¹Johnson, B.C.,²Sori, M.M.,³Evans, A.J.
Nature Astronomy (in Press) Link to Article [DOI: 10.1038/s41550-019-0885-x]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, United States
²Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, United States
³Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, United States

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¹Ke Zhu (朱柯),^1,2Frédéric Moynier,³Daniel Wielandt,³Kirsten K. Larsen,⁴Jean-Alix Barrat,³Martin Bizzarro
The Astrophysical Journal Letters 877, L13 Link to Article [DOI
https://doi.org/10.3847/2041-8213/ab2044]
¹Institut de Physique du Globe de Paris, Université de Paris, CNRS, 1 rue Jussieu, Paris F-75005, France
²Institut Universitaire de France, 103 boulevard Saint-Michel, Paris F-75005, France
³Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5–7, Copenhagen DK-1350, Denmark
⁴Laboratoire Géosciences Océan (UMR CNRS 6538), Université de Bretagne Occidentale et Institut Universitaire Européen de la Mer, Place Nicolas Copernic, F-29280 Plouzané, France

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¹Yasuhiro Hasegawa,²Bradley M. S. Hansen,¹Gautam Vasisht
The Astrophysical Journal Letters, 876, L32 Link to Article [https://doi.org/10.3847/2041-8213/ab1b5a]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Mani L. Bhaumik Institute for Theoretical Physics, Department of Physics & Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA

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¹W. Rapin,^1,2B. L. Ehlmann,³G. Dromart,⁴J. Schieber,¹N. H. Thomas,¹W. W. Fischer,¹V. K. Fox,¹N. T. Stein,⁵M. Nachon,⁶B. C. Clark,⁷L. C. Kah,⁸L. Thompson,¹H. A. Meyer,⁹T. S. J. Gabriel,⁹C. Hardgrove,¹⁰ N. Mangold,¹¹F. Rivera-Hernandez,¹²R. C. Wiens,¹³A. R. Vasavada
Nature Geoscience 12, 889-895 Link to Article [https://doi.org/10.1038/s41561-019-0458-8]
¹California Institute of Technology, Pasadena, CA, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
³Laboratoire de Géologie de Lyon, Université de Lyon, Lyon, France
⁴Indiana University, Bloomington, IN, USA
⁵Texas A&M University, College Station, TX, USA
⁶Space Science Institute, Boulder, CO, USA
⁷University of Tennessee, Knoxville, TN, USA
⁸University of New Brunswick, Fredericton, Canada
⁹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
¹⁰Laboratoire de Planétologie et Géodynamique, UMR6112, CNRS,
Université Nantes, Université Angers, Nantes, France
¹¹Dartmouth College, Hanover, NH, USA
¹²Los Alamos National Laboratory, Los Alamos, NM, USA
¹³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

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^1,2Michael J. Henehan,^3,4Andy Ridgwell,^1,5Ellen Thomas,¹Shuang Zhang,⁶Laia Alegret,⁷Daniela N. Schmidt,⁸James W. B. Rae,^9,10James D. Witts,⁹Neil H. Landman,¹¹Sarah E. Greene,¹²Brian T. Huber,¹James R. Super,¹Noah J. Planavsky,¹Pincelli M. Hull
Proceedings of the National Academy of Sciences of teh United States of America (PNAS) (in Press) Link to to Article [https://doi.org/10.1073/pnas.1905989116]
¹Department of Geology & Geophysics, Yale University, New Haven, CT 06520;
²Section 3.3, Deutsches GeoForschungsZentrum GFZ, 14473 Potsdam, Germany;
³School of Geographical Sciences, Bristol University, Bristol BS8 1SS, United Kingdom;
⁴Department of Earth Sciences, University of California, Riverside, CA 92521;
⁵Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459;
⁶Instituto Universitario de Investigación en Ciencias Ambientales de Aragón, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain;
⁷School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom;
⁸School of Earth & Environmental Sciences, University of St. Andrews, St. Andrews KY16 9AL, United Kingdom;
⁹Division of Paleontology, American Museum of Natural History, New York, NY 10024;
¹⁰Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131;
¹¹School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, United Kingdom;
¹²Department of Paleobiology, Smithsonian Institution, Washington, DC 20560

Mass extinction at the Cretaceous–Paleogene (K-Pg) boundary coincides with the Chicxulub bolide impact and also falls within the broader time frame of Deccan trap emplacement. Critically, though, empirical evidence as to how either of these factors could have driven observed extinction patterns and carbon cycle perturbations is still lacking. Here, using boron isotopes in foraminifera, we document a geologically rapid surface-ocean pH drop following the Chicxulub impact, supporting impact-induced ocean acidification as a mechanism for ecological collapse in the marine realm. Subsequently, surface water pH rebounded sharply with the extinction of marine calcifiers and the associated imbalance in the global carbon cycle. Our reconstructed water-column pH gradients, combined with Earth system modeling, indicate that a partial ∼50% reduction in global marine primary productivity is sufficient to explain observed marine carbon isotope patterns at the K-Pg, due to the underlying action of the solubility pump. While primary productivity recovered within a few tens of thousands of years, inefficiency in carbon export to the deep sea lasted much longer. This phased recovery scenario reconciles competing hypotheses previously put forward to explain the K-Pg carbon isotope records, and explains both spatially variable patterns of change in marine productivity across the event and a lack of extinction at the deep sea floor. In sum, we provide insights into the drivers of the last mass extinction, the recovery of marine carbon cycling in a postextinction world, and the way in which marine life imprints its isotopic signal onto the geological record.

¹Yves Marrocchi,¹Johan Villeneuve,²Emmanuel Jacquet,¹Maxime Piralla,³Marc Chaussidon
Proceedings of the National Academy of Sciences of the United States of America (PNAS) (in Press) Link to Article [DOI:https://doi.org/10.1073/pnas.1912479116]
¹Centre de Recherches Pétrographiques et Géochimiques (CRPG), CNRS, Université de Lorraine, UMR 7358, 54501 Vandoeuvre-lès-Nancy, France;
²Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), CNRS & Muséum national d’Histoire naturelle, UMR 7590, 75005 Paris, France;
³Institut de Physique du Globe de Paris, Université de Paris, CNRS, 75238 Paris, France

Chondritic meteorites are composed of primitive components formed during the evolution of the Solar protoplanetary disk. The oldest of these components formed by condensation, yet little is known about their formation mechanism because of secondary heating processes that erased their primordial signature. Amoeboid Olivine Aggregates (AOAs) have never been melted and underwent minimal thermal annealing, implying they might have retained the conditions under which they condensed. We performed a multiisotope (O, Si, Mg) characterization of AOAs to constrain the conditions under which they condensed and the information they bear on the structure and evolution of the Solar protoplanetary disk. High-precision silicon isotopic measurements of 7 AOAs from weakly metamorphosed carbonaceous chondrites show large, mass-dependent, light Si isotope enrichments (–9‰ < δ³⁰Si < –1‰). Based on physical modeling of condensation within the protoplanetary disk, we attribute these isotopic compositions to the rapid condensation of AOAs over timescales of days to weeks. The same AOAs show slightly positive δ²⁵Mg that suggest that Mg isotopic homogenization occurred during thermal annealing without affecting Si isotopes. Such short condensation times for AOAs are inconsistent with disk transport timescales, indicating that AOAs, and likely other high-temperature condensates, formed during brief localized high-temperature events.

¹Steven J.Desch,²Katharine L.Robinson
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125546]
¹School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe AZ 85287, United States
²Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston TX 77058, United States
Copyright Elsevier

The Moon is thought to have formed after a planetary embryo, known as Theia, collided with the proto-Earth 4.5 billion years ago. This so-called Giant Impact was the last major event during Earth’s accretion, and its effects on the composition of the Earth and the newly forming Moon would be measureable today. Recent work on lunar samples has revealed that the Moon’s water was not lost as a result of this giant impact. Instead, the Moon appears to contain multiple hydrogen reservoirs with diverse deuterium-to-hydrogen (D/H) ratios. For the first time, we incorporate hydrogen isotopic measurements of lunar samples to help constrain the composition of Theia. We show that the Moon incorporated very low-D/H (δD ≈ -750‰) materials that only could have derived from solar nebula H₂ ingassed into the magma ocean of a large (∼0.4 M_E) planetary embryo that was largely devoid of chondritic water. We infer Theia was a very large body comparable in size to the proto-Earth, and was composed almost entirely of enstatite chondrite-like material. These conclusions limit the type of impact to a “merger” model of similarly-sized bodies, or possibly a “hit-and-run” model, and they rule out models that mix isotopes too effectively.

¹Maxime Piralla,¹Yves Marrocchi,^2,3Maximilien J.Verdier-Paoletti,^1,4Lionel G.Vacher,¹Johan Villeneuve,¹Laurette Piani,²David V.Bekaert,¹Matthieu Gounelle
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.10.041]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
²IMPMC, CNRS & Muséum national d’Histoire naturelle, UMR 7590, CP52, 57 rue Cuvier, 75005 Paris, France
³DTM, Carnegie Institution for Science, Washington, DC, 20015, USA
⁴Department of Physics, Washington University, St. Louis, MO, 63130, USA
Copyright Elsevier

As the chemical compositions of CI chondrites closely resemble that of the Sun’s photosphere, their oxygen isotopic compositions represent a powerful tool to constrain the origin and dynamics of dust and water ice grains in the protoplanetary disk. However, parent-body alteration processes make straightforward estimation of the primordial isotopic compositions of CI chondritic water and anhydrous minerals difficult. In this contribution, we used in-situ SIMS measurements to determine the oxygen isotope compositions of mechanically isolated olivine and carbonate grains from the CI chondrite Orgueil and carbonates in a polished section of the CI chondrite Ivuna. Most CI olivine grains have Earth-like O isotopic compositions (Δ¹⁷O ≈ 0‰) plotting at the intersection of the terrestrial fractionation line and the primitive chondrule minerals line. Ca-carbonates from Orgueil and Ivuna define a trend with δ¹⁷O = (0.50 ± 0.05) × δ¹⁸O + (0.9 ± 1.4) that differs from mass-independent variations observed in secondary phases of other carbonaceous chondrites. These data show that CIs are chemically solar but isotopically terrestrial for oxygen isotopes. This supports models suggesting that primordial Solar System dust was ¹⁶O-poor (Δ¹⁷O ≈ 0‰) relative to the ¹⁶O-rich nebular gas. Based on results, mass balance calculations reveal that the pristine O isotopic compositions of carbonaceous chondrite matrices differ significantly from the CI composition, except for CR chondrites (calculated Δ¹⁷O values of CM, CO, CV and CR matrices being –3.97 ± 1.19‰, –4.33 ± 1.45‰, –7.95 ± 1.95‰, and –0.07 ± 1.16‰, respectively). This confirms an open chondrule-matrix system with respect to oxygen isotopes where chondrule compositions reflect complex processes of chondrule precursor recycling and gas-melt interactions. As the Mg-Si-Fe chondrule budget is also partially controlled by gas-melt interactions, the complementary formation of chondrules and matrix from a single solar-like reservoir −if it exists− require that (i) this reservoir must have been in a closed system with the gas or (ii) the gas had a CI composition to satisfy the elemental mass balance.

^1,2J.Guignard,¹G.Quitté,²M.Méheut,¹M.J.Toplis,²F.Poitrasson,³D.Connetable,⁴M.Roskosz
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.10.028]
¹IRAP, Université de Toulouse, CNRS, UPS, CNES, Toulouse, France
²GET, Université de Toulouse, CNRS, UPS, IRD, CNES, Toulouse, France
³CIRIMAT, CNRS, INP, ENSIACET, 4 allée Emile Monso, BP44362, 31030 Toulouse cedex 4, France
⁴IMPMC, CNRS, UMR 7590, Sorbonne Universités, Université Pierre et Marie Curie, Muséum National d’Histoire Naturelle, CP 52, 57 rue Cuvier, Paris F-75231, France
Copyright Elsevier

Metal-silicate fractionation of nickel isotopes has been experimentally quantified at 1623 K, with oxygen fugacities varying from 10^-8.2 to 10^-9.9 atm and for run durations from 0.5 to 1 h. Both kinetic and equilibrium fractionations have been studied. A wire loop set-up was used in which the metal reservoir is a pure nickel wire holding a silicate melt droplet of anorthite-diopside eutectic composition. During the course of the experiment, diffusion of nickel from the wire to the silicate occurred. The timescale to reach chemical equilibrium was fO₂ dependent and decreased from 17 to 1 hour, as conditions became more reducing.

The isotopic composition of each reservoir was determined by Multicollector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICPMS) after Ni purification. The isotopic composition was found to be constant in the metallic wire, which therefore behaved as an infinite reservoir. On the contrary, strong kinetic fractionation was observed in the silicate melt (δNi down to -0.98 ‰.amu^-1 relative to the standard). Isotopic equilibrium was typically reached after 24 hours. For equilibrated samples at 1623 K, no metal-silicate fractionation was observed within uncertainty (2SD), with ΔNi_{Metal-Silicate} = 0.02 ± 0.04 ‰.amu^-1.

Theoretical calculations of metal-silicate isotope fractionation at equilibrium were also performed on different metal-silicate systems. These calculations confirm (1) the absence of fractionation at high temperature and (2) a weak temperature dependence for Ni isotopic fractionation for the metal-olivine and metal-pyroxene pairs with the metal being slightly lighter isotopically.

Our experimental data were finally compared with natural samples. Some mesosiderites (stony-iron meteorites) show a ΔNi_{Metal-Silicate} close to experimental values at equilibrium, whereas others exhibit positive metal-silicate fractionation that could reflect kinetic processes. Conversely, pallasites display a strong negative metal-silicate fractionation. This most likely results from kinetic processes with Ni diffusion from the silicate to the metal phase due to a change of Ni partition coefficient during cooling. In this respect we note that in these pallasites, iron isotopes show metal-silicate fractionation that is opposite direction to Ni, supporting the idea of kinetic isotope fractionation, associated with Fe-Ni interdiffusion.

^1,2Lidia Pittarello,^1,3Seann McKibbin,⁴Akira Yamaguchi,^5,6Gang Ji,⁵Dominique Schryvers,⁷Vinciane Debaille,⁷Philippe Claeys
American Mineralogist 104, 1663-1672 Link to Article [https://doi.org/10.2138/am-2019-7001]
¹Analytical, Environmental, and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
²Department of Mineralogy and Petrography, Natural History Museum Vienna, Burgring 7, A-1010 Vienna, Austria.
³Geowissenschaftliches Zentrum, Georg-August Universität, Goldschmidtstraße 1, 37073 Göttingen, Germany.
⁴National Institute of Polar Research, Antarctic Meteorite Research Center, 10-3 Midoricho, Tachikawa, Japan
⁵Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
⁶University of Lille, CNRS, INRA, ENSCL, UMR 8207, UMET, Unité Matériaux et Transformations, F-59000 Lille, France.
⁷Laboratoire G-Time (Géochemie: Traçage isotopique, minéralogique et élémentaire), Université Libre de Bruxelles, Av. F.D. Roosevelt 50, 1050 Brussels, Belgium B-1050 Brussels, Belgium
Copyright: The Mineralogical Society of America

Mesosiderite meteorites consist of a mixture of crustal basaltic or gabbroic material and metal. Their formation process is still debated due to their unexpected combination of crust and core materials, possibly derived from the same planetesimal parent body, and lacking an intervening mantle component. Mesosiderites have experienced an extremely slow cooling rate from ca. 550 °C, as recorded in the metal (0.25–0.5 °C/Ma). Here we present a detailed investigation of exsolution features in pyroxene from the Antarctic mesosiderite Asuka (A) 09545. Geothermobarometry calculations, lattice parameters, lamellae orientation, and the presence of clinoenstatite as the host were used in an attempt to constrain the evolution of pyroxene from 1150 to 570 °C and the formation of two generations of exsolution lamellae. After pigeonite crystallization at ca. 1150 °C, the first exsolution process generated the thick augite lamellae along (100) in the temperature interval 1000–900 °C. By further cooling, a second order of exsolution lamellae formed within augite along (001), consisting of monoclinic low-Ca pyroxene, equilibrated in the temperature range 900–800 °C. The last process, occurring in the 600–500 °C temperature range, was likely the inversion of high to low pigeonite in the host crystal, lacking evidence for nucleation of orthopyroxene.

The formation of two generations of exsolution lamellae, as well as of likely metastable pigeonite, suggest non-equilibrium conditions. Cooling was sufficiently slow to allow the formation of the lamellae, their preservation, and the transition from high to low pigeonite. In addition, the preservation of such fine-grained lamellae limits long-lasting, impact reheating to a peak temperature lower than 570 °C. These features, including the presence of monoclinic low-Ca pyroxene as the host, are reported in only a few mesosiderites. This suggests a possibly different origin and thermal history from most mesosiderites and that the crystallography (i.e., space group) of low-Ca pyroxene could be used as parameter to distinguish mesosiderite populations based on their cooling history.