We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ56Fe in olivines varies from 0.18‰ to -0.22‰ with an average of -0.01±0.18‰ (2SD, n=18), opx from 0.24‰ to -0.22‰ with an average of 0.04±0.20‰, and cpx from 0.24‰ to -0.16‰ with an average of 0.10±0.19‰. δ26Mg of olivines varies from -0.25‰ to -0.42‰ with an average of -0.34±0.10‰ (2SD, n=18), opx from -0.19‰ to -0.34‰ with an average of -0.25±0.10‰, and cpx from -0.09‰ to -0.43‰ with an average of -0.24±0.18‰. Although current precision (~±0.06‰ for δ56Fe; ±0.10‰ for δ26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ56Feol-cpx=-0.10±0.12‰ (2SD, n=18); Δ56Feol-opx=-0.05±0.11‰; Δ26Mgol-opx=-0.09±0.12‰; Δ26Mgol-cpx=-0.10±0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ56Fe of peridotites in this study is 0.01±0.17‰ (2SD, n=18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ26Mg is -0.30±0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ56Fe and δ26Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ56Fe, the larger deviations from average δ56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ56Fe and slightly heavy δ26Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt.
ASJC Scopus subject areas
- Geochemistry and Petrology