David L. Cook^a,b, Thomas S. Kruijer^a,b, Ingo Leya^c and Thorsten Kleine^a

^aInstitut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm Str. 10, 48149 Münster, Germany
^bInstitut für Geochemie und Petrologie, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
^cSpace Research and Planetology, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

We measured tungsten isotopes in 23 iron meteorites and the metal phase of the CB chondrite Gujba in order to ascertain if there is evidence for a large-scale nucleosynthetic heterogeneity in the p-process isotope¹⁸⁰W in the solar nebula as recently suggested by Schulz et al. (2013). We observed large excesses in ¹⁸⁰W (up to ≈ 6 ε) in some irons. However, significant within-group variations in magmatic IIAB and IVB irons are not consistent with a nucleosynthetic origin, and the collateral effects on ¹⁸⁰W from an s-deficit in IVB irons cannot explain the total variation. We present a new model for the combined effects of spallation and neutron capture reactions on ¹⁸⁰W in iron meteorites and show that at least some of the observed within-group variability is explained by cosmic ray effects. Neutron capture causes burnout of ¹⁸⁰W, whereas spallation reactions lead to positive shifts in ¹⁸⁰W. These effects depend on the target composition and cosmic-ray exposure duration; spallation effects increase with Re/W and Os/W ratios in the target and with exposure age. The correlation of¹⁸⁰W/¹⁸⁴W with Os/W ratios in iron meteorites results in part from spallogenic production of ¹⁸⁰W rather than from ¹⁸⁴Os decay, contrary to a recent study by Peters et al. (2014). Residual ε¹⁸⁰W excesses after correction for an s-deficit and for cosmic ray effects may be due to ingrowth of ¹⁸⁰W from ¹⁸⁴Os decay, but the magnitude of this ingrowth is at least a factor of ≈ 2 smaller than previously suggested. These much smaller effects strongly limit the applicability of the putative ¹⁸⁴Os-¹⁸⁰W system to investigate geological problems.

Reference
Cook DL, Kruijer TS, Leya I and Kleine T (in press) Cosmogenic 180W Variations in Meteorites and Re-assessment of a Possible 184Os-180W Decay System. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.013]
Copyright Elsevier

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Johanna K. Teske¹, Katia Cunha^1,2, Verne V. Smith³, Simon C. Schuler⁴ and Caitlin A. Griffith⁵

¹Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
²Observatório Nacional, Rua General José Cristino, 77, 20921-400 São Cristóvão, Rio de Janeiro, RJ, Brazil
³National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁴University of Tampa, 401 West Kennedy Boulevard, Tampa, FL 33606, USA
⁵Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA

The relative abundances of carbon and oxygen have long been recognized as fundamental diagnostics of stellar chemical evolution. Now, the growing number of exoplanet observations enable estimation of these elements in exoplanetary atmospheres. In hot Jupiters, the C/O ratio affects the partitioning of carbon in the major observable molecules, making these elements diagnostic of temperature structure and composition. Here we present measurements of carbon and oxygen abundances in 16 stars that host transiting hot Jupiter exoplanets, and we compare our C/O ratios to those measured in larger samples of host stars, as well as those estimated for the corresponding exoplanet atmospheres. With standard stellar abundance analysis we derive stellar parameters as well as [C/H] and [O/H] from multiple abundance indicators, including synthesis fitting of the [O i] λ6300 line and non-LTE corrections for the O i triplet. Our results, in agreement with recent suggestions, indicate that previously measured exoplanet host star C/O ratios may have been overestimated. The mean transiting exoplanet host star C/O ratio from this sample is 0.54 (C/O_☉ = 0.54), versus previously measured C/O_host star means of ~0.65–0.75. We also observe the increase in C/O with [Fe/H] expected for all stars based on Galactic chemical evolution; a linear fit to our results falls slightly below that of other exoplanet host star studies but has a similar slope. Though the C/O ratios of even the most-observed exoplanets are still uncertain, the more precise abundance analysis possible right now for their host stars can help constrain these planets’ formation environments and current compositions.

Reference
Teske JK, Cunha K, Smith VV, Schuler SC and Griffith CA (2014) C/O ratios of stars with transiting hot Jupiter exoplanets. The Astrophysical Journal  788:39.
[doi:10.1088/0004-637X/788/1/39]

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Michael S. Bramble^1,2, Roberta L. Flemming^1,3, Jeffrey L. Hutter², Melissa M. Battler^1,3, Gordon R. Osinski^1,2,3 and Neil R. Banerjee^1,3

¹Centre for Planetary Science and Exploration, The University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 5B7, Canada
²Department of Physics and Astronomy, The University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 3K7, Canada
³Department of Earth Sciences, The University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 5B7, Canada

A temperature-controlled sample stage with an operational range of ~60 °C above or below ambient laboratory temperature (~ −35 to 85 °C) was constructed for in situ X-ray diffraction of minerals and materials using a Bruker D8 Discover diffractometer with θ-θ geometry. The stage was primarily designed for characterizing mirabilite-bearing samples from a Mars analog High Arctic perennial cold spring at an in situ temperature. Operation of the stage was demonstrated through the analysis of a synthetic sample of the hydrated sodium sulfate, mirabilite (Na₂SO₄·10H₂O). Mirabilite was held at −25 °C for approximately two hours without significant dehydration and then incrementally warmed to ambient laboratory temperature at 5 °C intervals, during the acquisition of in situ diffraction data. At ambient laboratory temperature, the mirabilite dehydrated and only polycrystalline thenardite (Na₂SO₄) remained. Preliminary analysis of the cold spring precipitates demonstrates that when mirabilite is present in the sample the dehydration reaction is occurring between collection and analysis at ambient laboratory temperature. This temperature-controlled stage was designed for versatility and ease of X-ray access, with applications that can extend to many geological and planetary settings, including Mars analog environments.

Reference
Bramble MS, Flemming RL, Hutter JL, Battler MM, Osinski GR and Banerjee NR (2014) A temperature-controlled sample stage for in situ micro-X-ray diffraction: Application to Mars analog mirabilite-bearing perennial cold spring precipitate mineralogy. American Mineralogist  99:943-947.
[doi:10.2138/am.2014.4629]
Copyright: The Mineralogical Society of America

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