Mlange Signatures and Low Oxygen Fugacity in Eclogite Xenoliths From the Crust-Mantle Transition Below a Mesoproterozoic Collision Belt

Subduction zones represent the main interface between Earth's surface and its deep interior. Metamorphic reactions during subduction cause fluid or melt loss from seawater-altered oceanic crust and sediment, which enriches the overlying mantle, and possibly oxidizes it. This would explain why the mantle sources of subduction zone magmas appear to be more oxidized than in other tectonic settings. However, the details of the mass transfer in this deep environment are difficult to constrain because it is inaccessible. Using rare deep-seated magmas (kimberlites) as probes of a ca. 1.2 billion year old southern African subduction zone, we investigated eclogite fragments that originated as subducted oceanic crust and were much later plucked from the wallrocks by the ascending magma. These eclogites show elemental and isotopic signatures of interaction with subducted sediments, pointing to mingling processes similar to those observed in modern subduction zones. We also estimated their oxygen fugacity, a measure of the chemical potential of oxygen. We find that sulfur, which has been implicated in mantle oxidation, would have only been stable in these rocks in its reduced form, making even seawater-altered eclogites sinks rather than sources of oxygen, with implications for the transfer of sulfur-loving metals across the mantle-to-crust-boundary.