A new approach to reconstructing the composition and evolution of kimberlite melts: a case study of the archetypal Bultfontein kimberlite (Kimberley, South Africa)

The compositions of kimberlite melts at depth and upon emplacement in the upper crust remain elusive. This can be attributed to the unquantified effects of multiple processes, such as alteration, assimilation, xenocryst contamination, and fractional crystallisation. The inability to accurately constrain the composition and physical properties of kimberlite melts prevents a comprehensive understanding of their petrogenesis.

To improve constraints on the compositions of kimberlite melts, we have combined modal analysis including the discrimination of xenocrystic from magmatic phases, with mineral chemistry determinations to reconstruct a whole-rock composition. We apply this approach to a sample of "fresh" macrocrystic hypabyssal kimberlite (sample BK-1) from the Bultfontein mine (Kimberley, South Africa). The accuracy of this whole-rock reconstruction method is validated by the similarity between reconstructed and measured whole-rock compositions. A series of corrections are then applied to account for the effects of post-emplacement serpentinisation, pre-emplacement olivine crystallisation, and the inclusion and assimilation of mantle material. This approach permits discernment of melt compositions at different stages of kimberlite evolution.

The primitive melt parental to the Bultfontein kimberlite is estimated to contain 17.4–19.0 wt% SiO₂, 20.2–22.8 wt% MgO, 20.9–21.9 wt% CaO, 2.1–2.3 wt% P₂O₅, 1.2–1.4 wt% TiO₂, 0.9–1.1 wt% Al₂O₃, and 0.6–0.7 wt% K₂O, and has a Mg# of 83.4–84.4. Primary volatile contents (i.e., after an attempt to account for volatile loss) are tentatively estimated at ~2.1–2.2 wt% H₂O and ~22.9–25.4 wt% CO₂. This composition is deficient in SiO₂, MgO and H₂O, but enriched in CaO and CO₂ compared with most previous estimates of primitive kimberlite melts. We suggest that the primitive melt parental to the Bultfontein kimberlite was a transitional silicate-carbonate melt, which was progressively enriched in SiO₂, MgO, Al₂O₃, Cr₂O₃, and Na₂O through the assimilation of lithospheric mantle material. Comparisons with experimentally produced low-degree melts of carbonated lherzolite indicate that the Bultfontein kimberlite could have formed by ~0.5% melting of asthenospheric mantle at ~6.0–8.6 GPa (i.e., ~190–285 km) and ~1400–1500 °C. The low calculated Na₂O contents (<0.2 wt%), which are inconsistent with derivation from low-degree melting of lherzolite, suggest that an alkali-bearing, volatile-rich fluid was exsolved during ascent or released after emplacement, and subsequently removed.