Iron isotope fractionation during magmatic evolution and implications for isotope disequilibrium in the Bushveld Complex

Chemical variations in mafic intrusions are used to infer primary igneous processes such as magma injection, assimilation and fractional crystallization. However, such cryptic variations often tell a more intricate story reflective of the interplay between early crystal accumulation and later processes occurring within crystal mushes. Determining the extent to which the magmatic record has been overprinted by subsequent post-accumulation processes is thus key to understand how basaltic magmas evolve within crustal magma chambers. As an ubiquitous element, iron stable isotopes have successfully been used to address petrogenetic issues in igneous rocks including fractional crystallization, partial melting, liquid immiscibility and sub-solidus diffusional re-equilibration, as well as to reveal processes and time scales of magmatic evolution in volcanic rocks.

As the largest known mafic intrusion, the Bushveld Complex likely preserved a significant post- crystallization heat anomaly, with cooling rates estimated between 20 and 1000 °C/Ma. We measured the iron isotope compositions of bulk pyroxenites and chromitites as well as olivine, pyroxenes and Cr-spinel separates from the Critical Zone of the Bushveld Complex to investigate post-cumulus magmatic processes during cooling and crystallization. We observe large isotopic variations at mineral scales, with Cr-spinels displaying systematically lighter Fe isotope compositions than their coexisting silicate mineral assemblage. Overall, our dataset highlights significant inter-mineral isotope fractionation, inconsistent with high temperature equilibrium isotope partitioning. Instead, the iron isotope data reflects kinetic effects associated with diffusional processes and variable degrees of chemical re-equlibration during the prolonged cooling of the Bushveld Complex.