The controls on transition metal stable isotope fractionation during high-temperature magmatic processes

Abstract

Transition metal stable isotopes have the potential to trace a variety of high-temperature geological processes because they can respond to magmatic processes such as partial melting, fractional crystallisation, and changes in redox. However, since the precision required to apply transition metal isotopes to high-temperature settings has only been achieved recently, there are still significant gaps in our understanding. This thesis aims to expand our understanding of the controls on vanadium, titanium, iron, and zinc stable isotope fractionation during high-temperature magmatic processes.

Mantle xenoliths and mineral separates from the Diavik diamond mine were measured to examine the roles of oxygen fugacity, bonding environment, and metasomatism on V isotopes. This data represents the first δ51V measurements on natural mineral separates. Vanadium isotopes were found to positively correlate with fO2 in garnet and clinopyroxene, suggesting that fO2 does influence δ51V. However, metasomatism and bonding environment were found to be more significant controls on δ51V.

IODP Expedition 352 subduction initiation fore-arc basalts (FABs) and boninites were measured for V, Ti, Fe, and Zn stable isotopes to assess the comparative contributions of magmatic versus subduction processes in the formation of arc lavas. Mantle partial melting and source depletion modelling are suggested to explain the observed δ56Fe differences between FABs and boninites. Contrary to previous δ66Zn work on forearc serpentinites, slab-derived inputs are unlikely to create observable δ66Zn fractionation in arc lavas. Vanadium isotopes are shown to respond to differences in source depletion and potentially changes in redox during subduction initiation, and Ti isotopes suggests that the mantle may be heterogeneous in δ49Ti. Finally, both V and Ti isotopes correlate with fluid mobile elements in subduction-related boninites, suggesting that despite V and Ti being considered fluid immobile, halogen-rich slab-derived fluids have the potential to mobilise and fractionate δ51V and δ49Ti.