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

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 (delta Fe-57 = 0.007 to 0.169 parts per thousand) with an average of 0.083 +/- 0.044 parts per thousand (2SD/root 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 stronglymetasomatized 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 d57Fe ranging from -0.615 to -0.311%, and disequilibrium Fe isotope fractionation occurs between most of their coexisting minerals (Delta Fe-57(Opx-Ol) = -0.616 to 0.522%; Delta Fe-57(Cpx-Ol) = -0.003 to 0.230 parts per thousand; Delta Fe-57(Sp-Ol) = 0.191 to 0.522 parts per thousand; Delta Fe-57(Sp-Opx) = 0.206 to 0.510 parts per thousand; Delta Fe-57(Opx-Cpx) = 0.112 to 0.297 parts per thousand; Delta Fe-57(Phl-Ol) = 0.097 to 0.450 parts per thousand; and Delta Fe-57(Amp-Ol) = 0.048 to 0.147 parts per thousand). 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 percolatingmelt. This process results in the heterogeneous Fe isotopic composition of the lithosphericmantle beneath the Central Zone of the North China Craton. (C) 2015 Elsevier B.V. All rights reserved.