Kimberlite and the Evolution of the Mantle

Abstract

Kimberlites are extremely enriched in the incompatible elements and are therefore important in discussions of the trace-element inventory of the Earth's mantle. Such extreme enrichment implies that kimberlites represent a small degree of partial melting of a mantle silicate or a late-stage residual fluid of a crystallizing cumulate layer. The LIL (large-ion lithophile) elements in kimberlite show that they have been in equilibrium with a garnet-clinopyroxene rich source region, possibly an eclogite cumulate. The LIL contents of kimberlite and MORB (mid-ocean ridge basalts) are complementary. Removal of a kimberlite-like fluid from an eclogite cumulate gives a crystalline residue with the required geochemical characteristics of the depleted source region that provides MORB. Kimberlite and lunar KREEP are remarkably similar in their minor and trace element chemistry. The main differences can be attributed to plagioclase fractionation in the case of KREEP and eclogite fractionation in the case of kimberlite. KREEP has been interpreted as the residual fluid of a crystallizing magma ocean. In a small body the Al_2O_3 content of a crystallizing melt is reduced by rilagioclase crystallization and flotation. In a magma ocean on a large body, such as the Earth, the Al_2O_3 is removed by the sinking of garnet. Kimberlite, in fact, is depleted in eclogite elements including the HREE and Na. This suggests that kimberlite may represent the late-stage residual fluid of a crystallizing terrestrial magma ocean. A buried eclogite cumulate layer is the terrestrial equivalent of the lunar anorthositic crust.