THE CRATONIC MANTLE, ISOPYCNICITY, AND KIMBERLITE PROVINCES

The isopycnicity hypothesis assumes that the cratonic lithospheric mantle has a unique composition such that low-density composition of old cratonic mantle nearly compensates high density due to low lithosphere temperature. This hypothesis is supported by petrological studies of mantle xenoliths hosted in kimberlite magmas. However, any type of magmatism is atypical for stable regions, and thus the representativeness of the kimberlite sampling may be questioned. Other aspects of the isopycnicity hypothesis have also been challenged recently in a number of studies.

Here we calculate the density structure of the cratonic lithospheric mantle in Siberia and Kaapvaal and show that geochemically studied kimberlite-hosted xenoliths sample exclusively those parts of the mantle where the isopycnic condition is satisfied. We demonstrate that the pristine lithospheric mantle which has not been affected by magmatism has a significantly lower density than required by isopycnicity. We therefore conclude that our knowledge on the composition of cratonic mantle is biased by kimberlite sampling which provides deceptive basis for the isopycnicity hypothesis.