Chemical variations in upper mantle nodules from southern African kimberlites

Abstract

A summary of the rock and mineral nodules erupted with kimberlites is presented. Garnet-peridotites are separated into various categories according to: their relatively depleted and fertile chemical character; their deformed and undeformed nature; and the Ca/(Ca+Mg) ratio of their clinopyroxenes (with associated temperature characteristics). Some bulk chemical similarities are noted between high-temperature, deformed and less depleted peridotites and the wall rocks to minor intrusions. There is clear evidence of the occurrence of infiltration metasomatism (involving K, Ti and other incompatible elements) in some mantle xenoliths before their incorporation in the kimberlite. These metasomatic effects may be linked with earlier magmatic events. Extreme chemical heterogeneities may be produced by metasomatism, as with restricted partial melting. Attempts to find homogeneous rocks with average or pristine upper mantle compositions are considered unrealistic. Within the Kaapvaal craton there is no overall pattern of lateral variations in the nodule types from kimberlite, though there is evidence of local heterogeneity and regional changes at craton margins. Boyd & Nixon's models of vertical layering of rocks and minerals in the upper mantle are contrasted with a model in which horizontal variations in temperatures occur after magma intrusion. It is suggested that the high-temperature deformed peridotites and the megacrysts result from the intrusion of high-temperature magmas into overlying cooler mantle, and that xenoliths from kimberlite provide little direct evidence of mantle stratification with depth.