Kimberlite segregation from an uppermost asthenospheric thermal boundary and the longevity of cold craton roots

Long-lived (>2.5 Ga) cratons usually preserve ancient cold and refractory mantle roots, but how the deep roots survive from recycling back to the convective mantle remains open to debate. Here, the mechanism for preservation of Archean mantle roots is explored using the major-, trace-element and Sr[sbnd]Nd isotopic systematics of kimberlites, the asthenosphere-derived magmas under cratons. A case study on ∼480 Ma kimberlites of the North China Craton suggests that their segregation domains have pressures (∼5 GPa) shallower than the lower boundaries of typical craton roots and potential temperatures (T_p) between those of the ambient asthenosphere (T_p = ∼1400 °C) and the cold lithospheric roots of cratons (∼1200 °C). The dataset of primary kimberlites worldwide records similar temperature variation in their segregation domains, which likely represent the lowermost (asthenospheric) part of a thick thermal boundary layer between conductive lithosphere and convective asthenosphere. Our calculation on mantle viscosity suggests that the asthenospheric part of the thermal boundary layer would show marked viscosity increase due to thermal offset from normal mantle adiabat. The resultant resistant uppermost asthenosphere can serve as a protective sheath that can protect the cratonic roots from being eroded and removed. Our proposed model emphasizes the longevity of cratons provided simply by the thermal contrast between the cold craton roots and the asthenosphere.