Iron and Zinc isotope fractionation during magmatism in the continental crust: Evidence from bimodal volcanic rocks from Hailar basin, NE China

Abstract

This study presents Fe–Zn isotope data for a suite of well–characterized bimodal volcanic rocks from Hailar Basin, northeast China to understand the mechanism of Fe isotope fractionation in highly differentiated igneous rocks. The samples range from basaltic trachyandesites to trachytes–rhyodacites, and rhyolites. The δ56Fe values increase with increasing SiO2 contents with the rhyolites having the highest δ56Fe (up to 0.64±0.02‰) among the previously reported data for igneous rocks at a similar SiO2. The lack of correlation between δ56Fe and Rb/La argues against the effect of fluid exsolution on Fe isotopes. The δ56Fe do not show a clear correlation with δ66Zn and radiogenic isotopes, suggesting that thermal diffusion or crustal contamination cannot produce the high δ56Fe in Hailar volcanic rocks.Fe isotopic variation in Hailar volcanic rocks can be explained by two steps of magmatism. During the first step, partial melting of basaltic trachyandesites with an average δ56Fe of 0.09±0.14‰ produced trachytes–rhyodacites with an average δ56Fe of 0.24±0.27‰. Modelling using rhyolite–MELTS shows that Fe isotopes can be fractionated by preferential partitioning of isotopically different Fe3+ and Fe2+ between the solid residue and partial melt. The second step involves formation of rhyolites with significantly high δ56Fe through partial melting or extensive crystallization of crust materials, during which isotopically heavy Fe preferentially partition into the rhyolitic melt. Therefore, fractionation of Fe isotopes between melts and minerals can result in high δ56Fe in SiO2-rich igneous rocks and apparent Fe isotope heterogeneity within the continental crust.