The purpose of this paper is to provide, discuss, and interpret a comprehensive set of geochemical data (involving major elements as well as Ni, Ti, Sr, Y, Zr, Nb, Hf and the rare earth elements) for peridotitic garnets in diamonds from Premier and Finsch, with a view on the nature of the metasomatic processes operating up to the time of diamond crystallisation, and the location of these two diamondiferous kimberlites within and outside the region of low seismic velocity in the Kaapvaal lithosphere. Trace element data were acquired using an ion microprobe, and a new method for the analysis of Ni in garnet by ion microprobe is presented. Peridotitic garnets in diamonds from the Premier mine are characterised by a significantly higher proportion of the lherzolite paragenesis relative to diamonds from other South African mines, such as Finsch, Venetia and De Beers Pool. Based on Ni-in-garnet thermometry, inclusion encapsulation temperatures of 1055 °C to 1669 °C are calculated for peridotitic garnets from Premier, with an average temperature of 1215 °C. Calculated temperatures for garnets from Finsch range from 1036 °C to 1167 °C, and are generally lower than for Premier, with an average of 1098. °C. The garnets in the diamonds from Premier and Finsch reflect contrasting styles of metasomatism associated with diamond crystallisation, with a low temperature fluid-type metasomatism prevalent in the case of the Paleoarchean diamonds from Finsch, and a higher temperature melt-related metasomatism occurring in the case of the Paleoproterozoic diamonds from Premier. The metasomatic agent accompanying diamond crystallisation at Finsch is effective at introducing Sr, the light rare earth elements, and some Zr into the lithosphere, but is ineffective at transporting much Ca, Ti, Y and heavy rare earth elements. In the case of Premier the metasomatic agent is highly effective at element transport, introducing e.g. Ca, Fe, Ti, Zr, Y and the rare earth elements. The location of the Premier kimberlite within a region of lithosphere characterised by low seismic velocity, and which is considered to have formed through the infiltration of melts associated with the intrusion of the Bushveld Complex, is here linked to the trace element chemistry of peridotitic garnets in diamonds from this locality. The location of the Finsch kimberlite outside of the seismic velocity anomaly can also be linked to the composition of peridotitic garnets in diamonds from Finsch, where a contrasting, lower temperature fluid dominated style of metasomatism is seen.