Credit: Chad Paton

Kimberlites — magnesium-rich igneous rocks that are sometimes diamond-bearing — are generally thought to originate by melting of the mantle layer that forms the lower part of continental plates, the subcontinental lithospheric mantle. A recent geochemical study suggests that these magmas might instead come from the underlying mantle layer, the asthenosphere, perhaps from depths over 400 km.

Sampling two different fields of kimberlitic rocks on the Dharwar Craton in southern India, Chad Paton of the University of Melbourne and colleagues1 obtained high-resolution strontium isotopic data on individual crystals of the mineral perovskite, which are expected to reflect the composition of the rocks' mantle source. The remarkable homogeneity in the strontium isotopic composition of crystals within each of the fields is hard to reconcile with derivation of the magmas from the subcontinental lithospheric mantle, which is usually heterogeneous. Together with the mineralogy of the kimberlites and constraints from models of magma movement, the observed homogeneity is more likely to result from melting upwelling asthenospheric mantle, possibly from great depth.

The findings give us a rare insight into the nature of sources involved in the genesis of these peculiar magmas and shed additional light on their transport to the Earth's crust.