Petrogenesis of Group I Kimberlites from Kimberley, South Africa: Evidence from Bulk-rock Geochemistry

Fresh samples of hypabyssal kimberlite from the five major kimberlitepipes in the Kimberley area of South Africa have been analysedfor their bulk-rock major and trace element geochemistry. Thegeochemical data allow identification of the influence of crustalcontamination in certain samples, best illustrated in termsof elevated SiO₂, Al₂O₃, Pb and heavy rare earth element (HREE)contents. Samples devoid of such crustal contamination showcoherent major and fluid-immobile trace element variations,whereas fluid-mobile trace elements are scattered. Kimberlitesrich in macrocrysts are shown to reflect substantial (up to35%) entrainment of mantle peridotite, with Ni–SiO₂ andSc–SiO₂ variations defining mixing trajectories towardsgarnet lherzolite. The likely primary magma(s) parental to theKimberley kimberlites is suggested to have a composition of26–27 wt % MgO, 26–27 wt % SiO₂,