Low-metallicity CO + He WD post-merger models for RCB stars as a source of pre-solar graphite grains

1Menon, A.,1Karakas, A.I.,2Lugaro, M.,1,2Doherty, C.L.,3,4,5Ritter, C.
Monthly Notices of the Royal Astronomical Society 482, 2320-2335 Link to Article [DOI: 10.1093/mnras/sty2606]
1Monash Centre for Astrophysics (MoCA), School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
2Konkoly Observatory, Hungarian Academy of Sciences, Konkoly-Thege Miklos ut 15-17, Budapest, 1121, Hungary
3Astrophysics Group, Keele University, Keele, Staffordshire, ST5 5BG, United Kingdom

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Origins of meteoritic nanodiamonds investigated by coordinated atom‐probe tomography and transmission electron microscopy studies

1Josiah B. Lewis,1Christine Floss,2Dieter Isheim,1,3Tyrone L. Daulton,2David N. Seidman,1Ryan Ogliore
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13373]
1Laboratory for Space Sciences, Washington University, St. Louis, MO, 63130 USA
2Northwestern University Center for Atom‐Probe Tomography, Evanston, IL, 60208 USA
3Institute for Materials Science and Engineering, Washington University, St. Louis, MO, 63130 USA
Published by arrangement with John Wiley & Sons

To constrain the origins of meteoritic nanodiamonds, the abundance ratios of stable C isotopes in acid residues from the carbonaceous chondritic meteorite Allende CV3 were measured using coordinated atom‐probe tomography (APT) and transmission electron microscopy (TEM). We combined our data with previously published APT data. A statistical analysis of this combined data set suggests an upper bound of 1 in 102 on the subpopulation that could have a large isotopic enrichment in 13C relative to 12C, consistent with the possible detection by secondary ion mass spectrometry of a similar enrichment in a 1 in 105 fraction, abundant enough to account for the Xe‐HL anomalous isotopic component carried by the acid residues. Supernovae are believed to be the source of Xe‐HL, leading to the mystery of why all other supernova minerals do not carry Xe‐HL. The lack of Xe‐HL in low‐density disordered supernova graphite suggests that the isotopically anomalous component is the nanodiamonds, but the disordered C in the residue is not ruled out. We discuss possible origins of the disordered C and implications of our results for proposed formation scenarios for nanodiamonds. At least 99% of the meteoritic acid residue exhibits no unambiguous evidence of presolar formation, although production with solar isotope ratios in asymptotic giant branch stars is not ruled out. Comparison of TEM and APT results indicates that a minority of the APT reconstructions may preferentially sample disordered C rather than nanodiamonds. If this is the case, a presolar origin for a larger fraction of the nanodiamonds remains possible.

Northwest Africa 10414, a pigeonite cumulate shergottite

1,2R.H. Hewins,1,3B. Zanda,1S. Pont,4P.‐M. Zanetta
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13374]
1Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum National d’Histoire Naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD UMR 206, 61 rue Buffon, 75005 Paris, France
2Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, 08854 USA
3IMCCE, Observatoire de Paris, CNRS UMR 8028, 77 Av. Denfert Rochereau, 75014
Paris, France
4Université Lille, CNRS, UMR 8207, UMET, 59000 Lille, France
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 10414 is an unusual shergottite with a cumulate texture. It contains 73% coarse prismatic pigeonite, plus 18% interstitial maskelynite, 2% Si‐rich mesostasis, 2% merrillite, and minor chromite‐ulvöspinel. It contains no olivine, and only ~3% augite. Phase compositions are pigeonite (En68‐43Fs27‐48Wo5‐15) and maskelynite An~54‐36, more sodic than most maskelynite in shergottites. Chromite‐ulvöspinel composition plots between the earliest and most fractionated spinel‐group minerals in olivine‐phyric shergottites. NWA 10414 mineralogically resembles the contact facies between Elephant Moraine 79001 lithologic units A and B, with abundant pigeonite phenocrysts, though it is coarser grained. Its most Mg‐rich pigeonite also has a similar composition to the earliest crystallized pyroxenes in several other shergottites, including Shergotty. The Shergotty intercumulus liquid composition crystallizes pigeonite with a similar composition range to NWA 10414 pigeonite, using PETROLOG. Olivine‐phyric shergottite NWA 6234, with a pure magma composition, produces an even better match to this pigeonite composition range, after olivine crystallization. These observations suggest that after the accumulation of olivine from an olivine‐phyric shergottite magma, the daughter liquid could precipitate pigeonite locally to form this pigeonite cumulate, before the crystallization of overlying liquid as a normal basaltic shergottite.

New simulants for martian regolith: Controlling iron variability

1Nisha K.Ramkissoon,1Victoria K.Pearson,1Susanne P.Schwenzer,2Christian Schröder,3Thomas Kirnbauer,2Deborah Wood,1Robert G.W.Seidel,4Michael A.Miller,1Karen Olsson-Francis
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2019.104722]
1STEM Faculty, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
2Biological and Environmental Sciences, Faculty of Natural Sciences, University of Stirling, Stirling, FK9 4LA, UK
3Technische Hochschule Georg Agricola, Herner Straße 45, 44787 Bochum, Germany
4Southwest Research Institute, 6220 Culebra Road, San Antonio, TX, 78238, USA

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Pb isotope evidence for rapid accretion and differentiation of planetary embryos

1J.N.Connelly,1M.Schiller,1M.Bizzarro
Earth and Planetary Science Letters 525, 115722 Link to Article [https://doi.org/10.1016/j.epsl.2019.115722]
1Centre for Star and Planet Formation, GLOBE Institute, University of Copenhagen, Øster Voldgade 5-7, Copenhagen, 1350, Denmark
Copyright Elsevier

Group IVA iron and siliate-iron meteorites record a large range of cooling rates attributed to an impact-related disruption of a molten and differentiated ca. 1000 km diameter planetary embryo of chondritic composition before re-accretion of mainly the metallic core with minor silicates. To better understand the timing of primary accretion, disruption, re-accretion and cooling of the Group IVA parent body, we have determined Pb-Pb and Al-Mg ages for the Group IVA silicate-iron Steinbach meteorite. A Pb-Pb age based on multiple fractions of late-phase, slowly-cooled orthopyroxene from Steinbach yields an absolute age of 4565.47 ± 0.30 Ma corresponding to a relative age of 1.83 ± 0.34 Myr after formation of calcium-aluminium-rich inclusions (CAIs). This is the oldest U-corrected Pb-Pb absolute age for a differentiated meteorite. We use the deficit Al-Mg dating method on one whole rock sample and two mineral separates to produce a model age of 1.3−0.3+0.5 Myr after CAI formation corresponding to the depletion age of Al relative to Mg in the source material for Steinbach. Assuming this fractionation event occurred in the pre-impact parent body, this provides a maximum time after CAI formation for the disruption of the original Group IVA parent body. Together, these ages require that the original parent body accreted very early and differentiated prior to the impact-related break up, re-accretion and cooling between 1.3−0.3+0.5
Myr and 1.83 ± 0.34 Myr after CAI formation. These ages are fully consistent with a growing body of evidence from meteorites and astronomical observation supporting the early and efficient growth of planetary embryos and with numerical models of pebble accretion that predict rapid growth of embryos in the presence of chondrules. This time frame for the efficient formation of planetary embryos by chondrule accretion is inconsistent with a proposed ∼1.5 Myr delay in chondrule formation, a contradiction that is resolved by a non-canonical abundance of 26Al in the inner Solar System during at least the first million years of the protoplanetary disk.

Heating duration of igneous rim formation on a chondrule in the Northwest Africa 3118 CV3oxA carbonaceous chondrite inferred from micro-scale migration of the oxygen isotopes

1Nozomi Matsuda,2Naoya Sakamoto,1,3Shogo Tachibana,1,4Hisayoshi Yurimoto
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.07.006]
1Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
2Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
3UTokyo Organization for Planetary and Space Science (UTOPS), University of Tokyo, Tokyo 113-0033, Japan
4ISAS/JAXA, Sagamihara, Kanagawa, 252-210, Japan
Copyright Elsevier

Due to their common occurrence in various types of chondrites, igneous rims formed on pre-existing chondrules throughout chondrule-forming regions of the solar nebula. Although the peak temperatures are thought to reach similar values to those achieved during chondrule formation events, the heating duration in chondrule rim formation has not been well defined. We determined the two-dimensional chemical and oxygen isotopic distributions in an igneous rim of a chondrule within the Northwest Africa 3118 CV3oxA chondrite with sub-micrometer resolution using secondary ion mass spectrometry and scanning electron microscopy. The igneous rim experienced aqueous alteration on the CV parent body. The aqueous alteration resulted in precipitation of the secondary FeO-rich olivine (Fa40―49) and slightly disturbed the Fe-Mg distribution in the MgO-rich olivine phenocrysts (Fa11―22) at about a 1 µm scale. However, no oxygen isotopic disturbances were observed at a scale greater than 100 nm. The MgO-rich olivine, a primary phase of igneous rim formation, has δ17O = ―6 ± 3‰ and δ18O = ―1 ± 4 ‰, and some grains contain extreme 16O-rich areas (δ17O,□δ18O = ˜―30‰) nearly 10 µm across. We detected oxygen isotopic migration of approximately 1 µm at the boundaries of the extreme 16O-rich areas. Using oxygen self-diffusivity in olivine, the heating time of the igneous rim formation could have continued from several hours to several days at near liquidus temperatures (˜2000 K) in the solar nebula suggesting that the rim formed by a similar flash heating event that formed the chondrules.

Formation and destruction of magnetite in CO3 chondrites and other chondrite groups

1,2Alan E.Rubin,1,3,4YeLia
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.07.009]
1Department of Earth, Planetary & Space Sciences, University of California, Los Angeles, CA, 90095-1567, USA
2Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Bethel, ME, 04217, USA
3Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210034, China
4Chinese Academy of Sciences Center for Excellence in Comparative Planetology, China
Copyright Elsevier

Primitive CO3.00–3.1 chondrites contain ˜2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H2 and reacts with SiO2 to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ≲3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr2O3 in a few R4-6 chondrites. Magnetite grains with high Cr2O3 behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ˜4 vol.% magnetite with substantial Cr2O3 (up to ˜15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7.