High-Mg and low-Mg mantle eclogites from Koidu (West African craton) linked by Neoproterozoic ultramafic melt metasomatism of subducted Archaean plateau-like oceanic crust

Bimineralic eclogites and pyroxenites (n = 75 +/- accessory rutile, ilmenite, sulphide, apatite) from the Koidu kimberlite (West African Craton) were investigated for mineral major and trace elements and mineral Sr-Nd isotope compositions to constrain (1) the nature and age of their basaltic to picritic protoliths, and (2) the effect, timing and source of mantle metasomatism. Consistent with published work, samples are grouped into low-Mg eclogites with Mg# from 0.49 to 0.73 (median 0.59; n = 40) and high-Mg eclogites with Mg# from 0.60 to 0.88 (median 0.75; n = 14), plus pyroxenites [clinopyroxene Na/(Na + Ca) <0.2; n = 8] and gabbroic eclogites and pyroxenite (Eu/Eu* of reconstructed bulk-rocks >1.05; n = 8), with five unclassifiable samples. Reconstructed low-Mg and gabbroic eclogites have major and trace element systematics (Eu/Eu*-heavy rare earth elements-Y) indicating crustal protolith crystallisation, confirming an origin as subducted oceanic crust. Their high FeO contents at MgO > similar to 10 wt % require an Fe-rich source, the high melt productivity of which led to the formation of thicker crust, perhaps in a plateau-like setting. This is consistent with SiO2-MgO relationships indicating differentiation at similar to 0.5 GPa. Unradiogenic Sr in some clinopyroxene (Sr-87/Sr-86 of 0.7010-0.7015), combined with light rare earth element (LREE) depletion relative to normal mid-ocean ridge basalt (N-MORB) for the majority of samples (average N-MORB-normalised Nd/Yb of unmetasomatised samples = 0.51), suggests eclogitisation and partial melt loss in the Neoarchaean, possibly coeval with and parental to 2.7Ga overlying continental crust. Most reconstructed high-Mg eclogites and some pyroxenites formed by metasomatic overprinting of low-Mg eclogites and gabbroic eclogites, as indicated by the preservation of positive Eu anomalies in some samples, and by the Mg-poorer composition of included versus matrix minerals. Coupled enrichment in MgO, SiO2 and Cr2O3 and in incompatible elements (Sr, LREE, Pb, Th and U) is ascribed to metasomatism by a kimberlite-like, small-volume, carbonated ultramafic melt, mediated by addition of clinopyroxene from the melt (i.e. stealth metasomatism). Strontium-Nd isotope systematics suggest a Neoproterozoic age for this metasomatic event, possibly linked to Rodinia break-up, which facilitated intrusion of asthenospheric carbonated melts with an ocean island basalt-like Sr-87/(86)Sri of similar to 0.7035. Cretaceous kimberlite magmatism (including Koidu), with more radiogenic Sr-87/Sr-86 (similar to 0.7065, intermediate between Kaapvaal kimberlites and orangeites), may have been partially sourced from associated Neoproterozoic metasomes. The presence of diamonds in low-Mg eclogites, but absence in high-Mg eclogites, indicates the diamond-destructive nature of this event. Nevertheless, the moderate proportion of affected eclogites (similar to 35%) suggests preservation of a sizeable diamond-friendly mantle eclogite reservoir beneath Koidu.