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Molybdenum and Platinum Isotope Anomalies in Iron Meteorites – Constraints on Solar Nebula Heterogeneities and Parent Body Processes

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Molybdenum and Platinum Isotope Anomalies in Iron Meteorites – Constraints on Solar Nebula Heterogeneities and Parent Body Processes Molybdenum and platinum isotope anomalies in iron meteorites – constraints on solar nebula heterogeneities and parent body processes Graeme M. Poole Imperial College London Department of Earth Science and Engineering A thesis submitted for the degree of Doctor of Philosophy (PhD) 2016 Abstract Recent investigations revealed systematic nucleosynthetic Mo isotope anomalies in meteorites, affording clear evidence for variable excesses in p- and r-process nuclides, and hence deficits in s-process nuclides. These anomalies were interpreted as reflecting selective destruction/removal of unstable presolar components, within the framework of thermal processing models that also take into account data for other elements (e.g., Ru, Zr, Os). To test such models, this study has undertaken extensive measurements of Mo and Pt isotopes in iron meteorites, providing the most precise data for the broadest range of samples analysed to date. The data presented here are in agreement with previous studies, with all groups analysed (except the IAB/IIICD complex) displaying deficits in s-process Mo nuclides, with the extent varying between groups. This unique dataset allows, for the first time, resolution of decoupled p-process and r-process isotope effects, providing the basis for an updated thermal processing model. Mass-independent Pt isotope anomalies were also observed, but these are interpreted as entirely cosmogenic in origin, resulting from exposure of the meteoroids to galactic cosmic rays. No nucleosynthetic Pt isotope anomalies are resolvable, in accord with predictions from the updated thermal processing model. Systematic variations in the stable isotope compositions of Mo (δ98Mo) and Pt (δ198Pt) within iron meteorite groups were found, reflecting internal processes within the parent bodies. In detail, these result from isotope fractionation during metal–sulphide partitioning of Mo, and solid–liquid metal partitioning of Pt, respectively. Significantly, a previously undetected correlation between the magnitude of the nucleosynthetic Mo isotope anomalies and δ98Mo values of iron meteorite parent bodies provides novel support for the thermal processing model. However, no signatures of elemental processing in the solar nebula are resolvable in δ198Pt, as any such effects were overprinted by the isotopic fractionation that accompanied partitioning of Pt between solid and liquid metal. Copyright Declaration The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work. Declaration of Originality The contents of this thesis are entirely my own work, except where explicitly indicated by citation or acknowledgement. Graeme Poole March 2016 Table of Contents Chapter 1 Introduction 19 1.1 Rationale and aim of study ........................................................................................ 20 1.2 Solar system formation .............................................................................................. 20 1.3 Iron meteorites ........................................................................................................... 22 1.4 Mass-independent isotope effects .............................................................................. 24 1.4.1 Nucleosynthetic isotope anomalies .............................................................. 24 1.4.2 Cosmogenic isotope effects .......................................................................... 29 1.4.3 Measuring mass-independent isotope effects ............................................... 30 1.5 Mass-dependent isotope effects ................................................................................. 31 1.5.1 Measuring mass-dependent isotope effects .................................................. 32 1.6 Preliminary laboratory work ...................................................................................... 34 1.6.1 Reagents and materials ................................................................................. 34 1.6.2 Iron meteorite sample preparation ................................................................ 34 Chapter 2 Nucleosynthetic molybdenum isotope anomalies in iron meteorites 37 2.1 Introduction ................................................................................................................ 38 2.2 Modelling of nucleosynthetic Mo isotope anomalies ................................................ 41 2.2.1 Normalisation to 98Mo/96Mo ......................................................................... 42 2.2.2 Normalisation to 92Mo/98Mo ......................................................................... 43 2.2.3 Normalisation to 97Mo/95Mo ......................................................................... 43 2.3 Analytical techniques ................................................................................................. 44 2.3.1 Ion-exchange chromatography ..................................................................... 44 2.3.2 Mass spectrometry ........................................................................................ 45 2.4 Results ........................................................................................................................ 47 2.4.1 Standard solutions and reference materials .................................................. 47 2.4.2 Iron meteorites .............................................................................................. 52 2.5 Discussion .................................................................................................................. 63 2.5.1 Molybdenum heterogeneity of solar nebula ................................................. 63 2.5.2 Correlation with other elements ................................................................... 69 2.5.3 Origin of isotopic heterogeneity ................................................................... 72 2.6 Conclusions ................................................................................................................ 78 Chapter 3 Molybdenum stable isotopes in iron meteorites 79 3.1 Introduction ................................................................................................................ 80 3.2 Analytical techniques ................................................................................................. 81 3.2.1 Molybdenum double spike preparation and calibration ............................... 81 3.2.2 Ion-exchange chromatography ..................................................................... 82 3.2.3 Mass spectrometry ........................................................................................ 82 3.2.4 Resolving mass-dependent and mass-independent Mo isotope effects ........ 84 3.3 Results ........................................................................................................................ 86 3.3.1 Standard solutions and reference materials .................................................. 86 3.3.2 Iron meteorites .............................................................................................. 88 3.4 Discussion .................................................................................................................. 93 3.4.1 Non-magmatic iron meteorite parent bodies ................................................ 93 3.4.2 Magmatic iron meteorite parent bodies ........................................................ 93 3.4.3 Bulk δ98Mo of the iron meteorite parent bodies ........................................... 96 3.4.4 Stable Mo isotope evidence for thermal processing in the solar nebula ...... 98 3.4.5 δ98Mo of Earth ............................................................................................ 102 3.5 Conclusions .............................................................................................................. 103 Chapter 4 Mass-independent platinum isotope anomalies in iron meteorites 105 4.1 Introduction .............................................................................................................. 106 4.2 Modelling of mass-independent Pt isotope effects .................................................. 107 4.2.1 Nucleosynthetic Pt isotope anomalies ........................................................ 108 4.2.2 Cosmogenic Pt isotope anomalies .............................................................. 109 4.2.3 Resolving nucleosynthetic from cosmogenic Pt isotope effects ................ 110 4.3 Analytical techniques ............................................................................................... 112 4.3.1 Ion-exchange chromatography ................................................................... 112 4.3.2 Mass spectrometry ...................................................................................... 113 4.4 Results ...................................................................................................................... 115 4.4.1 Standard solutions and reference materials ................................................ 115 4.4.2 Iron meteorites ............................................................................................ 117 4.5 Discussion
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