High-temperature inter-mineral magnesium isotope fractionation in eclogite from the Dabie orogen, China

Abstract

To investigate the magnitude and mechanism of inter-mineral Mg isotope fractionation at high temperatures, we report high-precision analyses of Mg isotopes for 10 whole rocks and 13 mineral separates for a set of eclogites from Bixiling in the Dabie orogen, China. Magnesium isotopic compositions of whole rocks (δ 26Mg of − 0.44 to − 0.26‰) are similar to the estimated δ 26Mg values of the mantle, suggesting Mg isotopic inheritance from a gabbroic protolith with limited Mg isotope fractionation during eclogite-facies metamorphism. By contrast, mineral separates are highly heterogeneous, with δ 26Mg values ranging from + 0.30 to + 0.60‰ in phengite, from + 0.16 to + 0.40‰ in omphacite and from − 0.95 to − 0.74‰ in garnet. Phengite and omphacite are > 1‰ heavier in δ 26Mg than coexisting garnet, indicating large high-temperature inter-mineral Mg isotope fractionations. The constant Δ 26Mg omphacite–garnet (= δ 26Mg omphacite − δ 26Mg garnet) value (1.14 ± 0.04‰), together with homogeneous mineral chemistry and equilibrium oxygen isotopic partitioning between omphacite and garnet, suggests an equilibrium Mg isotope fractionation, controlled by the difference in coordination number of Mg between omphacite (six) and garnet (eight). The 1.14‰ fractionation is the largest high-temperature equilibrium inter-mineral Mg isotope fractionation observed so far and makes the omphacite-garnet Mg isotope fractionation a potential geothermometer. By contrast, Mg isotope fractionations between phengite and garnet and between phengite and omphacite vary from 1.25 to 1.47‰ and from 0.14 to 0.32‰, respectively. This implies Mg isotopic disequilibria between phengite and garnet/omphacite, which might result from the Mg isotopic variation in phengites due to Mg isotope exchange between phengites and retrograde fluids in Bixiling eclogites.