grand Tack scenario that invokes orbital migration of Jupiter during the gaseous disk phase (Walsh et al., 2011; Raymond et al., 2018). We find that collisional erosion of the early crust is unlikely to explain the proposed superchondritic Sm/Nd ratio of the Earth for most simulations. Only Grand Tack simulations in which the last giant impact on Earth occurred later than 50 million years after the start of Solar System formation can account for this Sm/Nd ratio. This time frame is consistent with current cosmochemical and dynamical estimates of the Moon forming impact (Chyba, 1991; Walker, 2009; Touboul et al., 2007, 2009, 2015; Pepin and Porcelli, 2006; Norman et al., 2003; Nyquist et al., 2006; Boyet et al., 2015). However, such a late fractionation in the Sm/Nd ratio is unlikely to be responsible for a 20-ppm 142Nd excess in terrestrial rocks due to the half life of the radiogenic system. Additionally, such a large and late fractionation in the Sm/Nd ratio would accordingly induce non-observed anomalies in the 143Nd/144Nd ratio. Considering our results, the Grand Tack model with a late Moon-forming impact cannot be easily reconciled with the Nd isotopic Earth contents.

¹Jamie E. Elsila,¹Natasha M. Johnson,¹Daniel P. Glavin,^1,2José C. Aponte,¹Jason P. Dworkin
Meteoritics & Planetary Science (in PRess) Link to Article [https://doi.org/10.1111/maps.13638]
¹NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771 USA
²Department of Physics, Catholic University of America, Washington, District of Columbia, 20064 USA
Published by arrangement with John Wiley & Sons

The organic compositions of carbonaceous chondrite meteorites have been extensively studied; however, there have been fewer reports of other meteorite classes, and almost none from iron meteorites, which contain much less carbon than carbonaceous chondrites but make up ~4% of observed meteorite falls. Here, we report the bulk amino acid content of three iron meteorites (Campo del Cielo, IAB; Canyon Diablo, IAB; and Cape York, IIIAB) and both the metal and silicate portions of a pallasite (Imilac). We developed a novel method to prepare the samples for analysis, followed by hot water extraction and analysis via liquid chromatography‐mass spectrometry. Free amino acid abundances ranging from 301 to 1216 pmol g−1 were observed in the meteorites, with the highest abundance in the silicate portion of the pallasite. Although some of the amino acid content could be attributed to terrestrial contamination, evidence suggests that some compounds are indigenous. A suite of C5 amino acids was observed with a distinct distribution favoring a straight chain (n‐pentanoic acid) structure; this straight chain dominance is suggestive of that observed in thermally altered stony meteorites. Amino acids were also observed in terrestrial iron granules that were milled and analyzed in the same way as the meteorites, although the distribution of detected amino acids was different. It is possible that similar formation mechanisms existed in both the meteorites and the terrestrial iron, or that observed amino acids resulted from reactions of precursors during sample preparation. This work suggests that iron meteorites should not be overlooked for contributions of amino acids and likely other soluble organic molecules to the early Earth. Future studies of iron–nickel meteorites and asteroids, such as Psyche, may provide further insights into their potential organic inventory.

¹James M. D. Day,¹Marine Paquet
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13646]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, 92093‐0244 USA
Published by arrangement with John Wiley & Sons

Highly siderophile element (HSE: Au, Re, Pd, Pt, Rh, Ru, Ir, Os) abundances in planetary silicate mantles provide constraints on accretion and differentiation, as well as late accretion additions after core formation. The first in situ analyses of the HSE in mare basalt sulfide and metal phases enable the determination of the distribution of these elements during fractional crystallization processes. Metals have low Ni/Co (<8) and strong HSE inter‐element fractionation (Pd/Ir = 18–100) and, with troilite (Ni/Co < 4.2, Pd/Ir = 14–74), host 75–100% of the HSE in mare basalts. The compositions of these metal and sulfide grains are inconsistent with assimilation of impact‐contaminated HSE‐rich regolith materials. Furthermore, regolith contamination cannot explain the threshold of abundances of iridium (~50 ppt) in mare basalt bulk rock compositions. Instead, bulk rock mare basalts have HSE patterns consistent with inheritance from partial melting of mantle sources with no residual metal or sulfide. Mare basalt HSE and siderophile element abundances can be accounted for by their mantle sources recording prior separation of a small lunar core, fractionating Ni/Co and Hf/W and removing ~99% of the HSE after lunar formation. Following this event, late accretion of ~0.02% of lunar mass led to limited enrichment of the HSE in chondritic‐relative abundances. Signatures of core formation preclude elevated HSE abundances in the lunar silicate interior and indicate that disproportional late accretion in the Earth–Moon system may, in part, be due to later core formation in the Moon relative to Earth.

^1,2Bidong Zhang,³Yangting Lin,¹Desmond E.Moser,³Jialong Hao,⁴Yu Liu,³Jianchao Zhang,¹Ivan R.Barker,⁴Qiuli Li,¹Sean R.Shieh,^5,1Audrey Bouvier
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.03.012]
¹Department of Earth Sciences, the University of Western Ontario, London, Ontario N6A 5B7, Canada
²Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, California 90095–1567, USA
³Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
⁴State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
⁵Universität Bayreuth, Bayerisches Geoinstitut, Bayreuth 95447, Germany
Copyright Elsevier

In situ U-Pb radiometric dating of zircons is regarded as one of the most widely used and reliable methods to acquire geochronologic ages. However, it has been recently reported that radiogenic Pb (Pb*) mobilization within zircon may, in some cases, cause inaccurate age determinations with no geological significance. Such Pb* mobilization can be caused by deformation, α-coil damage, fluid-assisted annealing, and recrystallization. In this study, we report an investigation of Pb* mobilization in shock metamorphosed lunar zircons. NanoSIMS (nanoscale secondary ion mass spectrometry) and IMS 1280HR ion microprobe dating, EBSD (electron backscatter diffraction) and CL (cathodoluminescence) mapping, and scanning ion imaging (SII) were applied to micro-zircon grains from the Apollo 72255 Civet Cat norite clast. Based on the large number of grains with similarities in internal zoning, habit and trace element geochemistry, and host mineral context, the Civet Cat norite zircons are interpreted to be primary, igneous grains. The chronology obtained for three consecutive surfaces (at different depths) by NanoSIMS, SII, and IMS 1280HR, respectively, indicates that the radiogenic Pb distribution of the Civet Cat norite zircons is heterogeneous among different polished or sputtering surfaces. Forty-two NanoSIMS U-Pb ages (beam size of 5 μm) are concordant in a Wetherill Concordia diagram, and their corresponding 207Pb/206Pb ages spread from 4015 Ma to 4459 Ma. More notably, the six oldest spots of the 42 define a concordant U-Pb age of 4460 ± 31 Ma (2σ, MSWD = 0.47, P = 0.92) and a weighted mean 207Pb/206Pb age of 4453 ± 34 Ma (MSWD = 0.056, P = 0.998). These dates are among the oldest in the lunar highland rocks. However, the 207Pb/206Pb ages of repolished surfaces of these zircons by IMS 1280HR (beam size of 5 μm) do not reproduce the NanoSIMS results (up to 300 Ma younger). The SII (spatial resolution of 2 μm) confirms a heterogeneous distribution of radiogenic Pb within single grains. The EBSD mapping of these zircon grains shows that they have 3−20° of cumulative lattice misorientation. It is proposed that shock-related deformation has facilitated Pb* migration after primordial crystallization. With currently available data, we cannot preclude the possibility that the large errors of the U-Pb ages obscure reverse discordance that would bias our oldest 207Pb/206Pb ages to older values. Conversely, our data could be explained by mixing of Pb-retention and Pb-loss nanodomains as seen in shocked terrestrial zircon such that U-Pb date of 4460 ± 31 Ma approximates the norite formation.

¹Timo Hopp,¹Thorsten Kleine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.03.016]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

Primitive achondrites derive from the residual mantle of incompletely differentiated planetesimals, from which partial silicate and metallic melts were extracted. As such, primitive achondrites are uniquely useful to examine the early stages of planetesimal melting and differentiation. To better understand the nature of this early melting and melt segregation as well as the nature of the melts involved, we obtained mass-dependent Ru isotopic compositions of 17 primitive achondrites, including winonaites, acapulcoite-lodranites, ureilites, brachinites, and two ungrouped samples. Most primitive achondrites with subchondritic Ru concentrations are characterized by heavy Ru isotopic compositions relative to chondrites, likely reflecting the extraction of isotopically light partial metallic melts. While the segregation of early-formed S-rich partial Fe-Ni-S melts likely had no effect on the Ru isotope compositions, extraction of S-free partial metallic melts at higher temperatures after removal of the early formed S-rich partial melts provides a viable mechanism for producing the observed Ru isotopic fractionation and fractionated highly siderophile element ratios among primitive achondrites. Together, these observations indicate that differentiation of primitive achondrite parent bodies involved the segregation of distinct partial metallic melts over a range of temperatures, and that these melts ultimately formed a partial core with fractionated and light Ru isotopic composition. This contrasts with the unfractionated Ru isotope signatures previously estimated for bulk iron meteorite cores, which therefore indicates quantitative metal segregation during core formation in the iron meteorite parent bodies. The less efficient metal segregation in primitive achondrite parent bodies most likely reflects lower initial amounts of heat-producing 26Al due to later accretion or impact disruption of the parent bodies during differentiation.

¹M. Arif,²Saumitra Misra
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13643]
¹Indian Institute of Geomagnetism, Navi Mumbai, 410218 India
²Discipline of Geological Sciences, SAEES, University of KwaZulu‐Natal, Durban, 4000 South Africa
Published by arrangement with John Wiley & Sons

The continuous ejecta deposit around the rim of Lonar impact crater, central India, contains angular basaltic boulders of size ≤5 m. These boulders experienced varying level of shock between 2–30 GPa due to impact, as indicated by the extreme fracturing of these basaltic boulders, fragmentation of plagioclase and titanomagnetite constituents of these ejected boulders, and the presence of maskelynite in them. We measure some rock magnetic properties, e.g., NRM/χ (natural remanent magnetization [NRM]/bulk magnetic susceptibility [χ]), REM (=NRM/saturation isothermal remanent magnetization [SIRM] ratio expressed in %), and anisotropy of magnetic susceptibility (AMS) on 53 subsamples from 18 oriented drill cores of the shocked ejected basaltic boulders from the eastern half of ejecta deposit in the present study. The measured data are similar in many respects to our previous observations on Lonar crater rim shocked basalts (Arif et al. 2012b). For example, a small population of the ejected basaltic boulder samples show very high NRM/χ (between 378 and 989 Am−1; n = 7) and REM (between 1.5 and 7%; n = 4) and the AMS axes of these ejected basaltic boulders show triaxial distributions in stereographic projections. Moreover, some of the ejected basaltic boulders show higher values of squareness ratio (Mrs/Ms) and median destructive field (MDF) suggesting permanent changes in the intrinsic magnetic properties due to impact shock pressure. In stereographic plot, the high coercivity and high temperature (HC_HT) magnetization component of these ejected basaltic boulders are distributed in discrete clusters on the periphery of a small enveloping circle whose center (D = 108.0°, I = +69.2°) lies close to the HC_HT cluster of the crater rim shocked basalts. The center of this enveloping circle and the average HC_HT component of Lonar crater rim shocked basalts have the same statistical orientation, although the former has steeper dip. This distribution suggests the possibility that the ejected basaltic boulders, which were deposited during the modification stage of Lonar crater evolution, were magnetized in an impact‐induced magnetic field that was rapidly decaying just after the impact. Our present study suggests that the ejected basaltic boulders and Lonar crater rim shocked basalts experienced high shock pressure (≥2 GPa) magnetization during impact.

^1,2Ekaterina V. Korochantseva,¹Alexei I. Buikin,¹Jens Hopp,³Alexander B. Verchovsky,¹Alexander V. Korochantsev,^3,4Mahesh Anand,¹Mario Trieloff
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13632]
¹Institut für Geowissenschaften, Klaus‐Tschira‐Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 234‐236, 69120 Heidelberg, Germany
²Vernadsky Institute of Geochemistry, Kosygin St. 19, 119991 Moscow, Russia
³School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA UK
⁴Department of Earth Sciences, The Natural History Museum, London, SW7 5BD UK
Published by arrangement with John Wiley & Sons

The lunar meteorite Dhofar 1436 is dominated by solar wind type noble gases. Solar argon is equilibrated with “parentless” 40Ar commonly known as lunar orphan argon. Ar‐Ar isochron analyses determined the lunar trapped 40Ar/36Ar ratio to 2.51 ± 0.04, yielding a corrected plateau age of 4.1 ± 0.1 Ga, consistent with the lunar Late Heavy Bombardment period. Lunar trapped and radiogenic argon components are all released at high temperatures (1200–1400 °C). Surprisingly, solar noble gases and lunar trapped argon can largely be released by crushing. Initial crushing steps mainly release elementally fractionated solar wind gases, while in advanced crushing steps, cosmogenic components dominate. Cosmogenic noble gases indicate irradiation at the lunar surface; they are less fractionated than solar wind species. We favor a scenario in which both solar and a large fraction of cosmogenic gases were acquired before the 4.1 Ga event, which caused shock metamorphism and formation of the regolith breccia. Sintering and agglutination along grain boundaries resulted in mobilization of solar wind, reimplanted, radiogenic, and cosmogenic noble gases, and resulted in their partial homogenization, fractionation, and retrapping in voids and/or defects accessible by crushing. An alternative scenario would be complete reset of the K‐Ar system 4.1 Ga ago accompanied by loss of all previously accumulated solar and cosmogenic noble gases. Later, the precursor of Dhofar 1436 became lunar regolith and accumulated solar and cosmogenic noble gases and reimplanted 40Ar before its final formation of the polymict impact breccia. The C abundance of the step‐combusted Dhofar 1436 is 555.3 ppm, with δ13C of −28‰ to +11‰. Nitrogen contents released by crushing and combustion are 3.2 ppm and 20.8 ppm, respectively. The lightest nitrogen composition (δ15N = −79‰) is likely due to release from voids of shock metamorphic phases and is rather a result of the mobilization of nitrogen components that accumulated prior to the 4.1 Ga event.

¹J. Brossier,¹M.S. Gilmore,¹K. Toner,¹A.J. Stein
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006722]
¹Wesleyan University, Department of Earth and Environmental Sciences, Planetary Sciences Group, 265 Church Street, Middletown, CT, 06459 USA
Published by arrangement with John Wiley & Sons

NASA’s Magellan mission revealed that many Venus highlands exhibit low radar emissivity values at higher altitudes. This phenomenon is ascribed to the presence of minerals having high dielectric constants, produced or stabilized by temperature‐dependent chemical weathering between the rocks and the atmosphere. Some large volcanoes on Venus have multiple reductions of radar emissivity at varying altitudes. We present morphological maps of major lava flow units at Maat, Ozza and Sapas montes and compare them to radar emissivity. Sapas has a single reduction in emissivity values at 6054.6 km, while Maat and Ozza have several reductions at altitudes of 6052.5–6056.7 km. Emissivity values are highly spatially correlated to individual lava flows indicating that minerals in the rocks control the emissivity signature. The emissivity patterns at these volcanoes require at least 4 individual ferroelectric mineral compositions in the rocks that are highly conductive at Curie temperatures of 693–731 K. These temperatures are compatible with chlorapatite and some perovskite oxides. Modeling the minimum volumes of ferroelectrics (10s–100s ppm) shows the volume and type of ferroelectric may vary over the lifetime of a single volcano. The modeled volumes of ferroelectrics in Ozza and Sapas are greater than in Maat, consistent with the production of ferroelectrics via weathering over a longer period of time, and supporting the idea that Maat has younger volcanic activity. The stratigraphic relationship of Maat’s youngest flows with impact craters may indicate the timeframe of the production of specific ferroelectrics via chemical weathering is over 9–60 Ma.

¹Romain Tartèsea,²Paolo A. Sossi,³Frédéric Moynier
Proceedings of the National Academy of Sciences of teh United States of America (PNAS) (in Press) Link to Article [DOI: https://doi.org/10.1073/pnas.2023023118]
¹Department of Earth and Environmental Sciences, The University of Manchester, M13 9PL Manchester, United Kingdom;
²Institute of Geochemistry and Petrology, ETH Zürich, CH-8092 Zürich, Switzerland;
³Université de Paris, Institut de Physique du Globe de Paris, CNRS UMR 7154, 75005 Paris, France

Rocks from the lunar interior are depleted in moderately volatile elements (MVEs) compared to terrestrial rocks. Most MVEs are also enriched in their heavier isotopes compared to those in terrestrial rocks. Such elemental depletion and heavy isotope enrichments have been attributed to liquid–vapor exchange and vapor loss from the protolunar disk, incomplete accretion of MVEs during condensation of the Moon, and degassing of MVEs during lunar magma ocean crystallization. New Monte Carlo simulation results suggest that the lunar MVE depletion is consistent with evaporative loss at 1,670 ± 129 K and an oxygen fugacity +2.3 ± 2.1 log units above the fayalite-magnetite-quartz buffer. Here, we propose that these chemical and isotopic features could have resulted from the formation of the putative Procellarum basin early in the Moon’s history, during which nearside magma ocean melts would have been exposed at the surface, allowing equilibration with any primitive atmosphere together with MVE loss and isotopic fractionation.

^1,2Mingming Zhang,³Brett Clark,^1,4Ashley J. King,¹Sara S. Russell,²Yangting Lin
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13635]
¹Department of Earth Sciences, The Natural History Museum, Cromwell Road, SW7 5BD London, UK
²Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
³Core Research Laboratories, The Natural History Museum, Cromwell Road, SW7 5BD London, UK
⁴School of Physical Sciences, The Open University, Walton Hall, MK7 6AA Milton Keynes, UK
Published by arrangement with John Wiley & Sons

Refractory calcium‐aluminum‐rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in chondritic meteorites are the earliest solids of our solar system, bearing the information of nebular condensation as well as accretion and asteroidal shock and metasomatism processes. While the compositions of refractory inclusions have been intensely studied for ~50 years, their physical properties such as shape and porosity are poorly constrained. Here, we present a microcomputed tomography (µCT) study on 16 refractory inclusions of condensate origin in five CV3 chondrites. We find that they are prolate or triaxial in shape with very rough morphologies. The CAIs have nodular textures and are thought to form by agglomerating individual nodules via collision‐induced bouncing and/or fragmentation, where the nodules were grown by gas–solid reactions during condensation. On the parent body, refractory inclusions from the CVR meteorite Leoville experienced intense shocks that led to the flattening of their shapes and lowering of their porosities. High‐temperature metasomatism in CVOxA meteorites and low‐temperature metasomatism in CVOxB meteorites do not seem to have large effects on the porosities of their refractory inclusions, which have similar ranges and pore‐size distributions. Instead, we infer that their pores are mostly inherited from the gas–solid condensation and subsequent agglomeration processes. The porosities of CAIs are higher than those of AOAs, which is mainly due to the high‐temperature sintering process of AOAs.