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

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 δ26Mg ranging from − 0.29 to − 0.22‰ in Ol and from − 0.28 to − 0.22‰ in Opx (Δ26MgOpx–Ol = δ26MgOpx − δ26MgOl = − 0.04 to + 0.04‰; n = 11). By contrast, Mg isotopic compositions of Cpx and Phl are variable and slightly heavier than coexisting Ol (Δ26MgCpx–Ol = 0 to + 0.13‰, n = 13; Δ26MgPhl-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 MgO bond strength, with stronger bonds favoring heavier Mg isotopes. Cpx and Phl have shorter and thus stronger MgO bonds, and hence are isotopically heavier than coexisting Ol. Compared with coexisting silicates, δ26Mg values of spinels are more variable and much heavier, ranging from + 0.03 to + 0.28‰. The Δ26MgSpl–Ol values vary significantly from + 0.25 to + 0.55‰ (n = 10) and show an excellent, positive, linear correlation with 106/T2(K) [Δ26MgSpl–Ol = 0.63(± 0.12) × 106/T2(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.

Research highlights
► Mg isotopes are analyzed for Ol, Opx, Cpx, Spl and Phl in mantle xenoliths.
► Large equilibrium isotope fractionation occurs between Spl and Ol.
► The Spl–Ol fractionation is temperature-dependent over a range of > 350 °C.
► Spl–Ol Mg isotope geochemistry can be a great geothermometry.