Kimberlites: Strange bodies?



Several years ago I asked a well-known mantle petrologist why kimberlites had been studied relatively little in comparison with kimberlite-transported xenoliths from the upper mantle and lower crust. He replied, ‘When I see a bus full of people travel by, I'm more interested in the passengers than in the bus.’ For many years kimberlites were relegated to the status of the vehicle for a petrologically intriguing load of xenoliths.

There are several reasons for the initial lack of attention to kimberlites. (1) They are highly brecciated and inclusion-rich. Many petrologists viewed them as a confusing potpourri of exogenous and endogenous fragments. In fact, some were uncertain whether kimberlites had any primary magmatic/liquid component. (2) Kimberlites usually have undergone a high degree of alteration, principally serpentinization and carbonation, but in some cases localized phlogopitization. Initially, most of the alteration was viewed as very late-stage, probably post-magmatic, and some of it perhaps due to weathering. (3) Because of their brecciated, altered condition, kimberlites often are porous, friable, and difficult to make into thin sections.