¹V. Vinogradoff, ²C. Le Guillou, ³S. Bernarda, L. Binet, ⁴P. Cartigny, ⁵A.J. Brearley, ¹L. Remusat
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.009]
¹Institut de minéralogie, de physique des matériaux et de cosmochimie (IMPMC), Sorbonne Universités, UMR CNRS 7590, MNHN, UPMC, UMR IRD 206, CP 52, 57 rue Cuvier, 75005 Paris, France
²Unité matériaux et transformation (UMET), CNRS UMR 8207, Université Lille 1, France
³PSL Research University, Chimie-ParisTech, Institut de Recherche de Chimie-Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France
⁴Institut de physique du globe de Paris (IPGP), Sorbonne Paris Cité, Université Paris Diderot, UMR CNRS 7154, 1 rue Jussieu, 75238 Paris cedex 05, France
⁵Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

Unravelling the origin of organic compounds that were accreted into asteroids requires better constraining the impact of asteroidal hydrothermal alteration on their isotopic signatures, molecular structures, and spatial distribution. Here, we conducted a multi-scale/multi-technique comparative study of the organic matter (OM) from two CM chondrites (that originate from the same parent body or from identical parent bodies that accreted the same mixture of precursors) and underwent a different degree of hydrothermal alteration: Paris (a weakly altered CM chondrite – CM 2.8) and Murchison (a more altered one – CM 2.5). The Paris insoluble organic matter (IOM) shows a higher aliphatic/aromatic carbon ratio, a higher radical abundance and a lower oxygen content than the Murchison IOM. Analysis of the OM in situ shows that two texturally distinct populations of organic compounds are present within the Paris matrix: sub-micrometric individual OM particles and diffuse OM finely distributed within phyllosilicates and amorphous silicates. These results indicate that hydrothermal alteration on the CM parent body induced aromatization and oxidation of the IOM, as well as a decrease in radical and nitrogen contents. Some of these observations were also reported by studies of variably altered fragment of Tagish Lake (C2), although the hydrothermal alteration of the OM in Tagish Lake was apparently much more severe. Finally, comparison with data available in the literature suggests that the parent bodies of other chondrite petrologic groups could have accreted a mixture of organic precursors different from that accreted by the parent body of CMs.

^1,2Rebecca Bast et al. (>10)*
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.043]
¹Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, D-48149 Münster, Germany
²Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Str. 49b, D-50674 Köln, Germany
*Find the extensive, full author and affiliation list on the publishers website
Copyright Elsevier

The long-lived 176Lu-176Hf and 147Sm-143Nd radioisotope systems are commonly used chronometers, but when applied to meteorites, they can reveal disturbances. Specifically, Lu-Hf isochrons commonly yield dates up to ∼300 Myr older than the solar system and varying initial 176Hf/177Hf values. We investigated this problem by attempting to construct mineral and whole rock isochrons for eucrites and angrites. Meteorites from different parent bodies exhibit similar disturbance features suggesting that a common process is responsible. Minerals scatter away from isochron regressions for both meteorite classes, with low-Hf phases such as plagioclase and olivine typically being most displaced above (or left of) reference isochrons. Relatively Hf-rich pyroxene is less disturbed but still to the point of steepening Lu-Hf errorchrons. Using our Lu-Hf and Sm-Nd data, we tested various Hf and Lu redistribution scenarios and found that decoupling of Lu/Hf from 176Hf/177Hf must postdate the accumulation of significant radiogenic 176Hf. Therefore early irradiation or diffusion cannot explain the excess 176Hf. Instead, disturbed meteorite isochrons are more likely caused by terrestrial weathering, contamination, or common laboratory procedures. The partial dissolution of phosphate minerals may predominantly remove rare earth elements including Lu, leaving relatively immobile and radiogenic Hf behind. Robust Lu-Hf (and improved Sm-Nd) meteorite geochronology will require the development of chemical or physical methods for removing unsupported radiogenic Hf and silicate-hosted terrestrial contaminants without disturbing parent-daughter ratios.

^1,2Maria Lugaro, ^2,3Amanda I. Karakas, ¹Mária Pető, ¹Emese Plachy
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.006]
¹Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, H-1121 Budapest, Hungary
²Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800, Australia
³Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
Copyright Elsevier

We compare literature data for the isotopic ratios of Zr, Sr, and Ba from analysis of single meteoritic stardust silicon carbide (SiC) grains to new predictions for the slow neutron-capture process (the s process) in metal-rich asymptotic giant branch (AGB) stars. The models have initial metallicities Z=0.014 (solar) and Z=0.03 (twice-solar) and initial masses 2 – 4.5 M⊙, selected such as the condition C/O>1 for the formation of SiC is achieved. Because of the higher Fe abundance, the twice-solar metallicity models result in a lower number of total free neutrons released by the 13C(α,n)16O neutron source. Furthermore, the highest-mass (4 – 4.5 M⊙) AGB stars of twice-solar metallicity present a milder activation of the 22Ne(α,n)25Mg neutron source than their solar metallicity counterparts, due to cooler temperatures resulting from the effect of higher opacities. They also have a lower amount of the 13C neutron source than the lower-mass models, following their smaller He-rich region. The combination of these different effects allows our AGB models of twice-solar metallicity to provide a match to the SiC data without the need to consider large variations in the features of the 13C neutron source nor neutron-capture processes different from the s process. This raises the question if the AGB parent stars of meteoritic SiC grains were in fact on average of twice-solar metallicity. The heavier-than-solar Si and Ti isotopic ratios in the same grains are in qualitative agreement with an origin in stars of super-solar metallicity because of the chemical evolution of the Galaxy. Further, the SiC dust mass ejected from C-rich AGB stars is predicted to significantly increase with increasing the metallicity.

¹Simone Gerber, ¹Christoph Burkhardt, ¹Gerrit Budde, ¹Knut Metzler, ¹Thorsten Kleine
The Astrophysical Journal Letters 841 L17 Link to Article [https://doi.org/10.3847/2041-8213/aa72a2]
¹Institut für Planetologie, University of Münster, Wilhelm Klemm-Straße 10, D-48149 Münster, Germany

Chondrules formed by the melting of dust aggregates in the solar protoplanetary disk and as such provide unique insights into how solid material was transported and mixed within the disk. Here, we show that chondrules from enstatite and ordinary chondrites show only small 50Ti variations and scatter closely around the 50Ti composition of their host chondrites. By contrast, chondrules from carbonaceous chondrites have highly variable 50Ti compositions, which, relative to the terrestrial standard, range from the small 50Ti deficits measured for enstatite and ordinary chondrite chondrules to the large 50Ti excesses known from Ca–Al-rich inclusions (CAIs). These 50Ti variations can be attributed to the addition of isotopically heterogeneous CAI-like material to enstatite and ordinary chondrite-like chondrule precursors. The new Ti isotopic data demonstrate that isotopic variations among carbonaceous chondrite chondrules do not require formation over a wide range of orbital distances, but can instead be fully accounted for by the incorporation of isotopically anomalous “nuggets” into chondrule precursors. As such, these data obviate the need for disk-wide transport of chondrules prior to chondrite parent body accretion and are consistent with formation of chondrules from a given chondrite group in localized regions of the disk. Finally, the ubiquitous presence of 50Ti-enriched material in carbonaceous chondrites and the lack of this material in the non-carbonaceous chondrites support the idea that these two meteorite groups derive from areas of the disk that remained isolated from each other, probably through the formation of Jupiter.

^1,2,3,4Pierre Haenecour, ^1,3Christine Floss, ⁴Thomas J. Zega, ^1,3Thomas K. Croat, ^2,3Alian Wang, ^2,3Bradley L. Jolliff, ^2,3Paul Carpenter
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.004]
¹Laboratory for Space Sciences and Physics Department, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130-4899, USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130-4899, USA
³McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130-4899, USA
⁴Lunar and Planetary Laboratory and Department of Materials Science and Engineering, University of Arizona, 1629 E. University Blvd, Tucson, AZ 85721-0092, USA
Copyright Elsevier

To investigate the origin of fine-grained rims around chondrules (FGRs), we compared presolar grain abundances, elemental compositions and mineralogies in fine-grained interstitial matrix material and individual FGRs in the primitive CO3.0 chondrites Allan Hills A77307, LaPaz Icefield 031117 and Dominion Range 08006. The observation of similar overall O-anomalous (∼155 ppm) and C-anomalous grain abundances (∼40 ppm) in all three CO3.0 chondrites suggests that they all accreted from a nebular reservoir with similar presolar grain abundances. The presence of presolar silicate grains in FGRs combined with the observation of similar estimated porosity between interstitial matrix regions and FGRs in LAP 031117 and ALHA77307, as well as the identification of a composite FGR (a small rimmed chondrule within a larger chondrule rim) in ALHA77307, all provide evidence for a formation of FGRs by accretion of dust grains onto freely-floating chondrules in the solar nebula before their aggregation into their parent body asteroids. Our study also shows systematically lower abundances of presolar silicate grains in the FGRs than in the matrix regions of CO3 chondrites, while the abundances of SiC grains are the same in all areas, within errors. This trend differs from CR2 chondrites in which the presolar silicate abundances are higher in the FGRs than in the matrix, but similar to each other within 2σ errors. This observation combined with the identification of localized (micrometer-scaled) aqueous alteration in a FGR of LAP 031117 suggests that the lower abundance of presolar silicates in FGRs reflects pre-accretionary aqueous alteration of the fine-grained material in the FGRs. This pre-accretionary alteration could be due to either hydration and heating of freely floating rimmed chondrules in icy regions of the solar nebula or melted water ice associated with 26Al-related heating inside precursor planetesimals, followed by aggregation of FGRs into the CO chondrite parent-body.

^1,2,3,4Adam R. Sarafian, ^1,4Sune G. Nielsen, ^1,5Horst R. Marschall, ¹Glenn A. Gaetani, ⁶Erik H. Hauri, ⁷Kevin Righter, ^2,3Emily Sarafian
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.001]
¹Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02540
²Massachusetts Institute of Technology-Woods Hole Oceanographic Institution Joint Program in Oceanography/Applied Ocean Science and Engineering, Cambridge, MA 02139
³Geology and Geophysics Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543
⁴NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, USA
⁵Institut für Geowissenschaften, Goethe Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, Germany
⁶Carnegie Institution for Science, Department of Terrestrial Magnetism, Washington, DC 20015
⁷NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058
Copyright Elsevier

Earth and the other rocky bodies that make up the inner solar system are systematically depleted in hydrogen (H) and other cosmochemically volatile elements (e.g., carbon (C), fluorine (F), chlorine (Cl), and thallium (Tl)) relative to primitive undifferentiated meteorites known as carbonaceous chondrites. If we are to understand how and when Earth gained its life-essential elements, it is critical to determine the timing, flux, and nature of the delivery of condensed volatiles into the presumed hot and dry early inner solar system. Here we present evidence preserved in ancient basaltic angrite meteorites for an addition of volatiles to the hot and dry inner solar system within the first two million years of solar system history. Our data demonstrate that the angrite parent body was enriched in highly volatile elements (H, C, F, and Tl) relative to those predicted on the basis of the angrite parent body’s overall volatile depletion trend (e.g., H is enriched by up to a factor of 106).This relative enrichment is best explained by mixing of extremely volatile-depleted material, located well inside the snow line, with volatile-rich material derived from outside the snow line.

¹Shigeru Wakita, ²Takaya Nozawa, ³Yasuhiro Hasegawa
The Astrophysical Journal 836, 106 Link to Article [https://doi.org/10.3847/1538-4357/aa5b8c]
¹Center for Computational Astrophysics, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
²Division of Theoretical Astronomy, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Presolar grains are small particles found in meteorites through their isotopic compositions, which are considerably different from those of materials in the solar system. If some isotopes in presolar grains diffused out beyond their grain sizes when they were embedded in parent bodies of meteorites, their isotopic compositions could be washed out, and hence the grains could no longer be identified as presolar grains. We explore this possibility for the first time by self-consistently simulating the thermal evolution of planetesimals and the diffusion length of 18O in presolar silicate grains. Our results show that presolar silicate grains smaller than ~0.03 μm cannot keep their original isotopic compositions even if the host planetesimals experienced a maximum temperature as low as 600 °C. Since this temperature corresponds to that experienced by petrologic type 3 chondrites, isotopic diffusion can constrain the size of presolar silicate grains discovered in such chondrites to be larger than ~0.03 μm. We also find that the diffusion length of 18O reaches ~0.3–2 μm in planetesimals that were heated up to 700–800°C. This indicates that, if the original size of presolar grains spans a range from ~0.001 μm to ~0.3 μm like that in the interstellar medium, then the isotopic records of the presolar grains may be almost completely lost in such highly thermalized parent bodies. We propose that isotopic diffusion could be a key process to control the size distribution and abundance of presolar grains in some types of chondrites.

¹J. C. Gómez Martín, ¹D. L. Bones, ¹J. D. Carrillo-Sánchez, ¹A. D. James, ²J. M. Trigo-Rodríguez, ³B. Fegley Jr., ¹J. M. C. Plane
The Astrophysical Journal 836, 212 Link to Article [https://doi.org/10.3847/1538-4357/aa5c8f]
¹School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
²Meteorites, Minor Bodies and Planetary Science Group, Institute of Space Sciences (CSIC-IEEC). Campus UAB, C/Can Magrans s/n, E-08193 Cerdanyola del Vallés (Barcelona), Spain
³Washington University, St. Louis, MO, USA

A newly developed laboratory, Meteoric Ablation Simulator (MASI), is used to test model predictions of the atmospheric ablation of interplanetary dust particles (IDPs) with experimental Na, Fe, and Ca vaporization profiles. MASI is the first laboratory setup capable of performing time-resolved atmospheric ablation simulations, by means of precision resistive heating and atomic laser-induced fluorescence detection. Experiments using meteoritic IDP analogues show that at least three mineral phases (Na-rich plagioclase, metal sulfide, and Mg-rich silicate) are required to explain the observed appearance temperatures of the vaporized elements. Low melting temperatures of Na-rich plagioclase and metal sulfide, compared to silicate grains, preclude equilibration of all the elemental constituents in a single melt. The phase-change process of distinct mineral components determines the way in which Na and Fe evaporate. Ca evaporation is dependent on particle size and on the initial composition of the molten silicate. Measured vaporized fractions of Na, Fe, and Ca as a function of particle size and speed confirm differential ablation (i.e., the most volatile elements such as Na ablate first, followed by the main constituents Fe, Mg, and Si, and finally the most refractory elements such as Ca). The Chemical Ablation Model (CABMOD) provides a reasonable approximation to this effect based on chemical fractionation of a molten silicate in thermodynamic equilibrium, even though the compositional and geometric description of IDPs is simplistic. Improvements in the model are required in order to better reproduce the specific shape of the elemental ablation profiles.

¹Andreas Morlok, ²Stephan Klemme, ¹Iris Weber, ¹Aleksandra Stojic, ³Martin Sohn, ¹Harald Hiesinger
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.05.024]
¹Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Institut für Mineralogie, Corrensstraße 24, 48149 Münster, Germany
³Hochschule Emden/Leer, Constantiaplatz 4, 26723 Emden, Germany
Copyright Elsevier

In a study to provide ground-truth data for mid-infrared observations of the surface of Mercury with the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument onboard the ESA/JAXA BepiColombo mission, we have studied 17 synthetic glasses. These samples have the chemical compositions of characteristic Hermean surface areas based on MESSENGER data.

The samples have been characterized using optical microscopy, EMPA and Raman spectroscopy. Mid-infrared spectra have been obtained from polished thin sections using Micro-FTIR, and of powdered size fractions of bulk material (0-25, 25-63, 93-125 and 125-250 μm) in the 2.5-18 µm range.

The synthetic glasses display mostly spectra typical for amorphous materials with a dominating, single Reststrahlen Band (RB) at 9.5 µm – 10.7 µm. RB Features of crystalline forsterite are found in some cases at 9.5-10.2 µm, 10.4-11.2 µm, and at 11.9 µm. Dendritic crystallization starts at a MgO content higher than 23 wt.% MgO.

The Reststrahlen Bands, Christiansen Features (CF), and Transparency Features (TF) shift depending on the SiO2 and MgO contents. Also a shift of the Christiansen Feature of the glasses compared with the SCFM (SiO2/(SiO2+CaO+FeO+MgO)) index is observed. This shift could potentially help distinguish crystalline and amorphous material in remote sensing data. A comparison between the degree of polymerization of the glass and the width of the characteristic strong silicate feature shows a weak positive correlation.

A comparison with a high-quality mid-IR spectrum of Mercury shows some moderate similarity to the results of this study, but does not explain all features.

¹Xiaobin Cao, ¹Huiming Bao
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.028]
¹Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA
Copyright Elsevier

The equilibrium isotope fractionation factor αeq is a fundamental parameter in the study of stable isotope effects. Experimentally, it has been difficult to establish that a system has attained equilibrium. The three-isotope method, using the initial trajectory of changing isotope ratios (e.g. 16O, 17O, and 18O) to deduce the final equilibrium point of isotope exchange, has long been hailed as the most rigorous experimental approach. However, over the years some researchers have cautioned on the limitations of this method, but the foundation of three-isotope method has not been properly examined and the method is still widely used in calibrating αeq for both traditional and increasingly non-traditional isotope systems today. Here, using water-water and dissolved CO2-water oxygen exchange as model systems, we conduct an isotopologues-specific kinetic analysis of the exchange processes and explore the underlying assumptions and validity of the three-isotope method. We demonstrate that without knowing the detailed exchange kinetics a priori the three-isotope method cannot lead to a reliable αeq. For a two-reservoir exchanging system, α determined by this method may be αeq, kinetic isotope effect, or apparent kinetic isotope effect, which can all bear different values. When multiple reservoirs exist during exchange, the evolving trajectory can be complex and hard to predict. Instead of being a tool for αeq determination, three-isotope method should be used as a tool for studying kinetic isotope effect, apparent kinetic isotope effect, and detailed exchange kinetics in diverse systems.