¹Yogita Kadlag, ¹Harry Becker
¹Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany

Abundances of highly siderophile elements (HSE: Re, platinum group elements and Au), chalcogens (Te, Se and S), 187Os/188Os and the major and minor elements Mg, Ca, Mn, Fe, Ni and Co were determined in the components of Sahara 97072 (EH3, find) and Kota Kota (EH3, find) in order to understand the element fractionation processes. In a 187Re-187Os isochron diagram, most magnetic components lie close to the 4.56 Ga IIIA iron meteorite isochron, whereas most other components show deviations from the isochron caused by late redistribution of Re, presumably during terrestrial weathering. Metal- and sulfide rich magnetic fractions and metal-sulfide nodules are responsible for the higher 187Os/188Os in bulk rocks of EH chondrites compared to CI chondrites. The HSE and chalcogens are enriched in magnetic fractions relative to slightly magnetic and nonmagnetic fractions and bulk compositions, indicating that Fe-Ni metal is the main host phase of the HSE in enstatite chondrites. HSE abundance patterns indicate mixing of two components, a CI chondrite like end member and an Au-enriched end member. Because of the decoupled variations of Au from those of Pd or the chalcogens, the enrichment of Au in EH metal cannot be due to metal-sulfide-silicate partitioning processes. Metal and sulfide rich nodules may have formed by melting and reaction of pre-existing refractory element rich material with volatile rich gas. A complex condensation and evaporation history is required to account for the depletion of elements having very different volatility than Au in EH chondrites. The depletions of Te relative to HSE, Se and S in bulk EH chondrites are mainly caused by the depletion of Te in metal. S/Se and S/Mn are lower than in CI chondrites in almost all components and predominantly reflect volatility-controlled loss of sulfur. The latter most likely occurred during thermal processing of dust in the solar nebula (e. g., during chondrule formation), followed by the non-systematic loss of S during terrestrial weathering.

Reference
Kadlag Y, Becker H (2015) Fractionation of Highly Siderophile and Chalcogen Elements in Components of EH3 Chondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.022]

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^1,2Giulio F.D. Solferino, ^1,3Gregor J. Golabek, ⁴Francis Nimmo, ¹Max W. Schmidt
¹Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland
²School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland
³Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
⁴Department of Earth and Planetary Sciences, University of California Santa Cruz, 95064 Santa Cruz, CA, USA

Despite their relatively simple mineralogical composition (olivine + Fe-Ni metal + FeS +/- pyroxene), the origin of pallasite meteorites remains debated. It has been suggested that catastrophic mixing of olivine fragments with Fe-(Ni)-S followed by various degrees of annealing could explain pallasites bearing solely or prevalently fragmented or rounded olivines. In order to verify this hypothesis, and to quantify the grain growth rate of olivine in a liquid metal matrix, we performed a series of annealing experiments on natural olivine plus synthetic Fe-S mixtures. The best explanation for the observed olivine grain size distributions (GSD) of the experiments are dominant Ostwald ripening for small grains followed by random grain boundary migration for larger grains. Our results indicate that olivine grain growth in molten Fe-S is significantly faster than in solid, sulphur-free metal. We used the experimentally determined grain growth law to model the coarsening of olivine surrounded by Fe-S melt in a 100 to 600 km radius planetesimal. In this model, an impact is responsible for the mixing of olivine and Fe-(Ni)-S. Numerical models suggest that annealing at depths of up to 50 km allow for (i) average grain sizes consistent with the observed rounded olivine in pallasites, (ii) a remnant magnetization of Fe-Ni olivine inclusions as measured in natural pallasites and (iii) for the metallographic cooling rates derived from Fe-Ni in pallasites. This conclusion is valid even if the impact occurs several millions of years after the differentiation of the target body was completed.

Reference
Solferino GFD, Golabek GJ, Nimmo F, Schmidt MW (2015) Fast grain growth of olivine in liquid Fe-S and the formation of pallasites with rounded olivine grains. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.020]

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^1,2M. Schmieder, ¹E. Tohver, ²F. Jourdan, ^1,3S.W. Denyszyn, ⁴P.W. Haines
¹School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, 6009 WA, Australia
²Western Australian Argon Isotope Facility, Department of Applied Geology and John de Laeter Centre for Isotope Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
³Centre for Exploration Targeting, University of Western Australia, 35 Stirling Highway, Crawley, 6009 WA, Australia
⁴Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia

This study presents the first optical and scanning electron microscopic characterization and U–Pb SHRIMP dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large, ∼40–90 km eroded Ediacaran Acraman impact structure in South Australia. Microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism, from virtually unshocked crystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer-sized spots of ZrO2 (probably baddeleyite) that pseudomorph pre-impact zircon. The granular zircons correspond to the highest observed level of shock metamorphism and impact-induced recrystallization. ZrO2-bearing granular zircons indicate shock pressures in excess of ∼65–70 GPa, which are considerably higher than previous shock pressure estimates for the Acraman impactites. U–Pb systematics of untreated and chemically abraded melt rock zircons indicate that U–Pb ratios of the Acraman zircons were variably reset during impact. Weakly shocked crystalline grains yield ages on concordia at ∼1.59–1.60 Ga reflecting the magmatic age of the Gawler Range Volcanics. Only the entirely granular zircon population was apparently impact-reset, but based on an Ediacaran age from stratigraphic constraints on the ejecta layer, experienced significant post-impact Pb loss. The microcrystalline nature of granular zircons could have promoted Pb diffusion and α-recoil in post-impact time, as suggested by grain size-dependent diffusion and recoil modeling. A positive correlation of U concentration and shock level suggests that granularization might have preferentially occurred in initially U-rich, probably metamict, zircons. 40Ar/39Ar dating of a melted Yardea Dacite clast from the Acraman melt rock, as well as K-feldspar separated from shocked Yardea Dacite resulted in post-impact alteration plateau ages suggestive of hydrothermal events at ∼500 Ma and ∼450 Ma that selectively affected the impactites that outcrop in the central domain of the Acraman impact structure. Our study demonstrates that the Acraman impact is particularly difficult to date. In the absence of accurate and precise isotopic ages for Acraman, the Ediacaran ejecta-stratigraphic age of ∼635–541 Ma is considered the most reliable age constraint currently available for the timing of the large Acraman impact.

Reference
Schmieder M, Tohver T, Jourdan F, Denyszyn SW, Haines PW (2015) Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40Ar/39Ar systematics, and implications for the isotopic dating of impact Events. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.021]

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