Fig. 4: Relationship between olivine composition and diamond grade in kimberlites worldwide.

Olivine lamproites from South Africa and India are again included. a Diamond grade (carats per hundred tonnes or cpht) vs mean Mg# (= Mg/(Mg/Fe) as molar proportions) of olivine cores, and b olivine rims. The vertical light green line indicates the boundary between high and moderate to low-grade kimberlites, which is set at 50 cpht. The horizontal dark green lines, which corresponds to Mg# of 90.3 for the cores and 89.0 for the rims, indicate the lower limit of kimberlites with high diamond grades (≥50 cpht). The arrow labelled ‘Int’ shows olivine compositions from the Internationalnaya kimberlite, which has an estimated grade of 937 cpht (Supplementary Data 1). The error bars represent 1 standard deviation of the mean for olivine Mg#, and 10% of the reported value for diamond grades. (c) Mean Mg# vs CaO content for olivine cores colour-coded relative to diamond grade, showing that high diamond grades are limited to kimberlites in the upper left quadrant (Mg# ≥90.3 and CaO ≤0.06 wt.%). (d) Plot of kimberlites sorted by diamond grade and colour-coded relative to olivine Mg# score (see Methods). Tonguma and Newlands are exceptions discussed in the text.