Sources of cratonic metasomatic fluids: In situ LA-MC-ICPMS analysis of Sr, Nd, Hf and Pb isotopes in Lima from the Jagersfontein Kimberlite

Abstract

LIMA phases (LIndsleyite-MAthiasite: K-Ba-Zr-Cr titanates of the crichtonite series) in the Jagersfontein kimberlite (95 ± 5 Ma) are xenocrysts, related to the Phlogopite-K richterite-Peridotite (PKP) style of metasomatism that has affected the lithospheric mantle beneath the Kaapvaal Craton. The LIMA grains contain high levels of Sr, Hf, Nd, Pb and U, allowing a multi-isotope approach to the timing of metasomatism and the sources of metasomatic material. We provide a detailed discussion of the interference-correction and mass-bias issues involved in the in-situ isotopic analysis of these elements. Multiple in situ isotopic analyses of 11 grains by laser-ablation multi-collector ICPMS (and 2 by solution techniques) show significant within-grain and inter-grain variability in Sr and Pb isotopic composition, less variation in Nd-isotope composition, and very little variation in Hf-isotope composition. U-Pb dating, limited by high levels of initial Pb, gives a poorly constrained age of 162 ± 46 Ma, which may reflect an episode of PKP metasomatism. Back-calculation of the Pb-isotope data using measured 238U/204Pb values, and comparison with mantle evolution models, indicate a maximum age for the PKP metasomatism of 180 Ma, suggesting a link with the plume-related Karoo volcanism (179-183 Ma). The relationships between Pb, Sr, Hf and Nd isotopic compositions in LIMA, and published data on PKP–series xenoliths, suggest mixing between two components. One had high 87Sr/86Sr but low 206Pb/204Pb and 143Nd/144Nd, similar to the EM1 “reservoir.” This component probably represents lithospheric mantle that was enriched in Rb/Sr before the metasomatic episode that produced the LIMA phases. The other has isotopic characteristics similar to the “HIMU” reservoir, and may correspond to a plume-related fluid or melt responsible for at least the initiation of the metasomatism.