Stephan König^a,*, Jean-Pierre Lorand^b, Ambre Luguet^a and D. Graham Pearson^c

^aRheinische Friedrich-Wilhelms-Universität Bonn, Steinmann Institut für Geologie, Mineralogie und Paläontologie, Poppelsdorfer Schloss, 53115 Bonn, Germany
^bLaboratoire de Planétologie et Géodynamique de Nantes, CNRS UMR 6112, Université de Nantes, 44322 Nantes, France
^cUniversity of Alberta, Department of Earth and Atmospheric Sciences, Edmonton, Canada

The chalcophile and highly siderophile elements Se and Te, like the other Highly Siderophile Elements (HSE) in the terrestrial mantle, may constitute powerful key tracers for meteoritic materials that hit the Earth in its latest accretionary stages (“Late Veneer”). Here the Se and Te systematics of mantle-derived peridotites (orogenic peridotites, ophiolites, cratonic peridotite xenoliths) are assessed. Combined with published in-situ analyses of HSE host minerals, whole-rock data are modelled with respect to current petrogenetic models that affect mantle composition, for example partial melting and magmatic refertilisation. We demonstrate that the near-chondritic Se/Te signature (Se_N/Te_N≈9±4; N = CI-chondrite normalised) of “fertile” ophiolitic and orogenic lherzolites cannot be a primitive signature of the Earthʼs mantle. This signature can however be explained by simple refertilisation models. The HSE–Se–Te budget of these fertile rocks can be modelled by mixing various proportions of a residual assemblage of Fe–Ni monosulphide solid solutions (Mss) and/or refractory platinum group minerals (PGMs – Ru–Os–Ir sulphides + Pt–Ir–Os alloys) with a metasomatic assemblage comprising low-temperature Pt–Pd–Te phases and Cu–Ni-rich sulphides. On the other hand, the reported Se and Te ratios in fertile peridotites are not consistent with melt depletion alone. Additions of late-stage metasomatic S–Se–Te–HSE-rich phases render Primitive Upper Mantle (PUM) estimates for Se and Te highly debatable, especially without appropriate consideration of refertilisation and metasomatism. Our results indicate that there is currently no firm evidence for chondritic S–Se–Te signatures in the Primitive Upper Mantle. This conclusion challenges the simplistic perception that near-chondritic Se/Te ratios may readily trace the Late Veneer composition.

Reference
König S, Lorand J-P, Luguet A Pearson DG (2014) A non-primitive origin of near-chondritic S–Se–Te ratios in mantle peridotites; implications for the Earthʼs late accretionary history. Earth and Planetary Science Letters 385:110–121.
[doi:10.1016/j.epsl.2013.10.036]
Copyright Elsevier

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T.-C. Peng¹, E. M. L. Humphreys¹, L. Testi^1,2,3, A. Baudry^4,5, M. Wittkowski¹, M. G. Rawlings⁶, I. de Gregorio-Monsalvo^1,8, W. Vlemmings⁷, L.-A. Nyman⁸, M. D. Gray⁹ and C. de Breuck¹

¹ESO Garching, Karl-Schwarzschild Str. 2 85748 Garching, Germany
²Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
³INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁵CNRS, LAB, UMR 5804, 33270 Floirac, France
⁶National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁷Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁸Joint ALMA Observatory (JAO) and European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁹JBCA, Alan Turing Building, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

Aims. Galactic chemical evolution (GCE) is important for understanding the composition of the present-day interstellar medium (ISM) and of our solar system. In this paper, we aim to track the GCE by using the ²⁹Si/³⁰Si ratios in evolved stars and tentatively relate this to presolar grain composition.
Methods. We used the APEX telescope to detect thermal SiO isotopologue emission toward four oxygen-rich M-type stars. Together with the data retrieved from the Herschelscience archive and from the literature, we were able to obtain the ²⁹Si/³⁰Si ratios for a total of 15 evolved stars inferred from their optically thin ²⁹SiO and ³⁰SiO emission. These stars cover a range of masses and ages, and because they do not significantly alter ²⁹Si/³⁰Si during their lifetimes, they provide excellent probes of the ISM metallicity (or ²⁹Si/³⁰Si ratio) as a function of time.
Results. The ²⁹Si/³⁰Si ratios inferred from the thermal SiO emission tend to be lower toward low-mass oxygen-rich stars (e.g., down to about unity for W Hya), and close to an interstellar or solar value of 1.5 for the higher-mass carbon star IRC+10216 and two red supergiants. There is a tentative correlation between the ²⁹Si/³⁰Si ratios and the mass-loss rates of evolved stars, where we take the mass-loss rate as a proxy for the initial stellar mass or current stellar age. This is consistent with the different abundance ratios found in presolar grains. Before the formation of the Sun, the presolar grains indicate that the bulk of presolar grains already had ²⁹Si/³⁰Si ratios of about 1.5, which is also the ratio we found for the objects younger than the Sun, such as VY CMa and IRC+10216. However, we found that older objects (up to possibly 10 Gyr old) in our sample trace a previous, lower ²⁹Si/³⁰Si value of about 1. Material with this isotopic ratio is present in two subclasses of presolar grains, providing independent evidence of the lower ratio. Therefore, the ²⁹Si/³⁰Si ratio derived from the SiO emission of evolved stars is a useful diagnostic tool for the study of the GCE and presolar grains.

Reference
Peng T-C, Humphreys EML, Testi L, Baudry A, Wittkowski M Rawlings, MG, de Gregorio-Monsalvo I, Vlemmings W, Nyman L-A, Gray MD and de Breuck C (2013) Silicon isotopic abundance toward evolved stars and its application for presolar grains. Astronomy & Astrophysics 559:L8.
[doi:10.1051/0004-6361/201322466]
Reproduced with permission © ESO

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Katherine R. Compaan^1,2, Jay Agarwal¹, Bryson E. Dye¹, Yukio Yamaguchi¹ and Henry F. Schaefer, III¹

¹Center for Computational Quantum Chemistry, University of Georgia, Athens, GA 30602, USA
²Research Focus Area for Chemical Resource Beneficiation, North-West University, Hoffman Street, Potchefstroom 2520, South Africa

Highly reliable molecular properties have been computed for AlCH₂ and AlCH₂⁺ at the CCSD(T)/cc-pwCVQZ level of theory. These simple aluminum species are relevant to the chemistry of the interstellar medium (ISM) and circumstellar medium, and are proposed to form in regions where aluminum and methylene exist in appreciable concentrations because AlCH₂ and AlCH₂⁺ are thermodynamically stable with respect to dissociation. Herein, dissociation energies were computed for both species through extrapolation to the complete basis set limit using focal point analysis. For the neutral species, 86 kcal mol^-1 is required to heterolytically cleave the Al–C single bond, while the cationic species requires 38 kcal mol^-1. To aid in identification within the ISM, the anharmonic (fundamental) frequencies, spectroscopic constants, and vibrationally averaged properties for AlCH₂ and the corresponding cation are also reported.

Reference
Compaan KR, Agarwal J, Dye BE, Yamaguchi Y and Schaefer, III HF (2013) Toward Detection of AlCH2 and AlCH2+ in the Interstellar Medium. The Astrophysical Journal 778:125.
[doi:10.1088/0004-637X/778/2/125]

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Robin T. Garrod

Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853-6801, USA

The first off-lattice Monte Carlo kinetics model of interstellar dust grain surface chemistry is presented. The positions of all surface particles are determined explicitly, according to the local potential minima resulting from the pair-wise interactions of contiguous atoms and molecules, rather than by a pre-defined lattice structure. The model is capable of simulating chemical kinetics on any arbitrary dust grain morphology, as determined by the user-defined positions of each individual dust grain atom. A simple method is devised for the determination of the most likely diffusion pathways and their associated energy barriers for surface species. The model is applied to a small, idealized dust grain, adopting various gas densities and using a small chemical network. Hydrogen and oxygen atoms accrete onto the grain to produce H₂O, H₂, O₂, and H₂O₂. The off-lattice method allows the ice structure to evolve freely; the ice mantle porosity is found to be dependent on the gas density, which controls the accretion rate. A gas density of 2 × 10⁴cm^-3, appropriate for dark interstellar clouds, is found to produce a fairly smooth and non-porous ice mantle. At all densities, H₂ molecules formed on the grains collect within the crevices that divide nodules of ice and within micropores (whose extreme inward curvature produces strong local potential minima). The larger pores produced in the high-density models are not typically filled with H₂. Direct deposition of water molecules onto the grain indicates that amorphous ices formed in this way may be significantly more porous than interstellar ices that are formed by surface chemistry.

Reference
Garrod RT (2013) Three-dimensional, Off-lattice Monte Carlo Kinetics Simulations of Interstellar Grain Chemistry and Ice Structure. The Astrophysical Journal 778:158.
[doi:10.1088/0004-637X/778/2/158]

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Refers To

Cyrena A. Goodrich, Richard D. Ash, James A. Van Orman, Kenneth Domanik, William F. McDonough
Metallic phases and siderophile elements in main group ureilites: Implications for ureilite petrogenesis
Geochimica et Cosmochimica Acta, Volume 112, 1 July 2013, Pages 340-373

The publisher regrets the article is incorrectly labelled as “Response” when it is a full length research article. The publisher would like to apologise for any inconvenience caused.