Sr, Nd and Pb isotope and minor element geochemistry of lamproites and kimberlites

All samples studied here exhibit low ɛ _Nd (lamproites; Western Australia = -10 to -19.2 and Smoky Butte = -21.6 to -25.9: Group II kimberlites: Finsch Mine = -5.8 to -7.4) and ( ⁸⁷Sr/ ⁸⁶Sr) _i > Bulk Earth. They are all LREE enriched (up to 1000 × chondrite) and have high trace element contents (i.e. Rb, Rb, Ba, Th, K, Ta, Sr, P, Hf, Zr and Ti). Smoky Butte and Finsch rocks plot below the Pb-ore growth curve ( ²⁰⁶Pb ²⁰⁴Pb) = 16.02-16.64 and 17.74-17.99, and ²⁰⁷Pb ²⁰⁴Pb= 15.19-15.28 and 15.48-15.57, respectively) consistent with two-stage evolutions whereas the Western Australia rocks exhibit high ²⁰⁷Pb ²⁰⁴Pb (15.71-15.75) with unradiogenic ²⁰⁶Pb ²⁰⁴Pb (17.23-17.57) indicative of a more complex three-stage evolution. These lamproites and Group II kimberlites were at least predominantly derived from source regions which had had low, but variable UP/b and Rb/Sr for between 1 and 2.5 Ga. Such source regions are inferred to exist within the subcontinental mantle lithosphere. Different styles of trace element enrichment are recognised which with time develop different isotope-isotope trends. High Rb/Sr, Rb/Ba and K/Ti ratios observed in Group II kimberlites and metasomatised peridotites are probably related to the migration of H ₂O-rich fluids within the upper mantle, while the lamproites have features consistent with the introduction of small volume silicate melts to their source regions, i.e. high T/iK, Ta/Yb and low Rb/Ba. At Smoky Butte Rb/Sr was also low, but the Western Australia rocks have high Rb/Sr because they are relatively less enriched in Sr. Hence high Rb/Sr ratios (and with time high ⁸⁷Sr/ ⁸⁶Sr ) are developed in mantle source with both high and low Rb/Ba. Inferred μ for the last stage of evolution of their source regions is < 7.5 for all the rocks studied and there is a striking positive correlation between ɛ _Nd and ²⁰⁶Rb/ ²⁰⁴Pb , which indicates that low Sm/Nd and U/Pb ratios coexist (over long periods of time) within the subcontinental mantle lithosphere.