Reply to the Discussion by Mitchell and Tappe on “Kimberlites and aillikites as probes of the continental lithospheric mantle”

We thank Mitchell and Tappe for giving us the opportunity to discuss the nature of kimberlite magmas. We agree with Mitchell and Tappe about the importance of mineralogy, but the mineralogy of interstitial phases in kimberlites can shed little light on the origin of kimberlite magmas. Our use of cation units enables the interpretation of kimberlite whole-rock compositions in terms of the stoichiometry of their constituent minerals. When viewed in this manner, the variation in the whole-rock compositions of kimberlites clearly indicates that they are mixtures of harzburgitic lithospheric mantle and a carbonate-rich liquid. Aillikites are systematically more Fe-rich than kimberlites, but both contain less silica than olivine, such that they would evolve to lower Si contents via olivine fractionation. The opposite is true of olivine lamproites and meimechites. Although kimberlite whole-rock compositions do not reliably predict diamond grade on a local scale, there is a clear correlation between diamond grade and kimberlite composition on a global scale, which likely has the same explanation as the correlation between G10 garnets and diamonds. Considering that there is still no officially accepted definition of kimberlite, refusal to even consider the implications of their whole-rock compositional variation verges on lack of scientific due diligence.