Geochemical investigations of kimberlites from the Kimberley area, South Africa

Abstract

Fifteen major components and 21 trace elements (Li, F, S, Cl, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cd, Ba, Hg, Tl, Pb and Bi) were analyzed for 11 kimberlites and 3 xenoliths (garnet peridotite, eclogjte and gabbro) from South Africa. Selected major and trace element contents of olivine, serpentine, pyrox-ene, phlogopite, perovskite, spinel, magnetite, Fe-Ni-Cu-S ores and native copper in several kimberlites were also measured. Sulfur isotope compositions of 5 kimberlites and a garnet peridotite range from δ34 S -3 to +9.6%o- The concentrations of major elements and Cr, Co, Ni, Zn, Hg have a small, but those of Li, Na, K, Rb, Cl and T1 a wide range of variation. Good correlations of K-Rb have to be mentioned. Large Fe203/Fe0 ratios indicate relatively high oxygen fugacities during eruption. Additional data on about 60 elements in bulk kimberlites have been compiled from the literature to estimate the average chemical com-position of kimberlites. In order to know the accumulation and depletion of elements during magma for-mation, the elemental abundances of kimberlites were compared with those of undepleted mantle rocks. Kimberlites are characterized by very high concentrations of incompatible and volatile elements such as La, Ce, C, Nd, Th, U, F, Cs, Nb, Sm, Ta, Rb, Ba, K, P, Pb and Sr, and low concentrations of Si, Na, A1 and heavy REE. Concentrations of the former elements suggest that kimberlite magmas are produced by a small degree of partial melting in the presence of C02and H20 under upper mantle conditions. © 1983, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.