Galactic Cosmic Rays

Determining Rh/Pd and Ir/Pt to assess the effects of galactic cosmic rays on Pt and Pd isotope ratios in IAB, IIAB, IIIAB, IVB iron meteorites

Student : David Farsky

Supervisor : Dr Alison Hunt, Prof Dr Maria Schönbächler ; institute of Geochemistry and Petrology

When galactic cosmic rays (GCR) irradiate the surface of planetary bodies nuclear products are generated. In iron meteorites these products are intensively studied to more accurately assess the exposure history and the core formation age. Alterations in short lived decay systems like 182Hf – 182W are correlated with alterations in isotopic abundances of Pt and Pd (Kruijer et al., 2013). Secondary neutron capture happens in iron meteorites mostly in the resonance capture region with elements of large neutron capture cross section. Platinum and Palladium have proven to be good neutron dosimeters. In this work the elemental ratios of Ir/Pt and Rh/Pd in IAB, IIAB, IIIAB and IVB iron meteorites are measured using a Thermo Scientific Element ICP-MS. The measured ratios show good agreement with the literature data. The results are compared to Pt and Pd isotope data and a GCR model (Leya & Masarik, 2013). Platinum isotope data show good agreement with the GCR model when modelled using our new Ir/Pt ratios, for all iron meteorite groups studied. For our new Rh/Pd ratios, the Pd data only agree with the GCR model for the IAB and IIAB groups. IIIAB and IVB meteorites do not fit to the GCR model, possibly as a result of underlying nucleosynthetic variations. Further proof for these results were found in the Pd vs. Pt isotope plots. IAB und IIAB show no nucleosynthetic variations while IIIAB and IVB do, which agrees with earlier studies (Mayer, Wittig, Humayun, & Leya, 2015).