Mantle xenolith perspectives.

There are records of approx 3500 basaltic xenolith localities and a similar number of kimberlitic occurrences, although actual figures are much greater. The xenoliths group into: spinel-bearing peridotites from alkali basalts from oceanic, circum-oceanic, and young continental (rift) regions; spinel-garnet peridotites from peralkaline basaltic rocks in oceanic (plateau) and Phanerozoic-Proterozoic continental regions, and garnet peridotites together with shallower-seated xenoliths from kimberlitic intrusions in old continental (mainly cratonic) provinces. Xenoliths provide data on the constitution of the mantle, particularly beneath stable cratonic regions. Old events may be related to subduction, as indicated by eclogite suite xenoliths. A petrogenetic model for Group I kimberlites (with essentially an asthenospheric source) and Group II kimberlites including lamproites (essentially lithospheric) shows a paired relationship based on an ascending diapir with MORB-OIB characteristics. This results in early Group II lamproite eruptions from the lithosphere within its thermal aureole, followed by eruptions of Group I kimberlites when the diapir impinges on the base of the lithosphere. -J.M.H.