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

Abstract

Fresh samples of hypabyssal kimberlite from the five major kimberlite pipes in the Kimberley area of South Africa have been analysed for their bulk-rock major and trace element geochemistry. The geochemical data allow identification of the influence of crustal contamination in certain samples, best illustrated in terms of elevated SiO2, Al2O3, Pb and heavy rare earth element (HREE) contents. Samples devoid of such crustal contamination show coherent major and fluid-immobile trace element variations, whereas fluid-mobile trace elements are scattered. Kimberlites rich in macrocrysts are shown to reflect substantial (up to 35%) entrainment of mantle peridotite, with Ni–SiO2 and Sc–SiO2 variations defining mixing trajectories towards garnet lherzolite. The likely primary magma(s) parental to the Kimberley kimberlites is suggested to have a composition of 26–27 wt % MgO, 26–27 wt % SiO2, ∼2·2 wt % Al2O3 and Mg number 0·86. Subtle differences in chondrite-normalized REE abundance patterns can be explained by small variations in the degree of partial melting within the range 0·4–1·5%, leaving residual garnet. The data are satisfied by melting a source enriched relative to chondrites by a factor of ∼10 in light REE (LREE), with chondritic or lower HREE abundances. Extended normalized trace element diagrams exhibit significant negative K, Rb, Sr and Ti anomalies that are interpreted to be primary magma characteristics, despite evidence for secondary mobility of K, Sr and Rb. A model is proposed in which fluid or melt from a sub-lithospheric source region precipitates phlogopite en route to metasomatizing the overlying subcontinental mantle lithosphere, imprinting its geochemical signature on a source region previously depleted in HREE relative to primitive mantle. Subsequent ∼1% melting of the metasomatized source produces a kimberlite with compatible element characteristics strongly influenced by depleted lithospheric peridotite (high Mg number, high Ni, low HREE), but with incompatible elements (and their isotope ratios) characteristic of the deeper source. The similarity of incompatible element ratios (Nb/U, Nb/Th, Ce/Pb) in the kimberlite magmas to those of ocean island basalts from the South Atlantic suggests an ultimate origin in an upwelling mantle plume.