A Discussion on volcanism and the structure of the Earth - Kimberlites and their relation to the mantle

Abstract

Kimberlite is a rock-type that originates within the Earth’s upper mantle, and which has sampled en route to the surface a variety of upper mantle rocks that now occur as xenoliths, or, in a fragmented form, xenocrysts in the kimberlite. Kimberlite is, in fact, a hybrid rock resulting from the incorporation of the mantle xenoliths and xenocrysts into a fine-grained matrix with which they may interact. This matrix, which must result from modification of primary mantle material, consists mineralogically mainly of calcite, serpentine, ilmenite, perovskite, magnetite, phlogopite and apatite; chemically, the matrix is rich (relative to garnet peridotite) in TiO 2 , Al 2 O 3 , total iron, CaO, K 2 0 , Na 2 O, H 2 O, CO 2 and P 2 O 5 , and a large number of the so-called ‘ incompatible ’ trace elements. The kimberlite matrix focuses attention upon the presence of various elements the source of which in the upper mantle is not adequately explained by the currently popular garnet peridotite model. The source difficulties are to some extent resolved by accepting titaniferous phlogopite as an additional upper mantle phase, although the source of carbon, sulphur, nitrogen and phosphorus is still a matter for speculation.