46th Lunar and Planetary Science Conference (2015) 1835.pdf THE EFFECT OF COSMIC RAY IRRADIATION ON PLATINUM AND PALLADIUM ISOTOPES IN IRON METEORITES. A. C. Hunt1, M. Ek1 and M. Schönbächler1. 1ETH Zürich, Institute for Geochemistry and Petrology, Clausiusstrasse 25, 8092 Zürich, Switzerland. E-mail: [email protected] 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 182Hf-182W [e.g. 1] or 107 Pd-107Ag [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 . Platinum is a highly siderophile element with six naturally occurring isotopes (190Pt, 192Pt, 194Pt, 195Pt, 196 Pt and 198Pt). Platinum isotopes are a powerful neutron dosimeter as a result of the burnout of 195Pt on to 196 Pt . Platinum isotopes are also affected by neutron capture by 191Ir, which can result in the production of 192 Pt . Therefore, depending on the Ir/Pt ratio of the sample, as well as shielding and exposure age, it is possible to produce large 192Pt 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 (102Pd, 104Pd, 105Pd, 106 Pd, 108Pd and 110Pd). Neutron capture by 106Pd can potentially lead to disturbances to the 107Pd-107Ag chronometer (Fig. 1). Large excesses in 104Pd can also be generated if Rh/Pd ratios of the sample are sufficiently elevated, due to the burnout of 103Rh to 104Rh, followed by β-decay to 104Pd (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 104Pd and more established dosimeters such as 192Pt [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 . Platinum. A second ion exchange column was developed for further purification and removal of Ir from Pt . 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 ε192Pt/195Pt . 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 191Ir/195Pt 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, 191 195 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. Figure 1. Isotopic shifts predicted by  for an iron meteorite with a pre-atmospheric radius of 40 cm and an exposure age of 625 Ma. Variations in ε107Ag/109Ag vs. ε104Pd/105Pd for a range of Pd/Ag ratios and Rh/Pd = 2 are shown. Palladium. Ruthenium causes isobaric interferences on 102Pd and 104Pd. Doping experiments show that it is vital to achieve Ru/Pd ratios of < 0.005 in order to ensure an accurate interference correction when measuring by MC-ICPMS . Separation of Pd from Ru was achieved by drying the sample in a mixture of aqua regia and perchloric acid. The sample was then loaded onto anion exchange resin where Pd was successfully separated from remaining Ru and other matrix elements. This procedure typically yields Ru/Pd ratios < 0.0005. 46th Lunar and Planetary Science Conference (2015) For both Pt and Pd, yields after anion exchange chemistry are 70 % or better, and total procedural blanks are less than 1 ng. Isotopic analyses. All Pt and Pd analyses were made using a Thermo Scientific Neptune Plus MCICPMS with a Cetac Aridus II desolvating nebulizer and standard H cones, operated in low-resolution mode. Additionally, the MC-ICPMS is fitted with two 1012 Ω resistors which were employed for collecting 190 Pt and 188Os, or 101Ru and 111Cd for interference corrections. For Pt analyses, all isotopes were collected simultaneously. In addition, 188Os and 200Hg were monitored to correct for isobaric interferences, and 191Ir was monitored to check tailing effects onto Pt isotopes. Analyses were corrected for instrumental mass bias using the exponential law, and were internally normalized to both 198Pt/195Pt (‘8/5’) = 0.2145 and 196Pt/195Pt (‘6/5’) = 0.7464 [5, 6]. Samples were measured relative to NIST SRM 3140 Pt standard solution. The daily external precision (2 S.D.) of a 260 ppb Pt standard is typically better than 0.8 for ε192Pt/195Pt (8/5). Reproducibility for SRM 3140 passed through ion exchange chemistry is ε192Pt/195Pt (8/5) = 0.2 ± 0.8 (2 S.D., n = 6). 1835.pdf Discussion: Palladium. Our results using the Aridus II introduction system for the Pd SRM 3138 standard (both untreated and passed through ion exchange chemistry) and the Josephinite yielded identical Pd isotope compositions (Fig. 2). This shows that our new analytical method provides accurate and precise results. Platinum. New data for the IVA meteorite group confirm previous analyses by [5, 7], which suggest that these meteorites were weakly exposed to the effects of neutron capture due to GCR (Fig. 3). Many IIAB samples were also weakly exposed, but their low Ir/Pt ratios result in a limited spread in ε192Pt/195Pt. IVB samples show larger variations, in particular for ε192Pt/195Pt, which is due to their higher and more variable Ir/Pt ratios (Fig. 3). Our new data correlate well with GCR models of , and confirm that Pt isotopes can be used to correct GCR effects without prior knowledge of exposure time and depth in sample. Figure 3. ε196Pt/195Pt vs. ε192Pt/195Pt. Modelled GCR exposure trends from  are also shown for Ir/Pt ratios of the analysed samples. New data for the IVA, IVB and IIABs overlap with literature data [5, 6, 7]. Figure 2. The 104Pd/105Pd ratios for SRM 3138 measured using a Neptune MC-ICPMS in wet plasma mode, compared to an Aridus II introduction system. The Aridus II was tested with both standard H cones and an X skimmer cone. Also shown are data for SRM 3138 and a terrestrial Pd-bearing mineral (Josephinite) which had been passed through column chemistry. All Pd isotopes were simultaneously analysed. Additionally, 101Ru and 111Cd were monitored to correct for potential isobaric interferences. Instrumental mass bias was corrected for using the exponential law and 108 Pd/105Pd = 1.18899 . All samples were measured relative to the NIST SRM 3138 standard. The daily external precision (2 S.D.) of a 100 ppb Pd standard solution is typically better than 15 and 10 ppm for 104 Pd/105Pd and 106Pd/105Pd, respectively (Fig. 2). References:  Kleine T. et al. (2009) Geochim. Cosmochim. Acta, 73, 5805-5818  Chen J. H. and Wasserburg G. J. (1990) Geochim. Cosmochim. Acta, 54, 1729-1743  Theis K. et al. (2013) Earth. Planet. Sci. Letters, 361, 402-411  Leya I. and Masarik J. (2013) Meteoritics & Planet. Sci., 48, 665-685  Kruijer, T. S. et al. (2013) Earth. Planet. Sci. Letters, 361, 162-172  Wittig, N. et al. (2013) Earth. Planet. Sci. Letters, 361, 152-161  Kruijer et al. (2014) Science, 344, 1150-1154  Wittig N. et al. (2013) LPSC 44, Abstract #2355  Mayer B. et al. (2014) LPSC 45, Abstract #2581  Rehkämper M. and Halliday A. N. (1997) Talanta, 44, 663-672  Hunt et al. (2014) 77th Metsoc, Abstract #5260  Ek et al, (2014) 77th Metsoc, Abstract #5262  Kelly W. R. and Wasserburg G. J. (1978) Geophys. Res. Lett., 5, 1079-1082.
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