¹K. Righter,²A. Boujibar,³M. Humayun,³S. Yang,^4,5R. Rowland II,⁵K. Pando
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.06.014]
¹NASA JSC, Mailcode XI2, 2101 NASA Pkwy, Houston, TX 77058 USA
²Western Washington University, Geology Department ES – 240, Department of Physics and Astronomy, MS 9080, 516, High Street, Bellingham, WA, 98225 USA
³National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, FL 32310, USA
⁴Los Alamos National Laboratory, Mail Stop P952, Los Alamos, NM 87545, USA
⁵Jacobs JETS Contract, NASA JSC, Houston, TX 77058 USA
Copyright Elsevier

Understanding siderophile element partitioning between metal and silicate melts under diverse conditions can be used to place important constraints on the materials and conditions of planetary accretion and core formation, as well as post core formation processes. However, the effects of Si on the partitioning and activity coefficients for these elements are not well known, despite Si likely being one of the dominant light elements in Earth’s core. To address this gap in understanding, we have undertaken a systematic study of the highly siderophile elements Re, Pt, Os, and Ru, and the refractory lithophile elements Nb, Ta and Ti at 1600 °C and 1 GPa, to derive epsilon interaction parameters for these elements in FeSi metallic liquids. Positive epsilon interaction parameters were measured for Nb, Ta, Ti, Ru, Re, Pt, and Os, indicating that dissolved Si in Fe liquids causes a decrease in their metal/silicate partition coefficients (or ‘silicophobic’ behavior). Furthermore,
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which means Si causes a larger decrease in D(metal/silicate) and the chalcophile behavior expected from some elements will be completely masked by the presence of Si in a metallic liquid. The new parameters are used to update an activity model that now includes 36 siderophile elements in Fe-Ni-Si-S-C liquids (26 trace elements). Systematic assessment of these 26 elements shows which have the strongest affinity for Si, C, and S, and also how activity coefficients for these elements would vary during accretion and core formation in Earth, Mars, and Mercury of widely differing fO2 and core compositional conditions. The activity model is combined with new partitioning expressions for Mo, W, Cr, Re Ru, Pt, and Os and applied to aspects of post core formation mantle geochemistry of Earth, Mars, and Mercury. Our updated expressions show that the BSE Mo/W ratio can easily be achieved with metal/silicate partitioning during growth of the Earth, whereas Re, Os and Ru become lower than and highly fractionated compared with BSE values during core formation and accretion, and thus nearly 99% of their BSE abundances are likely contributed by late accretion. Ru isotopes should be a very good indicator of the source material for the late accretion. The high Pt/Os and Re/Os developed in a deepening magma ocean during the growth of the Earth, indicates 186Os and 187Os isotopes could be coupled if this ancient material remained isolated and subsequently became entrained in mantle plumes and measured in surficial lavas. The extent to which this occurred will be limited by the low Os content of this ancient material, thus requiring mixing as a major component in plume sources. Martian mantle Hf/W ratio stays low during accretion and core formation modelling, suggesting that W isotope anomalies are more likely due to solid/liquid silicate fractionation than to core formation. Finally, Ti contents measured by MESSENGER at Mercury’s surface can be explained by segregation of either a metallic core (IW-6 to -7.5) or metallic core + sulfide (IW-3.5 to -7) followed by mantle melting.

¹Frank Postberg et al. (>10)
Nature 618, 489–493 Open Access Link to Article [DOI https://doi.org/10.1038/s41586-023-05987-9]
¹Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany

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¹Emily M. Chiappe,¹Richard D. Ash,¹Richard J. Walker
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.06.009]
¹Department of Geology, University of Maryland, College Park, Maryland, 20742, USA
Copyright Elsevier

Chemical and isotopic data were obtained for ten iron meteorites classified as members of the IIIE group. Nine of the IIIE irons exhibit broadly similar bulk siderophile element characteristics. Modeling of highly siderophile element abundances suggests that they can be related to one another through simple crystal-liquid fractionation of a parent melt. Our preferred model suggests initial S, P, and C concentrations of approximately 12 wt.%, 0.8 wt.%, and 0.08 wt.%, respectively. The modeled IIIE parent melt composition is ∼4 times more enriched in highly siderophile elements than a non-carbonaceous (NC) chondrite-like parent body, suggesting a core comprising ∼22% of the mass of the parent body. Although chemically distinct from the other IIIE irons, formation of the anomalous IIIE iron Aletai can potentially be accounted for under the conditions of this model through the non-equilibrium mixing of an evolved liquid and early formed solid. Cosmic ray exposure-corrected nucleosynthetic Mo, Ru, and W isotopic compositions of four of the bona fide IIIE irons and Aletai indicate that they originated from the non-carbonaceous (NC) isotopic domain. Tungsten-182 isotopic data for the IIIE irons and Aletai yield similar model metal-silicate segregation ages of 1.6 ± 0.8 Myr and 1.2 ± 0.8 Myr, respectively, after calcium aluminum-rich inclusion (CAI) formation. These ages are consistent with those reported for other NC-type iron meteorite parent bodies. The IIIE irons are chemically and isotopically similar to the much larger IIIAB group. Despite some textural, mineralogical, and chemical differences, such as higher C content, the new results suggest they may have originated from a different crystallization sequence on the same or closely-related parent body.

¹Sarah S. Zeichner,^1,2Laura Chimiak,³Jamie E. Elsila,¹Alex L. Sessions,³Jason P. Dworkin,³José C. Aponte,¹John M. Eiler
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.06.010]
¹Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 90025, USA
²Department of Geological Sciences, UCB 399, University of Colorado, Boulder, CO 80309, USA
³Solar System Exploration Division, Code 691, NASA Goddard Space Flight Center, Greenbelt, MD, 20771. USA
Copyright Elsevier

The Murchison meteorite is a well-studied carbonaceous chondrite with relatively high concentrations of amino acids thought to be endogenous to the meteorite, in part because they are characterized by carbon isotope (δ13C) values higher than those typical of terrestrial amino acids. Past studies have proposed that extraterrestrial amino acids in the Murchison meteorite could have formed by Strecker synthesis (for α-amino acids), Michael addition (for β-amino acids), or reductive amination, but a lack of constraints have prevented confident discrimination among these possibilities, or assignment of specific formation pathways to each of several specific amino acids. Position-specific carbon isotope analysis differentiates amongst these mechanisms by relating molecular sites to isotopically distinct carbon sources and by constraining isotope effects associated with elementary chemical reactions. Prior measurements of the position-specific carbon isotopic composition of α-alanine from the Murchison CM chondrite demonstrated that alanine’s high δ13CVPDB value is attributable to the amine carbon (δ13CVPDB = +142±20‰), consistent with Strecker synthesis drawing on 13C-rich carbonyl groups in precursors (L. Chimiak et al., Geochim. Cosmochim. Acta 292, 188–202, 2021). Here, we measured the δ13C composition of fragment ions generated by electron impact ionization of derivatized ⍺-alanine, β-alanine, and aspartic acid from Murchison via gas chromatography-Fourier transform mass spectrometry. α-Alanine’s amine carbon yielded δ13CVPDB = +109±21‰, which is consistent with the previously measured value and with formation from 13C-rich precursors. β-Alanine’s amine carbon presents a lower δ13CVPDB = +33±24‰, which supports formation from 13C-rich precursors but potentially via a Michael addition mechanism rather than Strecker synthesis. Aspartic acid’s amine carbon has δ13CVPDB= –14±5‰, suggesting synthesis from precursors distinct from those that generated the alanine isomers. These measurements indicate that Murchison amino acids are a mixture of compounds made from different synthesis mechanisms, though some subsets likely drew on the same substrates; this conclusion highlights the complexity of extraterrestrial organic synthesis networks and the potential of emerging methods of isotope ratio analysis to elucidate the details of those networks.

^1,2Aryavart Anand,²Klaus Mezger
Chemie der Erde/Geochemistry (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2023.126004]
¹Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
²Institut für Geologie, Universität Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland
Copyright Elsevier

Age constraints on early solar system processes and events can be derived from meteorites and their components using different radioisotope systems. Due to the short time interval from the first formation of solids in the solar nebula to the accretion and differentiation of planetesimals and some planets, a high temporal resolution of the chronometers is essential and can be obtained in most cases only with short-lived isotope systems, particularly the decay schemes 26Al-26Mg, 182Hf-182W and 53Mn-53Cr. These chronometers provide highly resolved time constrains for the formation of the first solids (Ca-Al-rich inclusions or CAIs), chondrules, planetary cores, for the accretion and differentiation of planetesimals and hydrous/thermal alteration. Formation of Ca-Al-rich inclusions was restricted to the inner solar system and to a short time interval of ≪1 Ma, and marks the “beginning of the solar system”. It was immediately followed by planetesimal formation. The oldest planetesimals accreted within a few 105 a after the formation of CAIs. The accretion of early formed planetesimals and their subsequent differentiation into a metallic core and a silicate mantle was a continuous process that occurred at different times in different locations of the solar nebula and extended over a time interval of at least ~4 Ma. During this time interval the accretion process may have changed from planetesimal formation via streaming instability to pebble accretion. The earliest formed bodies that still needed to settle into stable orbits could have created bow shocks in the adjacent regions still composed of dust and gas which resulted in the formation of silicate chondrules in a narrow time interval from 1.8 to 3 Ma. The chondrule forming interval was immediately followed by the accretion of the chondrite parent bodies, which did not differentiate due to their late accretion when most of the heat producing 26Al had already decayed. Thus, the chondrite parent bodies are a second generation of planetesimals, but chemically they are the most primitive material preserved from the early solar system. Aqueous alteration of volatile rich planetesimals peaked at ca. 3.5 Ma and coincided with metamorphism recorded in ordinary chondrite parent bodies. The compilation of ages from different meteorites and their components demonstrates that similar accretion and differentiation processes do not follow an identical time line from dust to planetesimal formation and they do not correlate with the location in the disk. The accretion of matter into planetesimals was a local phenomenon with stochastic spatial distribution. The spatial distribution of accretion processes operating in the early solar system appears to be similar to those in some directly observable nascent exo-planetary systems.

¹Nicolas P. Walte,^1,2Christopher M. Howard,³Gregor J. Golabek
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2023.118247]
¹Heinz Maier-Leibnitz Center for Neutron Science (MLZ), Technical University Munich, 85748 Garching, Germany
²ISIS Neutron & Muon Spallation Facility, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Chilton, Oxfordshire OX11 0QX, United Kingdom
³Bayerisches Geoinstitut (BGI), University of Bayreuth, 95447 Bayreuth, Germany
Copyright Elsevier

Main group pallasites were likely formed by the collision of their parent body with a smaller impactor that caused a mixing of mantle and core material from the target and impactor, respectively. In order to better constrain the collision, we present particle size distribution (PSD) analyses of olivines in seven main group pallasites and of the Eagle station pallasite. The PSDs of two fragmental pallasites (Admire and Huckitta) contain linear segments in bi-logarithmic particle number versus size diagrams that are similar to terrestrial cataclasites or impact-related rocks. On the other hand, the PSDs of four angular pallasites only display linear segments above their respective average grain sizes. We argue that fragmental pallasites record brittle rock deformation close to the impact site of the collision, while angular pallasites represent deeper-lying mantle rocks of the target body that were disintegrated by the downward percolation of core metal from the impactor. High strain-rate deformation experiments with the system olivine – FeS melt ± Au melt produced microstructures and PSDs that are broadly similar to these two textural groups. The experiments also suggest that de-localized metal melt percolation and concomitant mantle disintegration as evidenced in angular pallasites is facilitated by weak grain boundaries caused by a small fraction of previously present metal melt in the mantle as opposed to localized diking that is dominant in a melt-free mantle. The former mechanism is expected to prevent efficient core-merging and instead causes mantle-impregnation with metal melt, which could be active when planetesimal mantles were still warm due to short-lived radiogenic heating. In addition to the parent body of the PMG, asteroid 16 Psyche may be an example of this inefficient core-merging mechanism.

¹Stephen M. Elardo,¹Daniel F. Astudillo Manosalva
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.05.020]
¹The Florida Planets Lab, Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA
Copyright Elsevier

Clasts of feldspathic lithologies such as magnesian anorthosites, anorthositic troctolites, and granulites in lunar meteorites have greatly expanded knowledge of the lithologic diversity of the lunar crust. However, their origins and relationships to other major crustal lithologies such as the ferroan anorthosites, magnesian-suite, and alkali-suite are not fully understood. Here we present the results of phase equilibrium modeling using the MELTS and MAGFOX programs designed to understand the origins of lunar crustal lithologies and petrologic connections between them. We show that the major and trace element compositions of the Mg- and alkali-suites are consistent with partial melting of hybridized sources and are inconsistent with decompression melting + assimilation models. Our results also show that the vertical trend in Mg# in mafic silicates vs. An# in plagioclase characteristic of feldspathic meteoritic lithologies and ferroan anorthosites can be produced by fractional crystallization of KREEP-free Mg-suite melts, but that these melts are not likely to contribute significantly to the composition of the global crust. Furthermore, we calculated potential parental melt compositions for these crustal lithologies using the abundances of REEs in plagioclase in FANs, the Mg- and alkali-suites, and clasts in feldspathic meteorites. Our results show that despite low bulk rock abundances of incompatible trace elements in many feldspathic lunar meteorites, parental melts with overall REE abundances similar to or in excess of KREEP are needed to reproduce the REE abundances in plagioclase in clasts from feldspathic lunar meteorites. However, the lack of a Na enrichment trend in their plagioclase compositions with decreasing Mg# requires very Na-depleted melts without a KREEP component. The available data regarding magnesian anorthosites, anorthositic troctolites, and granulites in lunar meteorites, and inferences made here regarding their parental melt compositions, lead to contradictory and ambiguous conclusions regarding their origins.

¹Morgano Maxime,¹Le Guillou Corentin,¹Leroux Hugues,¹Marinova Maya,²Dohmen Ralf
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115669]
¹Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207 – UMET – Unité Matériaux et Transformations, F-59000 Lille, France
²Ruhr-Universitaet Bochum, RUB, Institute of Geology, Geophysics and Mineralogy, 44780 Bochum, Germany
Copyright Elsevier

Amorphous silicates are abundant in extraterrestrial objects such as interplanetary dust particles and primitive chondrites. They are thought to be formed through condensation and possibly later exposed to thermal processes in the nebula before being accreted within an asteroid and/or comet.

We aim to constrain the conditions that prevailed during thermal events in the nebula, through experimental work on the chemical and structural evolution of condensed amorphous silicate.

We conducted coupled condensation and heating experiments of Fe-Mg-silicate thin films using the pulsed laser deposition technique. We compared samples condensed at room temperature and annealed in a second step with samples directly condensed on heated substrate, at 450 °C and 700 °C.

For both processes, at temperature as low as 450 °C, iron-rich nanoparticles and Mg-rich domains form, evidencing the high reactivity of the condensed amorphous silicate. This reactivity was found to be even higher for the process of condensation on heated substrate. We also evidence the persistence of amorphous silicate up to 700 °C, in spite of the chemical evolution and the demixion into MgO and SiO2 domains.

These results imply that amorphous silicates condensed from a plasma (and possibly from any process producing atoms in an excited state) are more reactive than quenched glasses of similar composition. In complement to high temperature events that occurred at the time of solar system formation and that formed chondrules for instance, this work emphasizes the importance of mild heating on dust evolution before accretion within parent(s) body(ies). It helps to place chemical and structural constraints on the thermal evolution of amorphous silicate found in primitive chondrites: i) iron segregation as metallic nanoparticles can be generated within a silicate groundmass at temperature as low as 450 °C (and possibly even below) ii) iron-rich chondritic amorphous silicate can persist up to 700 °C.

¹Flore Van Maldeghem,²Matthias van Ginneken,¹Bastien Soens,³Felix Kaufmann,⁴Seppe Lampe,¹Lisa Krämer Ruggiu,³Lutz Hecht,¹Philippe Claeys,¹Steven Goderis
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.06.002]
¹Analytical-, Environmental-, and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
²Centre for Astrophysics and Planetary Science, School of Physical Sciences, Ingram Building, University of Kent, Canterbury CT2 7NH, UK
³Museum für Naturkunde Berlin, Invalidenstrasse 43, Berlin 10115, Germany
⁴Hydrology and Hydraulic Engineering, Faculty of Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
Copyright Elsevier

Micrometeorites originate from the interplanetary dust complex and continuously fall to the Earth’s surface in large amounts. About 10 to 20% of micrometeorites are not melted upon reaching the Earth’s surface, preserving the primary features and characteristics of the parent material. Consequently, unmelted micrometeorites, together with scoriaceous micrometeorites, an intermediate form between cosmic spherules and unmelted micrometeorites, are pivotal in documenting the nature and evolution of interplanetary dust as well as the modifications experienced by micrometeorites during atmospheric entry. Based on their petrographic features, here we identified and characterized 64 scoriaceous and unmelted micrometeorites with diameters varying between 90 and 410 μm from fine-grained sediment sampled in the Sør Rondane Mountains of East Antarctica. Based on their size distribution, the micrometeorites from the Sør Rondane Mountains show a clear distinction between unmelted micrometeorites (< 300 μm) and cosmic spherules (> 400 μm) and imply an accumulation mechanism or exposure history distinct from other collections (e.g., Transantarctic Mountains). Different exposure windows, weathering processes and environmental factors (e.g., snow cover) could affect the size and composition of preserved particles.

A selection of the particles (n = 49) was further characterized for geochemical composition and high-precision oxygen isotope ratios to identify potential parent bodies and document their alteration histories. About 63% of the particles, exhibiting both coarse- and fine-grained textures, derive from carbonaceous chondritic precursors. Two particles (∼ 4%) display anomalously 16O-poor isotopic compositions similar to that previously observed for (giant) cosmic spherules and unmelted micrometeorites, classified as “group 4” particles. These particles are thought to originate from an unidentified chondritic parent body located in a specific region of the protoplanetary disk or may have been characterized by a distinct alteration history, with recent studies linking them to CY carbonaceous chondrites. Only a single fine-grained particle (∼ 2%) can be assigned to ordinary chondritic parentage with confidence. The partially hydrated fine-grained matrix suggests this particle might be consistent with a Semarkona-like parent body. Approximately 10% of the studied particles exhibit extensive evidence for secondary terrestrial weathering with formation of (hydr)oxides during residence in the Antarctic environment, preventing detailed parent body identification. Ten particles (∼ 20%) could not be assigned to a specific parent group due to ambiguous oxygen isotope values. Overall, the parent body statistics from this study agree with those reported for different collections of a similar size fraction. Clear associations between textural groups and parent bodies could not be established. Even though unmelted micrometeorites are generally considered pristine and often do not exhibit any obvious petrographic evidence of terrestrial weathering, the chemical and isotopic data obtained here confirm that alteration can occur at the microscale and any data on unmelted particles from Antarctic subaerial collections should be evaluated with caution.

¹Aiko Nakato et al.(>10)
Earth, Planets and Space 75, 45 Open Access Link to Article [DOI
https://doi.org/10.1186/s40623-022-01754-8%5D
¹Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Kanagawa, 252-5210, Japan

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