Magnesium isotope variations in cratonic eclogites: origins and implications

Cratonic eclogites play an important role in the formation and dynamic evolution of the ancient subcontinental lithospheric mantle. However, their origin, whether as fragments of subducted oceanic crust or high-pressure mantle cumulates, remains controversial. Here, we report Mg isotopic compositions (δ26Mg) for six cratonic eclogites from Kaalvallei and Bellsbank kimberlite pipes, South Africa. Clinopyroxene in the cratonic eclogite is 0.375 ± 0.069 to 0.676 ± 0.075‰ heavier than coexisting garnet, which reflects equilibrium isotope fractionation between these phases, primarily driven by the difference in Mg coordination between clinopyroxene and garnet. Bulk eclogites have strikingly variable Mg isotopic compositions, with δ26Mg ranging from -0.797 ± 0.075 to -0.139 ± 0.061‰ and most of them significantly lighter than global mantle peridotites (-0.25‰ ± 0.07). As significant Mg isotope fractionation is only known to take place during low-temperature water-rock interaction, our results provide further evidence for the derivation of cratonic eclogites from subducted altered oceanic crust. Results here also show that Mg isotopic composition of the mantle is heterogeneous and indicates Mg isotopes could be a great tracer for studying mantle processes and crustal recycling. Additionally, there is no correlation between Δ26Mg and Δ57Fe of cratonic eclogites. The lack of correlation suggests that high-temperature equilibrium inter-mineral Fe isotope fractionation is redox-controlled, while Fe coordination does not play an important role. Combined studies of Mg and Fe isotopes analyses on mineral pairs may therefore provide a novel way to identify the mechanism responsible for Fe isotope fractionation.