¹T.J.Barrett,¹A.Černok,¹G.Degli-Alessandrini,¹X.Zhao,^1,2M.Anand,¹I.A.Franchi,³J.R.Darling
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.03.018]
¹The Open University, School of Physical Sciences, Walton Hall, Milton Keynes, MK7 6AA, UK
²Department of Earth Sciences, Natural History Museum, London, SW7 5BD, UK
¹University of Portsmouth, School of the Environment, Geography and Geosciences, Burnaby Road Portsmouth, PO1 3QL, UK
Copyright Elsevier

The abundance and isotopic composition of volatile elements in meteorites is critical for understanding planetary evolution, given their importance in a variety of geochemical processes. There has been significant interest in the mineral apatite, which occurs as a minor phase in most meteorites and is known to contain appreciable amounts of volatiles (up to wt. % F, Cl, and OH). Impact-driven shock metamorphism, pervasive within many meteorites, can potentially modify the original signatures of volatiles through processes such as devolatilization and diffusion.

In this study, we investigate the microstructures of apatite grains from six eucrites across a broad range of shock stages (S1–S5) using electron backscatter diffraction (EBSD) to explore shock-induced crystallographic features in apatite. New Cl and H abundance and isotopic composition data were collected on moderate to highly shocked samples (S3-S5) by Nano Secondary Ion Mass Spectrometry (NanoSIMS). Previously reported volatile data for S1 and S2 eucrites were integrated with EBSD findings in this study.

Our findings indicate that apatite microstructures become increasingly more complex at higher shock stages. At low shock stages (S1–S2) samples display brecciation and fracturing of apatite. Samples in S3 and S4 display increasing crystal plastic deformation indicated by increasing spread in pole figures. At the higher shock stages (S4/S5) there is potential recrystallisation demonstrated by an increased density of subgrain boundaries.

The Cl content and δ37Cl values of highly-shocked apatite grains range from ∼ 940–1410 ppm and – 3.38 to + 7.70 ‰, respectively, within the range observed in less-shocked eucrites. In contrast, H2O abundances are more variable (from 186 to ∼ 4010 ppm), however, the measured water content still falls within the range previously reported for low-shock eucrites. The measured δD values range from – 157 to + 163 ‰, also within the range of values from known low-shock basaltic eucrites. Weighted averages for both isotopic systems (δD − 122 ± 20 ‰, δ37Cl + 1.76 ± 0.66 ‰) are consistent with the range displayed in other inner Solar System bodies.

NanoSIMS isotope images of apatite grains display heterogeneity in their Cl abundance at the nanoscale which increases in complexity with shock stage. This increasing complexity, however, does not correlate with deformation microstructures observed in EBSD or with the Cl isotopic composition at either an inter-grain or intra-grain scale. These findings are similar to analyses of variably shocked lunar apatite and, therefore, apatite appears to be a robust recorder of Cl and H (at least at spatial resolution and precision currently achievable by NanoSIMS) on airless bodies, despite intensive shock.

^1,2,5Derek Sears,^1,2,5Daniel Ostrowski,³Heather Smith,^1,6Adonay Sissay,⁴Mihir Trivedi
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114442]
¹Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA
²BAER Institute/NASA Ames Research Center, Moffett Field, CA 94035, USA
³USRA/NASA Ames Research Center, Moffett Field, CA 94035, USA
⁴NASA Ames Research Center, Moffett Field, CA 94035, USA
⁵Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA
Copyright Elsevier

In order to find an additional quantitative way to estimate the petrographic type of unequilibrated ordinary chondrites (UOC), and one that can be used remotely in the study of asteroids, we have analyzed the near-infrared spectra of a suite of UOC observed falls. We obtained spectra from the RELAB database at Brown University and applied several methods for determining the amount of clinopyroxene (CPX) as a percentage of the total pyroxene in the meteorites. The presence of low-Ca CPX has long been known to be characteristic of little-metamorphosed ordinary chondrites. The methods we used were (1) naked-eye determination of the wavelength of the absorption features at ~1 μm and ~2 μm, (2) determination of the wavelengths of these features by fitting polynomial equations, and (3) determining the relative intensities of the CPX and OPX features after isolation by a curve fitting procedure. The measurements were then “calibrated” using data from the literature to obtain values for the amount of CPX in the total pyroxene. We find that there is an empirical relationship between the amount of CPX detected by these methods of spectrum analysis and the petrologic type.

Petrologic type = +4.402–0.019 × CPX%

We explain this empirical relationship (1) as evidence that in pyroxene bearing rocks the spectrum of pyroxene dominates (this has been known in the 1970s), (2) that low-Ca CPX is so abundant in these meteorites (up to 40 vol%) that it is easily detected by reflectance spectroscopy, and (3) compositional effects caused by Ca and Fe in the pyroxenes partially cancel out or are small. We thus have a new method of quantitatively measuring the level of metamorphic alteration experienced by these important meteorites and of assigning them a petrologic type of 3.0 to 3.9. More importantly, unlike existing methods, this can be applied remotely so that chondritic asteroid surfaces (i.e. those of Q and S asteroids) can also be characterized in terms of their metamorphic history. As an example, (433) Eros and (25143) Itokawa were found to be types ~3.5 and ~3.4, respectively. We briefly discuss the implications of this for understanding the history of meteorites and asteroids.

¹Maxime Piralla,²Romain Tartèse,¹Yves Marrocchi,²Katherine H. Joy
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13639]
¹CRPG, CNRS, UMR 7358, Université de Lorraine, Vandœuvre‐lès‐Nancy, F‐54500 France
²Department of Earth and Environmental Sciences, School of Natural Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL UK
Published by arrangement with John Wiley & Sons

Apatite has been widely used for assessing the volatile inventory and hydrothermal fluid compositions of asteroidal and planetary bodies. We report the OH, F, and Cl abundances, as well as the hydrogen isotope composition, of apatite in the CM1‐2 chondrite Boriskino and in the C1‐ungrouped Bench Crater meteorite. Apatite in both meteorites is halogen‐poor, close to the hydroxylapatite endmember composition, and characterized by average δDSMOW values of −226 ± 59% and 233 ± 92%, respectively. Compared to apatite, the matrix in Bench Crater is depleted in D with a δDSMOW value of −16 ± 119‰. Comparing apatite and water H isotope compositions yields similar apatite‐water D/H fractionation ΔDApatite‐Water of approximately 120–150‰ for both chondrites, suggesting that apatite formed at similar temperatures. Combining a lattice strain partitioning model with apatite F and Cl abundances in Boriskino and Bench Crater yields low F and Cl abundances <300 μg g−1 in apatite‐forming fluids, and fluid F/Cl ratios that are roughly consistent with the bulk F/Cl ratios of other CI and CM chondrites. This suggests that hydrothermal alteration on these meteorite parent bodies took place under closed‐system conditions. Based on the OH abundance estimates for the apatite‐forming fluids, we estimated the pH values of alteration fluids to be of approximately 10–13. Such alkaline fluid compositions are consistent with previous modeling and suggest that apatite formed late, toward the end of completion of hydrothermal alteration processes on the Boriskino and Bench Crater parent bodies.

¹Marc M. Hirschmann,²Edwin A. Bergin,³Geoff A. Blake,^4,5Fred J. Ciesla,⁶Jie Li
Proceedings of the National Academy of Sciences of the United States of America [PNAS] (in Press) Link to Article [https://doi.org/10.1073/pnas.2026779118]
¹Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN 55455;
²Department of Astronomy, University of Michigan, Ann Arbor, MI 48109;
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125;
⁴Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637;
⁵Chicago Center for Cosmochemistry, University of Chicago, Chicago, IL 60637;
⁶Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109

During the formation of terrestrial planets, volatile loss may occur through nebular processing, planetesimal differentiation, and planetary accretion. We investigate iron meteorites as an archive of volatile loss during planetesimal processing. The carbon contents of the parent bodies of magmatic iron meteorites are reconstructed by thermodynamic modeling. Calculated solid/molten alloy partitioning of C increases greatly with liquid S concentration, and inferred parent body C concentrations range from 0.0004 to 0.11 wt%. Parent bodies fall into two compositional clusters characterized by cores with medium and low C/S. Both of these require significant planetesimal degassing, as metamorphic devolatilization on chondrite-like precursors is insufficient to account for their C depletions. Planetesimal core formation models, ranging from closed-system extraction to degassing of a wholly molten body, show that significant open-system silicate melting and volatile loss are required to match medium and low C/S parent body core compositions. Greater depletion in C relative to S is the hallmark of silicate degassing, indicating that parent body core compositions record processes that affect composite silicate/iron planetesimals. Degassing of bare cores stripped of their silicate mantles would deplete S with negligible C loss and could not account for inferred parent body core compositions. Devolatilization during small-body differentiation is thus a key process in shaping the volatile inventory of terrestrial planets derived from planetesimals and planetary embryos.

¹Netra S.Pillai et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114436]
¹Space Astronomy Group, U R Rao Satellite Centre, ISRO, Bengaluru, India
Copyright Elsevier

The Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS) onboard the Chandraayaan-2 spacecraft around the Moon, has been remotely measuring the lunar X-ray fluorescence spectra since September, 2019. The primary objective of the experiment is to provide global maps of O, Mg, Al, Si at a resolution of 12.5 km/pix and of Ca, Ti and Fe at localized regions during enhanced solar activity, using the lunar X-ray fluorescence measurements in the 0.5 to 10 KeV range. CLASS is an array of swept charge devices (SCDs), a variant of X ray Charge Coupled Devices (CCDs) that provide good spectral resolution and large area. The quality of X-ray measurements strongly depends on accuracy of its calibration techniques. In this work, the results from the pre-launch calibration of the instrument that combines experimental measurements and simulations are described. The spectral redistribution function of the swept charge device is simulated using an augmented version of a previously developed charge transport model (Athiray et al., 2015). Response matrices built from these models are verified with in-flight data. We study the background in SCDs arising from particles in the lunar orbit over many months and identify the sources. We demonstrate the in-flight performance of the instrument that enables generation of direct elemental maps. Elemental abundances for a region in the farside highland and in the nearside western mare are derived demonstrating the method and the instrument capability of deriving the elemental abundances at different spatial scales and at different solar activity levels.

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.