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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