¹Jan L.Hellmann,^1,2Thomas S.Kruijer,³James A.Van Orman,¹Knut Metzler,¹Thorsten Kleine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.040]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
²Nuclear & Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue (L-231), Livermore, CA 94550, USA
³Department of Earth, Environmental and Planetary Sciences, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
Copyright Elsevier

Fifteen H, L, and LL ordinary chondrites of petrologic types 4 to 6 have been analyzed for Hf-W isotope systematics to constrain the chronology, internal structure, and thermal history of their parent bodies. For most samples coarse-grained metals plot below the isochrons defined by silicate-dominated fractions which consist of variable mixtures of silicate minerals with tiny metal inclusions. This offset results from an earlier Hf-W closure in the large metal grains and provides a new means for simultaneously determining cooling rates and Hf-W closure ages for individual samples. For most type 5 and 6 samples, cooling rates and Hf-W ages are inversely correlated, indicating that these samples derive from concentrically zoned bodies in which more strongly metamorphosed samples derive from greater depth. These data, therefore, provide strong evidence for a common ‘onion shell’ structure for the H, L, and LL chondrite parent bodies. The cooling rates and Hf-W ages of some type 5 and 6 chondrites overlap, indicating that the Hf-W systematics provide a more robust measure of the thermal history and burial depth of a given sample than the simple petrographic distinction between types 5 and 6. Two type 6 samples deviate from the correlation between cooling rates and Hf-W ages and cooled much faster than expected for their Hf-W age. These samples likely were excavated by impacts that occurred during high-temperature metamorphism and prior to complete closure of the Hf-W system at ∼10 Ma after CAI formation. As these impacts would have disturbed the asteroid’s cooling history, these samples likely derive from different bodies than samples with undisturbed cooling histories, implying that ordinary chondrites derive from more than just three parent bodies. The Hf-W data reveal that metal-silicate fractionation among the H, L, and LL groups occurred between ∼2 and ∼2.7 Ma after CAI formation and, hence, was about coeval to chondrule formation. As both metal-silicate fractionation and chondrule formation occurred prior to chondrite parent body accretion, there should be no ordinary chondrite chondrules that are younger than ∼2.7 Ma. Finally, ordinary chondrite precursors had lower Hf/W ratios than carbonaceous chondrites, suggesting that inner and outer solar system materials, respectively, were chemically distinct even for refractory elements.

^1,2Nicole G.Lunning,¹Timothy J.Mccoy,^3,4Devin L.Schrader,⁵Kazu Nagashima,¹Catherine M.Corrigan,²JulianeGross,⁶AlfredKracher
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.032]
¹Department of Mineral Sciences, Smithsonian Institution, National Museum of Natural History, Washington, DC 20560, USA
²Department of Earth and Planetary Sciences, Rutgers, State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854-8066, USA
³Center for Meteorite Studies, Arizona State University, Tempe AZ 85287
⁴School of Earth and Space Exploration, Arizona State University, Tempe AZ 85287
⁵Hawai‘i Institute of Geophysics & Planetology, University of Hawai‘i at Manoa, 1680 East-West Road, POST 602 Honolulu, HI 96822 USA
⁶15837 Garden View Dr., Apple Valley, MN 55124-7006 USA1
Copyright Elsevier

The petrogenesis of the ungrouped iron meteorite Lewis Cliffs (LEW) 86211 and its proposed pair LEW 86498 has remained elusive in the decades since their discovery in Antarctica. Wasson, 1990, Kracher et al., 1998 noted the enrichment in the siderophile refractory elements, fine-grained texture, and high abundances of sulfides in LEW 86211 as features that are both difficult to explain and that set it apart from other iron meteorites. In this work, we investigate the pairing and formation of these two ungrouped iron meteorites using a combination of petrography, electron microprobe analyses, and secondary ion probe analyses of oxygen-three isotope of olivine. Similarities in petrographic features and phase compositions further support the initial pairing of LEW 86211 and 86498. The bulk composition of LEW 86211 (Wasson, 1990) closely resembles those of separated chondritic metallic components (e.g., Kong and Ebihara, 1997), which indicates this pairing group formed directly from this portion of a chondrite. The metal-sulfide cellular textures and mineral compositional trends are consistent with LEW 86211 and 86498 forming by rapid cooling of the FeNiS immiscible liquid of a larger chondritic impact melt unit. Previous bulk oxygen-three isotope analyses (Clayton and Mayeda, 1996) combined with the in situ oxygen-three isotope analyses from this work are consistent with LEW 86211 having a carbonaceous chondrite provenance. LEW 86211 is most similar to CR chondrites in its oxygen-three isotope signatures, but may not be from an established carbonaceous chondrite group. The silicate inclusions in LEW 86211 and 86498 record evidence of pre-impact metamorphism and later reduction related to contact with the metal-sulfide impact melt liquid. The silicate inclusions appear to have been engulfed by metal-sulfide liquids rather than part of the impact melted unit. Additionally, the size of this sulfide-dominated pairing group compared to the volume of sulfides and metal in unmelted CR chondrite suggests that these meteorites originated from a much larger carbonaceous chondrite impact melt body than has been previously recognized (e.g., Lunning et al., 2016).

Susanne Pfalzner^1,2 and Michele T. Bannister³
Astrophysical Journal Letters 874, L34 Link to Article [DOI: 10.3847/2041-8213/ab0fa0]
¹Jülich Supercomputing Center, Forschungszentrum Jülich, D-52428 Jülich, Germany
²Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
³Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK

The discovery of 1I/’Oumuamua confirmed that planetesimals must exist in great numbers in interstellar space. Originally generated during planet formation, they are scattered from their original systems and subsequently drift through interstellar space. As a consequence they should seed molecular clouds with at least hundred-meter-scale objects. We consider how the galactic background density of planetesimals, enriched from successive generations of star and system formation, can be incorporated into forming stellar systems. We find that at a minimum of the order of 10⁷ ‘Oumuamua-sized and larger objects, plausibly including hundred-kilometer-scale objects, should be present in protoplanetary disks. At such initial sizes, the growth process of these seed planetesimals in the initial gas- and dust-rich protoplanetary disks is likely to be substantially accelerated. This could resolve the tension between accretionary timescales and the observed youth of fully fledged planetary systems. Our results strongly advocate that the population of interstellar planetesimals should be taken into account in future studies of planet formation. As not only the Galaxy’s stellar metallicity increased over time but also the density of interstellar objects, we hypothesize that this enriched seeding accelerates and enhances planetary formation after the first couple of generations of planetary systems.

Prabal Saxena^1,2, Rosemary M. Killen¹, Vladimir Airapetian^1,3, Noah E. Petro¹, Natalie M. Curran^1,4, and Avi M. Mandell¹
Astrophysical Journal Letters 876, L16 Link to Article [DOI: 10.3847/2041-8213/ab18fb]
¹NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²CREST II/University of Maryland, College Park, MD 20742, USA
³American University, Washington, DC 20016, USA
⁴USRA, Columbia, MD, 21046, USA

While the Earth and Moon are generally similar in composition, a notable difference between the two is the apparent depletion in moderately volatile elements in lunar samples. This is often attributed to the formation process of the Moon, and it demonstrates the importance of these elements as evolutionary tracers. Here we show that paleo space weather may have driven the loss of a significant portion of moderate volatiles, such as sodium and potassium, from the surface of the Moon. The remaining sodium and potassium in the regolith is dependent on the primordial rotation state of the Sun. Notably, given the joint constraints shown in the observed degree of depletion of sodium and potassium in lunar samples and the evolution of activity of solar analogs over time, the Sun is highly likely to have been a slow rotator. Because the young Sun’s activity was important in affecting the evolution of planetary surfaces, atmospheres, and habitability in the early Solar System, this is an important constraint on the solar activity environment at that time. Finally, as solar activity was strongest in the first billion years of the Solar System, when the Moon was most heavily bombarded by impactors, evolution of the Sun’s activity may also be recorded in lunar crust and would be an important well-preserved and relatively accessible record of past Solar System processes.