Magnesium isotopic variations in cratonic eclogites: Origins and implications

Cratonic eclogites play an important role in the formation and dynamic evolution of the 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 (delta(26)mg) for cratonic eclogites from Kaalvallei and Bellsbank kimberlite pipes, South Africa. We find that clinopyroxene is 0.375 +/- 0.069 to 0.676 +/- 0.075 parts per thousand 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, which range from -0.797 +/- 0.075 to -0.139 +/- 0.061960, values that are significantly lighter than the range displayed by global mantle peridotites to date (-0.25 +/- 0.07 parts per thousand, 2SD). As significant Mg isotope fractionation is only known to occur during low-temperature water-rock interaction, our results provide further evidence for the derivation of cratonic eclogites from subducted altered oceanic crust. In addition, the lack of correlation between Delta Mg-26 and Delta Fe-57 provides evidence for redox control on equilibrium inter-mineral Fe isotope fractionation. (C) 2012 Elsevier B.V. All rights reserved.