High-temperature inter-mineral magnesium isotope fractionation in mantle xenoliths from the North China craton

Abstract

To investigate the magnitude and mechanism of inter-mineral Mg isotope fractionation at mantle temperatures, we measured Mg isotopic compositions of coexisting olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), phlogopite (Phl) and spinel (Spl) from harzburgite, lherzolite and clinopyroxenite xenoliths in the North China craton. These xenoliths are well-characterized and formed over a wide temperature range from ~ 800 to 1150 °C. The coexisting Opx and Ol have constant and indistinguishable Mg isotopic compositions, with δ 26Mg ranging from − 0.29 to − 0.22‰ in Ol and from − 0.28 to − 0.22‰ in Opx (Δ 26Mg Opx–Ol = δ 26Mg Opx − δ 26Mg Ol = − 0.04 to + 0.04‰; n = 11). By contrast, Mg isotopic compositions of Cpx and Phl are variable and slightly heavier than coexisting Ol (Δ 26Mg Cpx–Ol = 0 to + 0.13‰, n = 13; Δ 26Mg Phl-Ol = + 0.11 to + 0.20‰, n = 3). Isotope fractionations between coexisting Cpx and Ol are correlated with temperatures, implying equilibrium isotope fractionation. The degree and direction of isotope fractionations among these mantle silicates agree with theoretical predictions, suggesting that inter-mineral Mg isotope fractionation is primarily controlled by the Mg O bond strength, with stronger bonds favoring heavier Mg isotopes. Cpx and Phl have shorter and thus stronger Mg O bonds, and hence are isotopically heavier than coexisting Ol. Compared with coexisting silicates, δ 26Mg values of spinels are more variable and much heavier, ranging from + 0.03 to + 0.28‰. The Δ 26Mg Spl–Ol values vary significantly from + 0.25 to + 0.55‰ (n = 10) and show an excellent, positive, linear correlation with 10 6/T 2(K) [Δ 26Mg Spl–Ol = 0.63(± 0.12) × 10 6/T 2(K) − 0.03(± 0.08)], indicating equilibrium Spl–Ol isotope fractionation. The absence of intra-mineral isotopic variation and quantitative diffusion calculations further confirm isotope exchange equilibrium between coexisting Spl and Ol. Our results demonstrate the existence of measurable Mg isotope fractionation between mantle minerals and suggest that the large high-temperature equilibrium Spl–Ol Mg isotope fractionation in peridotite xenoliths can potentially be used as a geothermometer in mantle geochemistry.