A 187Re-187Os and highly siderophile element study of diamondiferous kimberlite melt-mantle interactions and the inferred age of continental lithosphere

Abstract

The ∼ 1.15-billion-year-old (Ga) Premier kimberlite pipe (Cullinan diamond mine), South Africa, is composed of several distinct kimberlite facies (Grey, Brown, Pale Piebald, Dark Piebald, Black Coherent [Type 3C], Blue/Brown Transitional, and Fawn). We report bulk rock Re-Os isotope data for Premier kimberlite facies, as well as for a suite of entrained peridotite and mafic xenoliths. These data are complemented by bulk rock highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os), major- and trace-element abundances. Measured ¹⁸⁷Os/¹⁸⁸Os for the kimberlite facies range from 0.1223 to 0.1672 (γOs_i of −2.5 to + 17.4), peridotite xenoliths range from 0.1096 to 0.1244 (γOs_i of −13.3 to −1.1), and pyroxenite xenoliths range from 0.1796 to 0.938 (γOs_i of + 27 to + 419). A single measured amphibolite xenolith has the most radiogenic measured ¹⁸⁷Os/¹⁸⁸Os of 2.86 (γOs_i of + 43). Harzburgite xenoliths yield time of rhenium depletion model ages (T_RD) of ∼ 1.5 to 2.8 Ga, consistent with average T_RD ages for Premier peridotites (2.4 ± 0.4 Ga). With these and published data, we considered the relationships between kimberlite and mantle xenoliths, compare estimates of relative peridotite incorporation to sampled diamond grade, and explore recratonization versus refertilization arguments with regards to T_RD model ages. Kimberlite melt infiltration into Premier peridotite xenoliths is evident from melt veins accounting for ∼ 2 and ∼ 14 modal % of samples, and has led to incompatible element enrichment, including elevated Re. In turn, kimberlites show geochemical evidence for addition of peridotite xenolith fragments, with Type 3C having > 30 % more peridotite contribution than the Brown volcaniclastic facies. Kimberlites and peridotites plot on a ¹⁸⁷Re/¹⁸⁸Os versus ¹⁸⁷Os/¹⁸⁸Os mixing line (R² = 0.92), with kimberlites having older apparent ages than the true age of crystallization. This mixing line provides estimates of lithospheric incorporation into the kimberlites, where the units with higher peridotite incorporation do not correlate with diamond grade. This is likely due to lithological and post-emplacement alteration heterogeneity within the kimberlite units, perhaps also reflecting the eclogitic paragenesis of many Premier diamonds. The peridotites provide evidence for the nature of the lithosphere beneath Premier prior to ∼ 1.15 Ga. Metasomatism of the peridotites is possibly linked to the Bushveld Igneous Event at ∼ 2 Ga, as well as to other magmatic events that affected the Kaapvaal craton from the Archean to the Mesoproterozoic. Premier peridotites do not suggest that the cratonic lithosphere beneath the region was completely replaced. Samples with Proterozoic T_RD eruption model ages may represent Archean lithosphere that experienced alteration by metasomatism and modification, such as during the Bushveld Igneous Event.