Iron isotope fractionation during fenitization: a case study of carbonatite dykes from Bayan Obo, Inner Mongolia, China

As a powerful tracer in high-temperature geochemistry, Fe isotopes have been studied for their behaviour during fluid exsolution and evolution related to felsic magma system, but that for carbonatite magma system remains unknown. Here we study the Fe isotope fractionation behaviour during fenitization–processes that widely occur associated with carbonatite or alkaline intrusions. Nine fenite/carbonatite samples from carbonatite dykes at Bayan Obo area are analyzed for their Fe isotope compositions as well as elemental compositions. Combined with previous reported carbonatite δ 56 Fe data, the results show that carbonatites range from − 0.35‰ to 0.28‰, with an average of -0.10‰ in δ 56 Fe values, while fenites range from − 0.17‰ to 0.30‰, with an average of 0.11‰ in δ 56 Fe values. This indicates that fenitizing fluids exsolved from carbonatite melts are enriched in heavier Fe isotopes. Such a Fe isotope fractionation trend is different from that for fluid exsolution from felsic magmatism. δ 56 Fe values in fenites are negatively correlated with indicators of fenitization intensity such as (Na + K), Ti, Ba, Th, Nb, U or Pb abundances, likely reflecting that Fe isotopes fractionate during the evolution of the fenitizing fluids. Thus, Fe isotopes are a valuable tool for tracing fluid exsolution and evolution relevant to carbonatite magmatism and related metal mineralization.