The mass-dependent stable osmium isotope system -
Method development and application to high-temperature systems

Abstract

This PhD thesis reports the development and application of a new geochemical tool; stable isotopes of osmium (Os) and their mass-dependent fractionation. Osmium is of interest because it holds some very specific characteristics (e.g. highly siderophile, refractory, chalcophile under mantle conditions) that makes it of interest for examining key processes in geo- and cosmo- sciences. Previous studies have explored the abundance of Os, the radiogenic Os isotopes, and mass-independent Os anomalies. However, no previous study has explored the stable isotopes of Os in a mass dependent manner. The first objective of this thesis, which will be presented in chapter 2, was to develop a method that is capable of resolving variations in Os stable isotopes. After a method was developed that has been shown to provide high precision and reproducible stable Os isotope compositions, it was applied to various rock types to examine key and current questions within geosciences. Specifically, focus was put on examining the abundance of highly siderophile elements in the Earth’s mantle and an attempt was made to unravel the processes associated with metal core crystallization. To this end, the stable Os isotope composition of chondrites, Earth’s mantle samples, and iron meteorites have been constrained. The outcome of these studies is presented in chapters 3 and 4. Chondrite data show that the bulk solar nebula composition holds a homogeneous composition. Earth’s mantle samples are on average similar to chondrites, which is consistent with the late veneer model for Earth’s highly siderophile element budget. Iron meteorites yield significant stable Os isotope variation, showing that planetary core solidification generates significant stable Os isotope variability. The latter provides new insights in the crystallization history of planetary cores, showing the strong potential of mass dependent Os isotopes as a new geochemical tool.