^1,2,3Philipp R. Heck,^1,2,3Jennika Greer,^1,2,3Levke Kööp,⁴Reto Trappitsch,^5,6Frank Gyngard,⁷Henner Busemann,⁷Colin Madeg,⁸Janaína N. Ávila,^1,2,3,9Andrew M. Davis,⁷Rainer Wieler
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [https://doi.org/10.1073/pnas.1904573117]
¹Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, IL 60605;
²Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL 60637;
³Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637;
⁴Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550;
⁵Physics Department, Washington University, St. Louis, MO 63130;
⁶Center for NanoImaging, Harvard Medical School, Cambridge, MA 02139;
⁷Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland;
⁸Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia;
⁹Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637

We determined interstellar cosmic ray exposure ages of 40 large presolar silicon carbide grains extracted from the Murchison CM2 meteorite. Our ages, based on cosmogenic Ne-21, range from 3.9 ± 1.6 Ma to ∼3 ± 2 Ga before the start of the Solar System ∼4.6 Ga ago. A majority of the grains have interstellar lifetimes of <300 Ma, which is shorter than theoretical estimates for large grains. These grains condensed in outflows of asymptotic giant branch stars <4.9 Ga ago that possibly formed during an episode of enhanced star formation ∼7 Ga ago. A minority of the grains have ages >1 Ga. Longer lifetimes are expected for large grains. We determined that at least 12 of the analyzed grains were parts of aggregates in the interstellar medium: The large difference in nuclear recoil loss of cosmic ray spallation products ³He and ²¹Ne enabled us to estimate that the irradiated objects in the interstellar medium were up to 30 times larger than the analyzed grains. Furthermore, we estimate that the majority of the grains acquired the bulk of their cosmogenic nuclides in the interstellar medium and not by exposure to an enhanced particle flux of the early active sun.

^1,2Timo Hopp,^1,3Gerrit Budde,¹Thorsten Kleine
Earth and Planetary Science Letters 534, 116065 Link to Article [https://doi.org/10.1016/j.epsl.2020.116065]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Origins Laboratory, Department of Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5743 South Ellis Avenue, Chicago, IL 60637, USA.
³The Isotoparium, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
Copyright Elsevier

The Mo and Ru isotopic compositions of meteorites and the bulk silicate Earth (BSE) hold important clues about the provenance of Earth’s building material. Prior studies have argued that non-carbonaceous (NC) and carbonaceous (CC) meteorite groups together define a Mo-Ru ‘cosmic’ correlation, and that the BSE plots on the extension of this correlation. These observations were taken as evidence that the final 10–15% of Earth’s accreted material derived from a homogeneous inner disk reservoir with an enstatite chondrite-like isotopic composition. Here, using new Mo and Ru isotopic data for previously uninvestigated meteorite groups, we show that the Mo-Ru correlation only exists for NC meteorites, and that both the BSE and CC meteorites fall off this Mo-Ru correlation. These observations indicate that the final stages of Earth’s accretion were heterogeneous and consisted of a mixture of NC and CC materials. The Mo-Ru isotope systematics are best accounted for by either an NC heritage of the late veneer combined with a CC heritage of the Moon-forming giant impactor, or by mixed NC-CC compositions for both components. The involvement of CC bodies in the late-stage accretionary assemblage of Earth is consistent with chemical models for core-mantle differentiation, which argue for the addition of more oxidized and volatile-rich material toward the end of Earth’s formation. As such, this study resolves the inconsistencies between homogeneous accretion models based on prior interpretations of the Mo-Ru systematics of meteorites and the chemical evidence for heterogeneous accretion of Earth.

¹Helffrich, G.,¹Brasser, R.,²Shahar, A.
Progress in Earth and Planetary Science 6, 66 Link to Article [DOI: 10.1186/s40645-019-0312-z]
¹Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan
²Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, DC 20015, United States

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^1,2C.C.Bedford,³S.P.Schwenzer,⁴J.C.Bridges,⁵S.Banham,⁶R.C.Wiens,⁷O.Gasnault,² E.B.Rampe,⁸J.Frydenvang,⁶P.J.Gasda
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113622]
¹Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Blvd., Houston, TX, USA
²Astromaterials and Exploration Science, NASA Johnson Space Center, Houston, TX, USA
³School of Environment, Earth and Ecosystem Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁴Space Research Centre, School of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁵Imperial College London, London, UK
⁶Los Alamos National Laboratory, Los Alamos, NM, USA
⁷Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, CNES, France
⁸Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
Copyright Elsevier

The Mars Science Laboratory Curiosity rover has encountered both ancient lithified and modern active aeolian dune deposits within Gale crater, providing an opportunity to study how aeolian processes have changed during Gale crater’s geological history. This study uses data from the Chemistry and Camera (ChemCam) and Chemistry and Mineralogy (CheMin) instrument suites onboard Curiosity to; (1) constrain the diagenetic processes that lithified and altered the ancient aeolian Stimson formation, (2) investigate whether the geochemical signature in the Stimson formation is consistent with the aeolian mafic-felsic mineral sorting trend identified in the modern Bagnold dune fields in Gale crater, and (3) discuss the provenance of the Stimson sediments, comparing it to those identified in the ancient river and lake deposits also analyzed along Curiosity’s traverse.
The ancient Stimson dune deposits that stratigraphically overlie the Gale fluvio-lacustrine units were analyzed in two locations; the Emerson and the Naukluft plateaus. ChemCam data show that the Stimson formation has subtle variations in MgO, Al2O3, Na2O and K2O between the two localities. An agglomerative cluster analysis of the constrained Stimson dataset reveals five clusters, four of which relate to different proportions of mafic and felsic minerals analyzed by ChemCam. In general, the cluster analysis shows that the Emerson plateau has a greater proportion of mafic minerals and fewer coarse, felsic grains relative to the Naukluft plateau. This variation in mafic and felsic minerals between localities suggests a southwest to northeast net sediment transport direction due to aeolian mineral sorting dynamics preferentially transporting mafic minerals that are easier to saltate than the elongate, often coarser, felsic minerals. This derived transport direction for the Stimson formation supports that determined by sedimentological evidence and is opposite to that previously determined for the active Bagnold dunes inferring a change in the wind regime with time. An opposite sediment transport direction between the ancient and modern dunes in Gale crater further supports geochemical and mineralogical evidence that suggests different basaltic source regions. Compositionally, the bulk Stimson formation is most similar to the subalkaline basalt source region that is inferred to be the dominant sediment source of the fluvio-lacustrine Bradbury group. This is likely the result of the Stimson formation and basaltic Bradbury group sediments sharing a similar local basaltic source region such as the rim and walls of Gale crater.

^1,2C.Cournède,¹J.Gattacceca,²P.Rochette,^3,4 D.L.Shuster
Earth & Planetary Science Letters 533, 116042 Link to Article [https://doi.org/10.1016/j.epsl.2019.116042]
¹Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, 150 John T. Tate Hall, 116 Church St SE, Minneapolis, MN 55455, USA
²CNRS, Aix Marseille Univ, IRD, Coll France, INRAe, CEREGE, Aix-en-Provence, France
³Department of Earth and Planetary Science, University of California–Berkeley, 307 McCone Hall, Berkeley, CA 94720, USA
⁴Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA
Copyright Elsevier

Different types of magnetic fields were at work in the early solar system: nebular fields generated within the protoplanetary nebula, solar fields, and dynamo fields generated within the solar system solid bodies. Paleomagnetic studies of extraterrestrial materials can help unravel both the history of these magnetic fields, and the evolution of solar system solid bodies. In this study we studied the paleomagnetism of two Rumuruti chondrites (PCA 91002 and LAP 03639). These chondrites could potentially bear the record of the different fields (solar, nebular, dynamo fields) present during the early solar system. The magnetic mineralogy consists of pseudo-single domain pyrrhotite in LAP 03639 and pyrrhotite plus magnetite in PCA 91002. Paleomagnetic analyses using thermal and alternating field demagnetization reveal a stable origin trending component of magnetization. Fields of 12 μT or higher are required to account for the magnetization in PCA 91002, but the timing and exact mechanism of the magnetization are unconstrained. In LAP 03639, considering various chronological constraints on the parent body evolution and on the evolution of the different sources of magnetic field in the early solar system, an internally-generated (dynamo) field of ∼5 μT recorded during retrograde metamorphism is the most likely explanation to account for the measured magnetization. This result indicates the existence of an advecting liquid core within the Rumuruti chondrite parent body, and implies that, as proposed for CV and H chondrites, this chondritic parent body is partially differentiated.

^1,2Crapster-Pregont, E.J.,^1,2Ebel, D.S.
Microscopy and Microanalysis (in Press) Link to Article [DOI: 10.1017/S1431927619015216]
¹Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, United States
²Department of Earth and Planetary Science, American Museum of Natural History, Central Park West, 79th Street, New York, NY 10024, United States

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^1,2,3E.S.Steenstra,²W.van Westrenen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113621]
¹The Geophysical Laboratory, Carnegie Institution of Science, Washington, DC, USA
²Faculty of Science, Vrije Universiteit Amsterdam, the Netherlands
³Institute of Mineralogy, University of Münster, Germany
Copyright Elsevier

Differentiation of Mercury and the aubrite parent body (AuPB) at extremely reducing conditions is implied from the low FeO and high S contents of their mantles. Here, the mantle abundances of these elements as derived from remote sensing (in the case of Mercury) and aubrite meteorite analysis are used in conjunction with new experimentally determined metal-silicate partition coefficients at reducing conditions presented in Steenstra et al. (2020a) to quantify the geochemical consequences of core formation in highly reduced planetary bodies. Plausible core compositions of Mercury and the AuPB are assessed, and the distribution of Si and other elements during core formation are quantified.

Combining the new results with previous remote sensing observations of the surface of Mercury it is found that its core is likely to be Si-rich (>4.5–21 wt%). The estimated core Si contents depend mostly on the assumption of C saturation in Mercury’s mantle and the type of bulk composition considered. Calculations for C-free conditions also yield significant quantities of Si in the AuPB core (>6–20 wt%, depending on bulk composition and redox state). The amount of Si in the AuPB core is greatly decreased if C-saturation is assumed.

The new experimental data and related thermodynamic parameterization for predicting the C contents at graphite saturation of FeSi alloys (CCGS) shows that Mercury’s mantle was likely C-saturated following formation of a Si-rich core, allowing for the separation of a graphitic flotation crust. Due to the lower Si contents calculated for the AuPB core under C-saturated conditions, a graphitic flotation crust is unlikely to form in smaller-sized reduced asteroids such as the AuPB.

The Si-rich nature of the AuPB core for C-undersaturated scenarios is expected to have resulted in preferential partitioning of the majority of the volatile siderophile elements (VSE) into the AuPB core. However, depletions for the most volatile VSE cannot be reconciled with core formation depletion only. Their depletions in aubrites require additional depletion from (I) segregating sulfide liquids during AuPB differentiation and/or (II) compatible behavior during mineral-melt fractionation and/or (III) loss of these elements in a vapor phase during and/or after AuPB differentiation.

^1,2,3E.S.Steenstra,²F.van Haaster,²R.van Mulligen, ³S.Flemetakis, ³J.Berndt, ³S.Klemme,²W.van Westrenen
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.006]
¹The Geophysical Laboratory, Carnegie Institution of Science, Washington D.C., the United States of America
²Faculty of Science, Vrije Universiteit Amsterdam, The Netherlands
³Institute of Mineralogy, University of Münster, Germany
Copyright Elsevier

¹Matsumoto, M. et al. (>10)
Science Advances 5, eaax5078 Link to Article [DOI: 10.1126/sciadv.aax5078]
¹Division of Earth and Planetary Sciences, Kyoto University, Kyoto, 606-8502, Japan

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¹Guda, L.V.,¹Kravtsova, A.N.,¹Guda, A.A.,²Kubrin, S.P.,³Mazuritskiy, M.I.,¹Soldatov, A.V.
Journal of Surface Investigation 13, 995-1004 Link to Article [DOI: 10.1134/S1027451019060089]
¹Smart Materials Research Institute, Southern Federal University, Rostov-on-Don, 344090, Russian Federation
²Research Institute of Physics, Southern Federal University, Rostov-on-Don, 344090, Russian Federation
³Physics Faculty, Southern Federal University, Rostov-on-Don, 344090, Russian Federation

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