METASOMATISM OF THE MANTLE LITHOSPHERE RECORDED BY RARE-EARTH ELEMENTS IN GARNETS

Garnets derived from peridotite and kimberlite concentrates in southern African kimberlites show normal and sinusoidal REE patterns. Both types of REE patterns are interpreted to have resulted from the metasomatism of previously existing mantle garnets by LREE-enriched melts or fluids. In this model, metasomatism proceeded in accordance with garnet-liquid K(d)s which increase by orders of magnitude from the LREE and MREE to the HREE, thus fractionating the REE into three groups. The development of sinusoidal REE patterns resulted from the HREE lagging behind the LREE during metasomatic readjustment in composition. Sinusoidal REE patterns represent a state of disequilibrium during the re-equilibration process and are retained by garnets of relatively refractory compositions. Garnets from off-craton and on-craton regions show both types of REE patterns, suggesting that the presence of refractory garnets typical of cratonic regions is not restricted to the craton. Calculated liquid compositions for the metasomatic melts resemble kimberlite and lamproite, but were probably unrelated to the host kimberlites. Melts from off-craton regions are less LREE rich than melts from cratonic regions. These results suggest a different lithosphere in the off-craton region, possibly reflecting extensional tectonics and a higher degree of mantle upwelling.