Introduction

Abstract

Kimberlite is a rare and volumentrically insignificant igneous rock. What, then, are the reasons for studying it? First, kimberlite and some rare xenoliths within it are the principal primary terrestrial source of the strategic mineral, diamond, and the study of kimberlite may eventually outline the conditions for the formation of diamond in nature. Second, it can now be demonstrated that the ultrabasic and basic xenoliths within kimberlite have originated within the upper mantle. Kimberlite has carried out a more extensive sampling of the upper mantle than any other type of magmatic activity, and it seems highly unlikely that the abandoned Mohole project would have provided even a small proportion of the information about the petrology and mineralogy of the upper mantle that has been gained from the study of xenoliths in kimberlite. In addition, the four-phase garnet Iherzolite xenoliths have received considerable attention from experimental petrologists in connection with the origin of basaltic magmas. The third principal reason for studying kimberlite is its chemistry. It contains high concentrations of lithophile elements in a rock that is essentially ultrabasic, and it also contains a much higher concentration of the so-called “incompatible” trace elements than other ultrabasic rocks. These features focus attention upon a source of these elements within the upper mantle and the processes concentrating them in kimberlite.