UC San Diego

The ~1.15 billion-year-old (Ga) Cullinan kimberlite pipe is composed of several lithologically distinct kimberlite facies (Grey, Brown, Pale Piebald, Dark Piebald, Black Coherent, Blue/Brown Transitional, and Fawn). In this study, I report bulk rock Os isotope data for these facies, as well as a suite of peridotites, pyroxenites and an amphibolite. Measured 187Os/188Os abundances of the different kimberlite facies range from 0.1223 to 0.1672, and peridotites range from 0.1096 to 0.1244. The pyroxenites have radiogenic measured 187Os/188Os ratios of 0.9376 and 0.1796 and the amphibolite 2.861. These data are complemented by bulk rock highly siderophile element abundances (HSE: Re, Pd, Pt, Ru, Ir, Os) and major element and trace element abundances. Kimberlite melt infiltration in the peridotite xenoliths is evident in the presence of melt veins, and enriched rare earth element and Re abundances. Negative La/Yb versus Re/Os correlations indicate that peridotites may have experienced metasomatism prior to kimberlite assimilation. The kimberlites and peridotites plot on a 187Re/188Os versus 187Os/188Os mixing line (R2=0.924) indicating that the range in measured187Os/188Os for the kimberlites reflects variable assimilation of cratonic lithospheric mantle (CLM), with the Brown kimberlite being the least affected by this process. The peridotites display a trend between higher Re concentration and lower TRD (Time of Rhenium Depletion) eruption ages. This suggests TRD eruption ages for some Proterozoic age samples are erroneously young, due to additional Re added prior to kimberlite entrainment, possibly linked to the Bushveld Igneous Event.