¹James A. D. Connolly
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005059]
¹Earth Sciences Department, Swiss Federal Institute of Technology, Zurich, Switzerland
Published by arrangement with John Wiley & Sons

This work explores the use of several van der Waals (vW) type equations of state (EoS) for predicting vaporous phase relations and speciation in the Si-O system, with emphasis on the azeotropic boiling curve of SiO2-rich liquid. Comparison with the observed Rb and Hg boiling curves demonstrates that prediction accuracy is improved if the a-parameter of the EoS, which characterizes vW forces, is constrained by ambient pressure heat capacities. All EoS considered accurately reproduce metal boiling curve trajectories, but absent knowledge of the true critical compressibility factor, critical temperatures remain uncertain by ~500 K. The EoS plausibly represent the termination of the azeotropic boiling curve of silica-rich liquid by a critical point across which the dominant Si oxidation state changes abruptly from the tetravalent state characteristic of the liquid to the divalent state characteristic of the vapor. The azeotropic composition diverges from silica toward metal-rich compositions with increasing temperature. Consequently, silica boiling is divariant and atmospheric loss after a giant impact would enrich residual silicate liquids in reduced silicon. Two major sources of uncertainty in the boiling curve prediction are: the heat capacity of silica liquid, which may decay during depolymerization from the near-Dulong Petit Limit heat capacity of the ionic liquid to values characteristic of the molecular liquid; and the unknown liquid affinity of silicon monoxide. Extremal scenarios for these uncertainties yield critical temperatures and compositions of 5200-6200 K and Si1.1O2-Si1.4O2. The lowest critical temperatures are marginally consistent with shock experiments and are therefore considered to be more realistic.

¹Z. Y. Cheng, ²D. C. Fernández-Remolar, ^3,4,5M. R. M. Izawa, ⁴D. M. Applin, ⁶M. Chong Díaz, ⁷M. T. Fernandez-Sampedro, ⁷M. García-Villadangos, ⁸T. Huang, ^8,9L. Xiao, ⁷V. Parro
Journal of Geophysical Research Biogeosciences (in Press) Link to Article [DOI: 10.1002/2016JG003439]
¹Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan, China
²Environmental Science Centre, British Geological Survey, Keyworth, UK
³Department of Earth Sciences, Brock University, St. Catharines, Ontario, Canada
⁴Hyperspectral Optical Sensing for Extraterrestrial Reconnaissance Laboratory, Department of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
⁵Planetary Science Institute, Tucson, Arizona, USA
⁶Department of Geological Sciences, Universidad Católica del Norte, Antofagasta, Chile
⁷Centro de Astrobiologia (INTA-CSIC), Torrejon de Ardoz, Spain
⁸Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan, China
⁹Space Science Institute, Macau University of Science and Technology, Macau, China

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¹Heather C. Watson, ²Frank Richter, ²Ankun Liu, ³Gary R. Huss
Earth and Planetary Science Letters 451, 159–167 Link to Article [doi:10.1016/j.epsl.2016.06.030]
¹Union College, Schenectady, NY, United States
²University of Chicago, United States
³University of Hawai‘i at Mānoa, United States
Copyright Elsevier

Mass-dependent, kinetic fractionation of isotopes through processes such as diffusion can result in measurable isotopic signatures. When these signatures are retained in geologic materials, they can be used to help interpret their thermal histories. The mass dependence of the diffusion coefficient of isotopes 1 and 2 can be written as (D1/D2)=(m2/m1)β(D1/D2)=(m2/m1)β, where D1D1 and D2D2 are the diffusion coefficients of m1m1 and m2m2 respectively, and β is an empirical coefficient that relates the two ratios. Experiments have been performed to measure β in the Fe–Ni alloy system. Diffusion couple experiments between pure Fe and Ni metals were run in a piston cylinder at 1300–1400 °C and 1 GPa. Concentration and isotopic profiles were measured by electron microprobe and ion microprobe respectively. We find that a single β coefficient of β=0.32±0.04β=0.32±0.04 can describe the isotopic effect in all experiments. This result is comparable to the isotope effect determined in many other similar alloy systems. The new β coefficient is used in a model of the isotopic profiles to be expected during the Widmanstätten pattern formation in iron meteorites. The results are consistent with previous estimates of the cooling rate of the iron meteorite Toluca. The application of isotopic constraints based on these results in addition to conventional cooling rate models could provide a more robust picture of the thermal history of these early planetary bodies.

¹J. Mortimer, ¹A.B. Verchovsky, ^1,2M. Anand
Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.08.006]
¹Planetary and Space Sciences, Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
²Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
Copyright Elsevier

Simultaneous static-mode mass spectrometric measurements of nitrogen, carbon, helium, neon, and argon, extracted from the same aliquot of sample by high-resolution stepped combustion, have been made for a suite of five lunar soils.

Noble gas isotope ratios show that the majority of noble gases are derived from a solar wind source; for example, at peak release temperatures of 500-600 °C, 21Ne/22Ne = 0.0313 ± 0.0007 to 0.0333 ± 0.0007, and 20Ne/22Ne = 11.48 ± 0.05 to 12.43 ± 0.07, with values at the lowest temperature steps less fractionated during implantation from, and therefore even closer to, solar values (21Ne/22NeSW = 0.03361 ± 0.00018 and 20Ne/22NeSW = 14.001 ± 0.042 (Pepin et al., 2012)). Despite the co-release of nitrogen and solar wind argon, measured nitrogen isotopic signatures at each temperature step, whilst variable, are significantly more enriched in 15N compared to the measured solar wind nitrogen value from the Genesis mission. Therefore, mixing between a 15N-enriched non-solar planetary nitrogen source with solar wind nitrogen is required to explain the measured isotopic values from the stepped combustion analysis of lunar soils. Binary mixing calculations, made under different assumptions about the degree of loss of solar wind 36Ar, reveal that the majority (up to 98%) of the nitrogen released is derived from a non-solar source. The range of modelled non-solar end-member nitrogen compositions required to satisfy the measured δ15N values varies between samples and temperature steps from +5 ‰ up to +300 ‰, or between +87 ‰ and +160 ‰ for bulk samples. This range of modelled isotopic compositions for the non-solar source of nitrogen encompasses measured values for several different groups of carbonaceous chondrite, as well as IDPs.

¹D.J. Cherniak, ¹E.B. Watson, ²R. Trappisch, ³J.B. Thomas, ⁴D. Chaussende
Geochimica et Cosmochmica Acta (in Press) Link to Article
[doi:10.1016/j.gca.2016.08.007]
¹Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180 USA
²Department of the Geophysical Sciences, The University of Chicago, and Chicago Center for Cosmochemistry, Chicago, IL 60637 USA
³Department of Earth Sciences, Syracuse University, Syracuse, NY 13244 USA
⁴Laboratoire des Matériaux et du Génie Physique, CNRS – Grenoble INP, 3 parvis Louis Néel, BP, 257, 38016 Grenoble, France
Copyright Elsevier

Diffusion of helium has been characterized in silicon carbide of cubic and hexagonal (4H and 6H) forms. Polished sections of SiC were implanted with 3He at 100 keV at a dose of 1×1015/cm2. The implanted SiC samples were sealed under vacuum in silica glass ampoules, and annealed in 1-atm furnaces. 3He distributions following all experiments were measured with Nuclear Reaction Analysis using the reaction 3He(d,p)4He. For He diffusion in cubic SiC and 4H hexagonal SiC we obtain the following Arrhenius relations:

Dcubic=1.83×10-6exp(-254±10kJmol-1/RT)m2sec-1Dcubic=1.83×10-6exp(-254±10kJmol-1/RT)m2sec-1

D4H=4.78×10-7exp(-255±29kJmol-1/RT)m2sec-1D4H=4.78×10-7exp(-255±29kJmol-1/RT)m2sec-1

While He diffusion is considerably slower in SiC than in many silicate phases, He retentivity may be limited under some conditions. For example, helium will be lost from SiC grains over much shorter timescales than potential survival times of SiC presolar grains in the solar nebula. When exposed to impact heating followed by slow cooling, nearly complete loss of He from SiC grains near the site of impact will occur within several hours to a few days. For SiC grains at greater distance from impact sites, He would be better retained, depending on the rapidity of cooling. At tens of km away from a large impactor, where peak T would be ∼800K, SiC grains would lose about 50% of their He if the grains cooled within a few thousand years, and 5% if they cooled within a few tens of years. At greater distances where heating is more modest (500K and lower), SiC grains would be quite retentive of He even for cases of very slow cooling. Helium would also be retained in cases of impact heating followed by very rapid cooling. For these short heating pulses, 10 μm diameter SiC grains would retain more than 50% of their He for peak heating temperatures of 2173, 1973 and 1773K for durations of 3, 10 and 60 seconds, respectively.

¹Alexander Hubbard
Monthly Notices of the Royal Astronomical Society 460, 1163-1172 Linkt to Article [doi: 10.1093/mnras/stw1005]
¹Department of Astrophysics, American Museum of Natural History, New York, NY 10024-5192, USA

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¹Frans J. M. Rietmeijer
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12695]
¹Department of Earth and Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, New Mexico, USA
Published by arrangement with John Wiley & Sons

¹Maartje F. Hamers,¹Gill M. Pennock,²Marco Herwegh,¹Martyn R. Drury
Meteoritics & Planetary Science (in Press)   Link to Article [DOI: 10.1111/maps.12711]
¹Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands
²Institute of Geological Sciences, University of Bern, Bern, Switzerland
Published by arrangement with John Wiley & Sons

Planar deformation features (PDFs) in quartz are one of the most reliable and most widely used forms of evidence for hypervelocity impact. PDFs can be identified in scanning electron microscope cathodoluminescence (SEM-CL) images, but not all PDFs show the same CL behavior: there are nonluminescent and red luminescent PDFs. This study aims to explain the origin of the different CL emissions in PDFs. Focused ion beam (FIB) thin foils were prepared of specific sample locations selected in composite color SEM-CL images and were analyzed in a transmission electron microscope (TEM). The FIB preparation technique allowed a direct, often one-to-one correlation between the CL images and the defect structure observed in TEM. This correlation shows that composite color SEM-CL imaging allows distinction between amorphous PDFs on one hand and healed PDFs and basal Brazil twins on the other: nonluminescent PDFs are amorphous, while healed PDFs and basal Brazil twins are red luminescent, with a dominant emission peak at 650 nm. We suggest that the red luminescence is the result of preferential beam damage along dislocations, fluid inclusions, and twin boundaries. Furthermore, a high-pressure phase (possibly stishovite) in PDFs can be detected in color SEM-CL images by its blue luminescence.

¹Yuchen Xu, ¹Yangting Lin, ¹Jianchao Zhang, ¹Jialong Hao
The Astrophysical Journal 825, 111 Link to Article [http://dx.doi.org/10.3847/0004-637X/825/2/111]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

Presolar graphite grains have been extensively studied, but are limited in carbonaceous chondrites, particularly in Murchison (CM2) and Orgueil (CI1), which sampled materials from the oxidizing regions in the solar nebula. Here, we report the first discovery of presolar graphite grains from the Qingzhen (EH3) enstatite chondrite which formed under a highly reducing condition. Eighteen presolar graphite grains were identified by C-isotope mapping of the low-density fraction (1.75–1.85 g cm−3) from Qingzhen acid residue. Another 58 graphite spherules were found in different areas of the same sample mount using a scanning electron microscope and were classified into three morphologies, including cauliflower, onion, and cauliflower–onion. The Raman spectra of these spherules vary from ordered, disordered, and glassy to kerogen-like, suggestive of a wide range of thermal metamorphisms. NanoSIMS analysis of the C- and Si-isotopes of these graphite spherules confirmed 23 presolar grains. The other 35 graphite spherules have no significant isotopic anomalies, but they share similar morphologies and Raman spectra with the presolar ones. Another three grains were identified during NanoSIMS analysis. Of all the 44 presolar graphite grains identified, six grains show 28Si-excesses, suggestive of supernovae origins, and four grains are 12C- and 29,30Si-rich, consistent with low-metallicity asymptotic giant branch star origins. Another two graphite spherules have extremely low 12C/13C ratios with marginal solar Si-isotopes. The morphologies, Raman spectra, and C- and Si-isotopic distributions of the presolar graphite grains from the Qingzhen enstatite chondrite are similar to those of the low-density fractions from Murchison carbonaceous chondrites. This study suggests a homogeneous distribution of presolar graphite grains in the solar nebula.

¹Deborah. L. Domingue, ¹Faith Vilas, ²Teck Choo, ³Karen R. Stockstill-Cahill, ⁴Joshua T.S. Cahill, ³Amanda R. Hendrix
Icarus (in Press) Link to Article [doi:10.1016/j.icarus.2016.07.011]
¹Planetary Science Institute
²Johns Hopkins University Applied Physics Laboratory
³Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA
⁴The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
Copyright Elsevier

Spectrophotometric examination of the Galileo Solid State Imager (SSI) observations from the Galileo spacecraft reveal surface compositional heterogeneities in mineral compositions not related to geologic unit. These include variations in olivine and orthopyroxene content of on the order of 15% and 25%, respectively. Opaque mineral phases across the inter-ridge regions vary in quantity, but consistently modeled better with ilmenite. The macroscale fraction of metallic iron varies subtly (0-10%) in quantity and in grain size (60 – 100 μm). Color properties also vary across the inter-ridge regions, indicating variations in regolith maturity. Comparisons of near-infrared ratio-reflectance suggest changes in regolith maturity that are different from those seen on the lunar surface and asteroid 433 Eros, commensurate with Gaspra’s higher olivine content. Visible to near-infrared slopes compared to near-ultraviolet to visible slopes are indicative of a nanophase iron content of 0.01% – 0.1%. Spectral mixing modeling studies of the SSI color spectra show results consistent with the presence of both microphase (> 50 nm) and nanophase (< 50nm) size iron particulates. While the quantity of microphase and nanophase iron appears to be constant within the sample areas studied, the grain size of the microphase component varies. Agglutinates are present in some areas of the inter-ridge regions, but at low abundances (∼5%).