^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.

¹Gregory A. Brennecka, ¹Thorsten Kleine
The Astrophysical Journal Letters 837, L9 Link to Article [https://doi.org/10.3847/2041-8213/aa61a2]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany

Precise knowledge of the abundances of short-lived radionuclides at the start of the solar system leads to fundamental information about the stellar environment of solar system formation. Previous investigations of the short-lived ${}^{135}\mathrm{Cs}\,\to {}^{135}\mathrm{Ba}$ system (t 1/2 = 2.3 Ma) have resulted in a range of calculated initial amounts of 135Cs, with most estimates elevated to a level that requires extraneous input of material to the protoplanetary disk. Such an array of proposed 135Cs/133Cs initial solar system values has severely restricted the system’s use as both a possible chronometer and as an informant about supernovae input. However, if 135Cs was as abundant in the early solar system as previously proposed, the resulting deficits in its daughter product 135Ba would be easily detectable in volatile-depleted parent bodies (i.e., having sub-chondritic Cs/Ba) from the very early solar system. In this work, we show that angrites and eucrites, which were volatile-depleted within ~1 million years of the start of the solar system, do not possess deficits in 135Ba compared to other planetary bodies. From this, we calculate an upper limit for the initial 135Cs/133Cs of 2.8 × 10−6, well below previous estimates. This significantly lower initial 135Cs/133Cs ratio now suggests that all of the 135Cs present in the early solar system was inherited simply from galactic chemical evolution and no longer requires an addition from an external stellar source such as an asymptotic giant branch star or SN II, corroborating evidence from several other short-lived radionuclides.

¹Conel M. O’D. Alexander,¹Larry R. Nittler,¹Jemma Davidson,²Fred J. Ciesla
Meteoritcs & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12891]
¹Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC, USA
²Department of Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois, USA
Published by arrangement with John Wiley & Sons

The timing and extent to which the initial interstellar material was thermally processed provide fundamental constraints for models of the formation and early evolution of the solar protoplanetary disk. We argue that the nonsolar (solar Δ17O ≈ −29‰) and near-terrestrial (Δ17O ≈ 0‰) O-isotopic compositions of the Earth and most extraterrestrial materials (Moon, Mars, asteroids, and comet dust) were established very early by heating of regions of the disk that were modestly enriched (dust/gas ≥ 5–10 times solar) in primordial silicates (Δ17O ≈ −29‰) and water-dominated ice (Δ17O ≈ 24‰) relative to the gas. Such modest enrichments could be achieved by grain growth and settling of dust to the midplane in regions where the levels of turbulence were modest. The episodic heating of the disk associated with FU Orionis outbursts were the likely causes of this early thermal processing of dust. We also estimate that at the time of accretion the CI chondrite and interplanetary dust particle parent bodies were composed of ~5–10% of pristine interstellar material. The matrices of all chondrites included roughly similar interstellar fractions. Whether this interstellar material avoided the thermal processing experienced by most dust during FU Orionis outbursts or was accreted by the disk after the outbursts ceased to be important remains to be established.

^1,2Lu Feng, ^3,4Masaaki Miyahara, ⁵Toshiro Nagase, ³Eiji Ohtani, ¹Sen Hu, ⁶Ahmed El Goresy, ¹Yangting Lin
American Mineralogist 102, 1254-1262 Link to Article [DOI: 10.2138/am-2017-5905]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029,China
²Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China
³Institute of Mineralogy, Petrology, and Economic Geology, Tohoku University, Sendai, 980-8578, Japan
⁴Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan
⁵Center for Academic Resources and Archives, Tohoku University, Sendai, 980-8578, Japan
⁶Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, 95447, Germany
Copyright: The Mineralogical Society of America

Akimotoite [(Mg,Fe)SiO₃-ilmenite] was encountered in shock-induced melt veins of Grove Mountains (GRV) 052082, a highly equilibrated low iron ordinary chondritic meteorite (L6). Coexistence of ringwoodite, majorite, and majorite-pyrope solid solution indicates the shock pressure at 18–23 GPa and temperature of 2000–2300 °C during the natural dynamic event. Most low-Ca pyroxene clasts entrained in the melt veins have been partially or entirely transformed into akimotoite-pyroxene glass assemblages, which contain micrometer-sized areas with various brightness in the backscattered electron images, different from the chemically homogeneous grains in the host-rock (Fs_20.5–21.3). The transmission electron microscopy study of a focused ion beam (FIB) slice from the heterogeneous areas shows that the assemblages are composed of FeO-depleted and heterogeneous akimotoite (Fs_6–19) crystals (100 nm up to 400 nm in size) scattered in FeO-enriched and relatively homogeneous pyroxene glass (Fs_31–39). All analyses of the akimotoite-pyroxene glass assemblages plot on a fractionation line in FeO-MgO diagram, with the host-rock pyroxene at the middle between the compositions of FeO-depleted akimotoite and the FeO-enriched pyroxene glass. These observations are different from previous reports of almost identical compositions of akimotoite, bridgmanite [(Mg,Fe)SiO₃-perovskite], or pyroxene glass to the host rock pyroxene (Chen et al. 2004; Ferroir et al. 2008; Ohtani et al. 2004; Tomioka and Fujino 1997), which is consistent with solid-state transformation from pyroxene to akimotoite and preexisting bridgmanite that could be vitrified. The observed fractionation trend and the granular shapes of akimotoite suggest crystallization from liquid produced by shock melting of the host-rock pyroxene, and the pyroxene glass matrix was probably quenched from the residual melt. However, this interpretation is inconsistent with the static experiments that expect crystallization of majorite [(Mg,Fe)SiO₃-garnet], instead of akimotoite, from pyroxene liquid (Sawamoto 1987). Our discovery raises the issue on formation mechanisms of the high-pressure polymorphs of pyroxene and places additional constraints on the post-shock high-pressure and high-temperature conditions of asteroids.

¹Danika F.Wellington et al. (>10)*
American Mineralogist 102, 1202-1217 Link to Article [DOI: 10.2138/am-2017-5760CCBY]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A.
*Find the extensive, full author and affiliation list on the publishers website
Copyright: The Mineralogical Society of America

The Mastcam CCD cameras on the Mars Science Laboratory Curiosity Rover each use an 8-position filter wheel in acquiring up to 1600 × 1200 pixel images. The filter set includes a broadband near-infrared cutoff filter for RGB Bayer imaging on each camera and 12 narrow-band geology filters distributed between the two cameras, spanning the wavelength range 445–1013 nm. This wavelength region includes the relatively broad charge-transfer and crystal-field absorption bands that are most commonly due to the presence of iron-bearing minerals. To identify such spectral features, sequences of images taken with identical pointings through different filters have been calibrated to relative reflectance using pre-flight calibration coefficients and in-flight measurements of an onboard calibration target. Within the first 1000 sols of the mission, Mastcam observed a spectrally diverse set of materials displaying absorption features consistent with the presence of iron-bearing silicate, iron oxide, and iron sulfate minerals. Dust-coated surfaces as well as soils possess a strong positive reflectance slope in the visible, consistent with the presence of nanophase iron oxides, which have long been considered the dominant visible-wavelength pigmenting agent in weathered martian surface materials. Fresh surfaces, such as tailings produced by the drill tool and the interiors of rocks broken by the rover wheels, are grayer in visible wavelengths than their reddish, dust-coated surfaces but possess reflectance spectra that vary considerably between sites. To understand the mineralogical basis of observed Mastcam reflectance spectra, we focus on a subset of the multispectral data set for which additional constraints on the composition of surface materials are available from other rover instruments, with an emphasis on sample sites for which detailed mineralogy is provided by the results of CheMin X-ray diffraction analyses. We also discuss the results of coordinated observations with the ChemCam instrument, whose passive mode of operation is capable of acquiring reflectance spectra over wavelengths that considerably overlap the range spanned by the Mastcam filter set (Johnson et al. 2016). Materials that show a distinct 430 nm band in ChemCam data also are observed to have a strong near-infrared absorption band in Mastcam spectral data, consistent with the presence of a ferric sulfate mineral. Long-distance Mastcam observations targeted toward the flanks of the Gale crater central mound are in agreement with both ChemCam spectra and orbital results, and in particular exhibit the spectral features of a crystalline hematite layer identified in MRO/CRISM data. Variations observed in Mastcam multi-filter images acquired to date have shown that multispectral observations can discriminate between compositionally different materials within Gale Crater and are in qualitative agreement with mineralogies from measured samples and orbital data.

¹Ryoji Tanaka, ¹Eizo Nakamura
Nature Astronomy 1, 0137 Link to Article [doi:10.1038/s41550-017-0137]
¹The Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry, Institute for Planetary Materials, Okayama University, Misasa, Tottori 682-0193, Japan

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