Kimberlites and lamproites: extreme products of mantle enrichment processes

Lamproites and kimberlites exhibit a wide range in Nb, Pb and Sr-isotope ratios which is at present much greater than that reported from mantle xenoliths and continental basalts. They contain at least a contribution from old (1-2,5 Ga) upper mantle source regions which had low Sm/Nd, but variable Rb/Sr and U/Pb ratios. Contrasting Pb-Pb and Sm-Nd whole rock ages for lamproites suggest that, on average, Sm/Nd was reduced by approximately 45 per cent during melting and fractionation, which in the simplest model implies very small (<1%) degrees of melting. High Rb/Sr ratios, and hence with time, high Epsilon sub (Sr) values can occur in rocks with both high and low Rb/Ba. The majority tend to have Rb/Ba = 0,1-0,03, and the inferred enrichment of trace elements in their source regions is consistent with the introduction of small volume partial melts. The comparatively rare high Rb/Ba rocks, in contrast, may reflect mantle metasomatism in the presence of H2O-rich fluids. A discussion of the fractionation of trace elements that might result from mantle metasomatism further suggests that it has been responsible for the development of distinctive trace element features in relatively few mantle-derived rocks