Characterising the Distinct Crustal Protoliths of Roberts Victor Type I and II Eclogites

The origin of the eclogites that reside in cratonic mantle roots has long been debated. In the classic Roberts Victor kimberlite locality in South Africa, the strongly contrasting textural and geochemical features of two types of eclogites have led to different genetic models. We studied a new suite of 63 eclogite xenoliths from the former Roberts Victor Mine. In addition to major- and trace-element compositions for all new samples, we determined O-18/O-16 for garnet from 34eclogites. Based on geochemical and textural characteristics we identify a large suite of Type I eclogites (n=53) consistent with previous interpretations that these rocks originate from metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, crystallised from melts of depleted mid-ocean ridge basalt (MORB) mantle. We identify a smaller set of Type II eclogites (n=10) based on geochemical and textural similarity to eclogites in published literature. We infer their range to very low delta O-18 values combined with their varied, often very low zirconium-hafnium (Zr-Hf) ratios and light rare earth element-depleted nature to indicate a protolith origin via low-pressure clinopyroxene-bearing oceanic cumulates formed from melts that were more depleted in incompatible elements than N-MORB. These compositions are indicative of derivation from a residual mantle source that experienced preferential extraction of incompatible elements and fractionation of Zr/Hf during previous melting.

Automated summary

The origin of eclogites in cratonic mantle roots has long been debated. In a classic kimberlite locality in South Africa, two types of eclogites with contrasting features have led to different genetic models. A new study of 63 eclogite xenoliths from the area identified two types: Type I likely originating from metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, and Type II possibly derived from low-pressure clinopyroxene-bearing oceanic cumulates formed from melts more depleted in incompatible elements than normal mid-ocean ridge basalt.