Effects of melt percolation on iron isotopic variation in peridotites from Yangyuan, North China Craton

• Fe isotope ratios of two suites of mantle xenoliths from Yangyuan were measured.
• Mg-lherzolites show a limited variation in Fe isotopic composition.
• Fe-rich lherzolites exhibit extremely light Fe isotopic compositions.
• Such variation appears to result from diffusion-driven kinetic isotope effects.

The petrology, geochemistry and Fe isotopic composition of two suites of mantle xenoliths (magnesium-lherzolites and Fe-rich lherzolites) and their associated mineral separates from Yangyuan, in the Central Zone of the North China Craton, were measured in detail to investigate the effects of metasomatism on Fe isotope fractionation and mantle Fe isotopic heterogeneity. The magnesium-lherzolites (Mg# = 89.8 to 90.9; TFeO = 8.3 to 8.9 wt.%) from Yangyuan are residual mantle lherzolites with no or only moderate metasomatism. They have a relatively limited Fe isotopic variation (δ57Fe = 0.007 to 0.169‰) with an average of 0.083 ± 0.044‰ (2SD/√n, n = 8), similar to the range reported previously for typical mantle lherzolites, and they show no significant inter-mineral disequilibria. In contrast, the Fe-rich lherzolites examined in detail in this study are strongly metasomatized and are enriched in iron (Mg# = 89.8 to 90.9; TFeO = 11.2 to 16.8 wt.%). These Fe-rich lherzolites exhibit extremely light Fe isotopic compositions, with δ57Fe ranging from − 0.615 to − 0.311‰, and disequilibrium Fe isotope fractionation occurs between most of their coexisting minerals (Δ57FeOpx–Ol = − 0.616 to 0.522‰; Δ57FeCpx–Ol = − 0.003 to 0.230‰; Δ57FeSp–Ol = 0.191 to 0.522‰; Δ57FeSp–Opx = 0.206 to 0.510‰; Δ57FeSp–Cpx = 0.112 to 0.297‰; Δ57FePhl–Ol = 0.097 to 0.450‰; and Δ57FeAmp–Ol = 0.048 to 0.147‰). Overall, the large disequilibrium isotope fractionations in Fe-rich lherzolites likely results from the kinetic nature of the isotopic exchange between the wall-rock mantle and the percolating melt. This process results in the heterogeneous Fe isotopic composition of the lithospheric mantle beneath the Central Zone of the North China Craton.