A. S. Pilski^1,*, J. T. Wasson², A. Muszyński³, R. Kryza⁴, Ł. Karwowski⁵ and M. Nowak³

¹Nicolaus Copernicus Museum, Frombork, Poland
²Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA
³Institute of Geology, Adam Mickiewicz University, Poznań, Poland
⁴Institute of Geological Sciences, University of Wrocław, Wrocław, Poland
⁵Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland

Differences in texture and discovery location prompted us to analyze 16 irons from Morasko; one from Seeläsgen, known to have a similar composition; and a new mass found at Jankowo Dolne. These were analyzed in duplicate by instrumental neutron-activation analysis (INAA). The results show that all 18 samples have very similar compositions, distinct from all other IAB irons except Burgavli; we conclude that they are all from a single shower. Eight of the samples were from regions with large amounts of cohenite (but were largely free of inclusions) and six were from samples with very little cohenite; we could find no resolvable difference in composition between these sets, a fact that suggests that the C contents of the metal phases were similar in the two areas. Although Morasko has been classified into the IAB main group (IAB-MG), its Ir plots well outside the main group field on an Ir-Au diagram. We considered the possibility that the low Ir reflected contamination by a melt from a IAB region that ponded and experienced fractional crystallization; however, because Morasko has Pt, W, and Ga values that are the same as the highest values in IAB-MG, we rejected this model. We therefore conclude that Morasko formed from a different melt than the IAB-MG irons; the Morasko melt was produced by impact heating, but one or more of the main Ir carriers did not melt, leaving much of the Ir in the unmelted residue. Copper is the only element that shows resolvable differences among Morasko samples. Most (13 of 18) samples have 149 ± 4 μg g⁻¹ Cu, but three have 213 ± 10 μg g⁻¹; we interpret this to mean that the low-Cu samples have equilibrated with a Cu-rich phase, whereas there was none of the latter phase within a few diffusion lengths of the samples with high Cu contents.

Reference
Pilski AS, Wasson JT, Muszyński A, Kryza R, Karwowski Ł and Nowak M (in press) Low-Ir IAB irons from Morasko and other locations in central Europe: One fall, possibly distinct from IAB-MG. Meteoritics & Planetary Science
[doi:10.1111/maps.12225]
Published by arrangement with John Wiley & Sons

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Thomas S. Kruijer^1,2,*, Peter Sprung^1,2, Thorsten Kleine², Ingo Leya³, Rainer Wieler¹

¹ETH Zürich, Institute of Geochemistry and Petrology, Zürich, Switzerland
²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany
³Space Research and Planetary Sciences, University of Bern, Bern, Switzerland

Cadmium is a highly volatile element and its abundance in meteorites may help better understand volatility-controlled processes in the solar nebula and on meteorite parent bodies. The large thermal neutron capture cross section of ¹¹³Cd suggests that Cd isotopes might be well suited to quantify neutron fluences in extraterrestrial materials. The aims of this study were (1) to evaluate the range and magnitude of Cd concentrations in magmatic iron meteorites, and (2) to assess the potential of Cd isotopes as a neutron dosimeter for iron meteorites. Our new Cd concentration data determined by isotope dilution demonstrate that Cd concentrations in iron meteorites are significantly lower than in some previous studies. In contrast to large systematic variations in the concentration of moderately volatile elements like Ga and Ge, there is neither systematic variation in Cd concentration amongst troilites, nor amongst metal phases of different iron meteorite groups. Instead, Cd is strongly depleted in all iron meteorite groups, implying that the parent bodies accreted well above the condensation temperature of Cd (i.e., ≈650 K) and thus incorporated only minimal amounts of highly volatile elements. No Cd isotope anomalies were found, whereas Pt and W isotope anomalies for the same iron meteorite samples indicate a significant fluence of epithermal and higher energetic neutrons. This observation demonstrates that owing to the high Fe concentrations in iron meteorites, neutron capture mainly occurs at epithermal and higher energies. The combined Cd-Pt-W isotope results from this study thus demonstrate that the relative magnitude of neutron capture-induced isotope anomalies is strongly affected by the chemical composition of the irradiated material. The resulting low fluence of thermal neutrons in iron meteorites and their very low Cd concentrations make Cd isotopes unsuitable as a neutron dosimeter for iron meteorites.

Reference
Kruijer TS, Sprung P, Kleine T, Leya I and Wieler R (in press) The abundance and isotopic composition of Cd in iron meteorites. Meteoritics & Planetary Science
[doi:10.1111/maps.12240]
Published by arrangement with John Wiley & Sons

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Maria Lugaro¹, Giuseppe Tagliente^2,8, Amanda I. Karakas³, Paolo M. Milazzo⁴, Franz Käppeler⁵, Andrew M. Davis^6,9,10, and Michael R. Savina^7,9

¹Monash Centre for Astrophysics (MoCA), Monash University, Clayton, VIC 3800, Australia
²Istituto Nazionale di Fisica Nucleare (INFN), Bari, Italy
³Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁴Istituto Nazionale di Fisica Nucleare (INFN), Trieste, Italy
⁵Karlsruhe Institute of Technology, Campus North, D-76021 Karlsruhe, Germany
⁶The Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
⁷Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
⁸Also at University of Ghent, Ghent, Belgium.
⁹Also at Chicago Center for Cosmochemistry, USA.
¹⁰Also at The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA.

We present model predictions for the Zr isotopic ratios produced by slow neutron captures in C-rich asymptotic giant branch (AGB) stars of masses 1.25-4 M_☉ and metallicities Z = 0.01-0.03, and compare them to data from single meteoritic stardust silicon carbide (SiC) and high-density graphite grains that condensed in the outflows of these stars. We compare predictions produced using the Zr neutron-capture cross sections from Bao et al. and from n_TOF experiments at CERN, and present a new evaluation for the neutron-capture cross section of the unstable isotope ⁹⁵Zr, the branching point leading to the production of ⁹⁶Zr. The new cross sections generally present an improved match with the observational data, except for the ⁹²Zr/⁹⁴Zr ratios, which are on average still substantially higher than predicted. The ⁹⁶Zr/⁹⁴Zr ratios can be explained using our range of initial stellar masses, with the most ⁹⁶Zr-depleted grains originating from AGB stars of masses 1.8-3 M_☉ and the others from either lower or higher masses. The ^90,91Zr/⁹⁴Zr variations measured in the grains are well reproduced by the range of stellar metallicities considered here, which is the same needed to cover the Si composition of the grains produced by the chemical evolution of the Galaxy. The ⁹²Zr/⁹⁴Zr versus ²⁹Si/²⁸Si positive correlation observed in the available data suggests that stellar metallicity rather than rotation plays the major role in covering the ^90,91,92Zr/⁹⁴Zr spread.

Reference
Lugaro M, Tagliente G, Karakas AI, Milazzo PM, Käppeler F, Davis AM and Savina MR (2014) The Impact of Updated Zr Neutron-capture Cross Sections and New Asymptotic Giant Branch Models on Our Understanding of the S Process and the Origin of Stardust. The Astrophysical Journal 780:95.
[doi:10.1088/0004-637X/780/1/95]

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