Nickel stable isotope fractionation in planetary materials
Nickel is a moderately siderophile and refractory first row transition metal, and a relatively new addition to the field of investigation of mass dependent stable isotope fractionation processes. Nickel is compatible in the magnesium-rich silicate minerals that constitute most upper mantle lithologies. This work provides the first detailed study of Ni stable isotopic compositions for such materials, with a particular focus on ultramafic xenoliths, oceanic basaltic rocks and lunar samples. The methods use ion exchange chromatography and multi-collector ICPMS with double spiking, adapting procedures of Gall et al., 2012. It is found that Ni isotopic compositions are not fractionated by partial melting or fractional crystallisation. There are no significant fractionations between mantle minerals. However, there are variations within the terrestrial mantle and in basalts from the Earth and Moon. On Earth, the deviations toward lighter compositions in enriched mid ocean ridge basalts show a relationship with the presence of enriched trace element chemical compositions. Ocean island basalts also display isotopic heterogeneities. In ultramafic samples it is the more metasomatised and iron-rich lithologies that are isotopically lightest, the most extreme being pyroxenitic material. This provides evidence that recycling of basaltic materials into the mantle leaves a signature of isotopically light Ni. Nickel isotopic compositions can therefore behave as a tracer for the composition of the mantle source region, uncomplicated by melting processes, redox changes, or time since enriched material was added to the mantle source region. On the Moon, variations in Ni isotopic composition have been shown not to relate to cosmogenic effects. The lunar Ni isotopic compositions appear to relate to the abundance of plagioclase and incompatible elements in the sample. Nickel isotopes, therefore, behaved differently in the lunar environment and were fractionated by processes that are insignificant or do not occur on Earth, probably in the lunar magma ocean.