Carbonaceous Achondrites Northwest Africa 6704/6693: Milestones for Early Solar System Chronology and Genealogy

1Matthew E.Sanborn, 1Josh Wimpenny, 1Curtis D.Williams, 1Akane Yamakawa, 2Yuri Amelin, 3Anthony J.Irving, 1Qing-ZhuYin
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.10.004]
1Department of Earth and Planetary Sciences, University of California-Davis, One Shields Avenue, Davis, CA 95616 USA
2Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601 Australia
3Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195 USA
Copyright Elsevier

Northwest Africa (NWA) 6704/6693 are medium- to coarse-grained achondrites with unique petrologic and geochemical traits that are distinct from the currently established meteorite groups. Here, we report on the extinct 26Al-26Mg and 53Mn-53Cr systems to establish fine-scale chronology of its formation and Cr and Ti isotopic anomalies to constrain the composition of the source reservoir of NWA 6704/6693. Excesses in the neutron-rich 54Cr and 50Ti isotopes, due to nucleosynthetic anomalies, separate NWA 6704/6693 from the vast majority of established achondrites and instead resemble the excesses seen among the carbonaceous chondrites; specifically, the CR-type chondrites. The excesses in these isotopes indicate a common feeding zone during accretion in the protoplanetary disk between the source of NWA 6704/6693 and that of the carbonaceous chondrites. The 26Al-26Mg data for pyroxene and plagioclase from NWA 6704 yield a (26Al/27Al)0 = (3.15 ± 0.38)×10-7 (MSWD = 0.49) and δ26Mg∗ = -0.004 ± 0.005 at the time of isotopic closure. This initial (26Al/27Al)0 translates to an absolute age of 4563.14 ± 0.38 Ma, relative to the D’Orbigny angrite. However, given the potential heterogeneity of 26Al, the D’Orbigny angrite might not be a good age anchor for the purpose of calculating 26Al-26Mg ages. The 26Al-26Mg age relative to another carbonaceous achondrite NWA 2976 is 4562.66 ± 0.60 Ma. The 53Mn-53Cr systematics of NWA 6704/6693 indicate a (53Mn/55Mn)0 of (2.59 ± 0.34)×10-6 (MSWD = 1.2) with an evolved initial ε53Cr of +0.14 ± 0.03. The (53Mn/55Mn)0 yields an 53Mn-53Cr age of 4562.17 ± 0.76 Ma relative to the D’Orbigny angrite. Concordant ages determined using the short-lived 26Al-26Mg and 53Mn-53Cr systems and extant 207Pb-206Pb system (4562.60±0.30 Ma for NWA 6704/6693; Amelin et al., 2018) indicate rapid cooling and nearly contemporaneous closing of multiple isotope systems. The ancient crystallization ages and positive 54Cr and 50Ti anomalies of NWA 6704/6693 indicate widespread melting and differentiation processes occurring in both non-carbonaceous (NC) and carbonaceous chondrite (CC) regions of the protoplanetary disk. Additionally, we report the Cr and Ti isotopic composition for a petrologic range of CR-type materials (CR2, CR6, and achondrites). The additional Cr and Ti isotopic data for these CR-type materials indicates a range in isotopic composition not previously observed based on CR2 chondrites alone.

Reassessing the origin and chronology of the unique achondrite Asuka 881394: Implications for distribution of 26Al in the early Solar System

1Josh Wimpenny, 1Matthew E.Sanborn, 2Piers Koefoed, 3Ilsa R.Cooke, 3Claudine Stirling, 2Yuri Amelin, 1Qing-ZhuYin
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.10.006]
1Department of Earth and Planetary Sciences, University of California at Davis, One Shields Avenue, Davis, CA 95616, USA
2Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia
3Department of Chemistry, University of Otago, Dunedin 9016, New Zealand
Copyright Elsevier

The achondrite Asuka 881394 has mineralogy broadly similar to that of eucrites but is isotopically, chemically and texturally distinct from them. Previous U-Pb chronology shows that it is very old; forming within the first 0.8 Ma of the formation of calcium-aluminum rich inclusions (CAIs). However, the age difference between Asuka 881394 and other very old Solar System materials (CAIs, quenched angrites) measured with the 26Al-26Mg and 53Mn-53Cr extinct radionuclide chronometers, and 207Pb/206Pb chronometer, is not the same. This could be interpreted to reflect heterogeneity in the distribution of 26Al and 53Mn in the early Solar System. Given the significant implications for the early Solar System chronology if 26Al and 53Mn are indeed heterogeneously distributed, in this study we further investigate the origin of Asuka 881394 and the apparent age discordance between short-lived and absolute chronometers, by focusing on measurement of its ε54Cr composition, renewed measurements of the absolute Pb-Pb age and new, high precision measurements of its 26Al-26Mg systematics.
New Cr isotope data places additional constraints on the origin of Asuka 881394; its ε 54Cr value of -0.37 ± 0.10ε is resolvable outside of uncertainty from HED meteorites (-0.72 ± 0.10ε), reinforcing evidence from oxygen isotopic analyses that suggest it originated from a distinct parent body, unlikely to be 4 Vesta. New Pb-Pb analyses, combined with using a directly measured 238U/235U ratio for age calculation, result in a recalculated Pb-Pb age of 4564.95 ± 0.53Ma, ∼1.5Ma younger than previously reported age. With this age Asuka 881394 remains one of the oldest known achondrites in our Solar System. New high precision 26Al-26Mg data produce an initial 26Al/27Al ratio of 1.48 ± 0.12 × 10-6, within error of previous data. This ratio corresponds to an Al-Mg age of 4563.69 ± 0.36 Ma or 4564.83 ± 0.21 Ma relative to CAIs and the D’Orbigny angrite, respectively. Thus, depending on which age anchor is used, the 26Al-26Mg age is either 1.26 Ma or 0.12 Ma younger than the new Pb-Pb age, the latter being unresolved within analytical uncertainty.
Though a potential age discrepancy between 26Al-26Mg and Pb-Pb could be a result of heterogeneous distribution of 26Al, we demonstrate with our new high precision Mg isotope data, in conjunction with petrographic evidence, that the Mg isotope system has been disturbed in Asuka 881394. We suggest that the 26Al-26Mg system closed to diffusion after the U-Pb system, either due to slow cooling on the parent body or low-grade metamorphic re-equilibration of Mg. Thus, we can satisfactorily explain the observed age discrepancy between 26Al-26Mg and U-Pb systems in Asuka 881394 without invoking heterogeneous distribution of 26Al/27Al ratio in the early Solar System.
Comparison of the Asuka 881394 data with that of other anomalous achondrites from distinct parent bodies suggests that these could also have evolved from a source region with a canonical 26Al/27Al ratio. Because these achondrites have significant differences in their ε54Cr-Δ17O systematics, which could be indicative of location within the early protoplanetary disk, it is consistent with homogeneous distribution of 26Al in the early Solar System. Angrites remain an outlier; either because they evolved from a source with a lower 26Al/27Al ratio or because their 26Al-26Mg or Pb-Pb data are problematic. In either case, this suggests that basaltic angrites are questionable as the age anchor for the entire Solar System as a whole, and other very old, well preserved achondrites should be considered for that role.

Fractionation of highly siderophile elements in metal grains from unequilibrated ordinary chondrites: Implications for the origin of chondritic metals

1Satoki Okabayashi, 1Tetsuya Yokoyama, 1Nao Nakanishi, 1,2HikaruIwamoria
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.10.003]
1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
2Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa 237-0061, Japan
Copyright Elsevier

To investigate the formation processes of metal grains in chondrites, we measured the abundances of highly siderophile elements (HSEs: Re, Os, Ir, Ru, Pt, Rh, Pd, and Au) using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on individual Fe-Ni metal grains from four petrologic type 3 ordinary chondrites: NWA 6910 (L3.3), NWA 4910 (LL3.1), Richfield (LL3.7), and SAH 97210 (L/LL3.2). Among HSEs, the abundances of Pd and Au in the metal grains had positive correlations with the measured Ni abundances, indicating equilibrium partitioning of Pd and Au between kamacite and taenite via thermal metamorphism. In contrast, the other HSEs (Re, Os, Ir, Ru, Pt, Rh) showed large variations in concentrations spanning nearly three orders of magnitude without evidence of redistribution between kamacite and taenite, suggesting that these elements preserved the initial compositions before kamacite-taenite segregation. The CI-normalized HSE patterns presented large depletions in Os and Ir with relatively large Os/Ir variations (0.29–3.2) and the Ru/Ir ratios also varied significantly (0.27–40). In addition, HSE abundances in fine metal grains showed wide variations compared to those of coarse metal grains. We suggest that the variation of refractory HSE compositions in Fe-Ni metal grains with characteristic Os-Ir depletions were most likely caused by solid metal-liquid metal partitioning during crystallization of a Fe-Ni metal melt containing 2 wt.% of C. The liquid metal is considered to be generated during multiple heating events related to chondrule formation. The lack of Fe-Ni metal grains exhibiting coexistence of liquid metal and solid metal composition within a single metal grain would suggest that solid metal grains were physically segregated from the liquid metal during the crystallization of Fe-Ni metals. Droplets of the segregated liquid metal collided and merged with other liquid metal droplets and solid metal grains to form coarser metal grains. The resultant larger metal grains have relatively homogeneous HSE abundances that are close to the bulk metal composition as a result of the mixing of liquid metal with solid metal. In contrast, molten metal droplets and solid metal grains that did not collide and merge formed finer metal grains formed finer metal grains with more variable HSE abundances.

FITS format for planetary surfaces: definitions, applications and best practices.

1C. Marmo, 2T. M. Hare, 3S. Erard, 4M. Minin, 5F.‐X. Pineau, 6,7A. Zinzi, 3B. Cecconi, 4A. P. Rossi
Earth and Planetary Science (in Press) Link to Article [https://doi.org/10.1029/2018EA000388]
1GEOPS, Univ. Paris‐Sud, CNRS, Univ. Paris‐Saclay, Orsay, France
2U. S. Geological Survey, Astrogeology Science Center, Flagstaff, AZ, USA
3LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, Meudon, France
4Jacobs University, Bremen, Germany
5Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, Strasbourg, France
6Space Science Data Center ‐ ASI, Rome, Italy
7INAF‐OAR, Monte Porzio Catone (RM), Italy

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Three-Dimensional Raman Tomographic Microspectroscopy: A Novel Imaging Technique

1Yesiltas, M.,2Jaret, S., 1Young, J., 3Wright, S.P., 2Glotch, T.D.
Earth and Space Science 5, 380-392 Link to Article [DOI: 10.1029/2018EA000369]
1Faculty of Aeronautics and Space Sciences, Kirklareli University, Kirklareli, Turkey
2Department of Geosciences, Stony Brook University, Stony Brook, NY, United States
3Planetary Science Institute, Tucson, AZ, United States

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Volatile loss following cooling and accretion of the Moon revealed by chromium isotopes

1Paolo A. Sossi, 1Frédéric Moynier, 1Kirsten van Zuilen
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [https://doi.org/10.1073/pnas.1809060115]
1Institut de Physique du Globe de Paris, Université Paris Diderot, Université Sorbonne Paris Cité, CNRS UMR 7154, 75238 Paris Cedex 05, France

With the exception of volatile elements, which are strongly depleted and isotopically fractionated, the Moon has chemical and isotopic signatures that are indistinguishable from Earth’s mantle. Reconciliation of these properties with Moon formation in a high-energy giant impact invokes evaporative loss of volatile elements, but at conditions that are poorly known. Chromium isotopic fractionation is sensitive to temperature variations and liquid–gas equilibration during evaporation. We measure an isotopic difference between Earth’s mantle and the Moon, consistent with the loss of a Cr-bearing, oxidized vapor phase in equilibrium with the proto-Moon. Temperatures of vapor loss required are much lower than predicted by recent models, implying that volatile elements were removed from the Moon following cooling rather than during a giant impact.