The Effect of Cosmic Ray Irradiation on Platinum and Palladium Isotopes in Iron Meteorites

Introduction: Dating of early solar system processes such as differentiation and core formation of planetesimals and planets frequently relies on shortlived decay systems such as Hf-W [e.g. 1] or Pd-Ag [e.g. 2,3]. However, these systems are susceptible to the effects of galactic cosmic rays (GCR) and hence, acquiring correct ages requires knowledge of the exposure history of the meteorite. Exposure to GCR causes secondary neutron capture on isotopes with large neutron cross sections, which can result in large shifts in the measured isotope ratios [4]. Platinum is a highly siderophile element with six naturally occurring isotopes (Pt, Pt, Pt, Pt, Pt and Pt). Platinum isotopes are a powerful neutron dosimeter as a result of the burnout of Pt on to Pt [4]. Platinum isotopes are also affected by neutron capture by Ir, which can result in the production of Pt [4]. Therefore, depending on the Ir/Pt ratio of the sample, as well as shielding and exposure age, it is possible to produce large Pt excesses. This relationship is demonstrated for the IVA, IVB, IIAB and IID iron meteorites [5, 6, 7], highlighting that Pt isotopes are a powerful tool for correcting the effects of GCR irradiation. Palladium is also a highly siderophile element with six naturally occurring isotopes (Pd, Pd, Pd, Pd, Pd and Pd). Neutron capture by Pd can potentially lead to disturbances to the Pd-Ag chronometer (Fig. 1). Large excesses in Pd can also be generated if Rh/Pd ratios of the sample are sufficiently elevated, due to the burnout of Rh to Rh, followed by β-decay to Pd (Fig. 1). Therefore Pd can potentially be used as a neutron dosimeter to correct for cosmogenic Ag. In this study we aim to collect Pt and Pd isotopes from the same sample aliquot in order to assess the effects of neutron capture on both elements in various iron meteorite groups. Previous studies reported a good agreement between variations in Pd and more established dosimeters such as Pt [8, 9]. We present new data for the IIAB, IIIAB, IVA and IVB iron meteorites, including the samples North Chile, Sikhote-Alin, Henbury, Cape York, Gibeon, Muonionalusta, Tawallah Valley and Santa Clara. Methods: Fusion crust and weathered edges were removed from all samples before digestion. Additionally, samples were leached in cold 2 M HCl, before dissolution in aqua regia. Primary separation of Pd from Pt (and general matrix elements) was achieved using AG1-X8 anion exchange resin following a procedure modified from [10]. Platinum. A second ion exchange column was developed for further purification and removal of Ir from Pt [11]. This is important as Ir causes tailing effects onto Pt isotopes during measurement by MC-ICPMS. In previous studies Ir tailing resulted in corrections of between 2 and 15 to εPt/Pt [5]. Iridium was reduced before the samples were loaded onto anion exchange resin and then separated from Pt. This procedure was repeated in order to achieve a Ir/Pt ratio of less than 0.16, thus minimizing the effect of tailing from Ir isotopes onto Pt. Finally, Pt cuts were dried in a mixture of aqua regia and perchloric acid in order to volatilize remaining Os, which generates isobaric interferences on Pt isotopes. After chemistry, both Os and Ir are removed to an adequate level. In particular, Ir/Pt ratios are less than 0.02 such that no additional correction for tailing of Ir onto Pt is necessary, therefore overcoming a major analytical uncertainty.