Tracing origins of cratonic eclogites by magnesium isotopes

Abstract:

Cratonic eclogites are samples of lithospheric mantle preserved beneath ancient continental cratons. Hence, the origin of cratonic eclogites is closely related to the formation and evolution of cratonic mantle. Here we report Mg isotopic compositions for 27 cratonic eclogites and 52 garnet and clinopyroxene mineral separates from Koidu, Bellsbank and Kaalvallei kimberlite pipes in South and West Africa. Whole-rock Mg isotopic compositions vary widely, with δ²⁶Mg ranging from -1.60 to +0.17, significantly different from the value (δ²⁶Mg = -0.25 ± 0.07) of peridotite xenoliths. Garnet and clinopyroxene in these cratonic eclogites record equilibrium inter-mineral Mg isotope partitioning at mantle temperatures, with Δ²⁶Mg_Cpx-Grt (= δ²⁶Mg_Cpx - δ²⁶Mg_Grt) in the range of 0.43 – 0.85 ‰. The constructed bulk δ²⁶Mg values based on mineral compositions are identical to the measured whole-rock values, indicating limited influence of kimberlite infiltration on Mg isotopic compositions of cratonic eclogites. As significant Mg isotope fractionation can only occur during low-temperature surface processes, the large Mg isotopic variations of cratonic eclogites suggest the incorporation of subducted materials in their protoliths. Therefore, our Mg isotopic data suggest the cratonic eclogites are the remnants of subducted oceanic crust within the lithospheric mantle. Collectively, Mg isotopes are potentially excellent tracers of the formation and evolution of sub-continental lithospheric mantle.