Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
6430405 | Earth and Planetary Science Letters | 2012 | 8 Pages |
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 (δ26Mg) for cratonic eclogites from Kaalvallei and Bellsbank kimberlite pipes, South Africa. We find that clinopyroxene is 0.375±0.069 to 0.676±0.075â° 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 â0.797±0.075 to â0.139±0.061â°, values that are significantly lighter than the range displayed by global mantle peridotites to date (â0.25±0.07â°, 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 Î26Mg and Î57Fe provides evidence for redox control on equilibrium inter-mineral Fe isotope fractionation.
⺠High-precision Mg isotopic data are reported for cratonic eclogites and minerals. ⺠The large Mg isotopic variation in eclogites indicates a crustal origin. ⺠The decoupled Mg and Fe isotopic variation in mantle rocks indicates a redox-controlled Fe isotope fractionation.