Evidence for Lithospheric Mantle Uniformity Beneath Cratons

Cratons are characterized by thick mantle roots that have experienced high degrees of partial melting, resulting in a cold, strong, and buoyant mantle compared to its oceanic counterpart. The extent of chemical variability within cratonic roots, the role of cratons in insulating the mantle over time and subsequent triggering of continental break-up, however, remains debated. To better understand the lithospheric and asthenospheric compositional variability of cratons, we combine phase equilibrium computations with the inversion of P-to-s and S-to-p receiver function waveforms and fundamental-mode Rayleigh wave dispersion data recorded at 53 globally distributed seismic stations in different tectonic settings with a focus on cratonic regions. Because existing binary basalt-harzburgite models are unable to account for the variability in Mg# (MgO/[MgO + FeO]) and Mg/Si ratios recorded in xenoliths from cratonic regions, we propose an extension of the binary model that is based on nominally pyroxenite, lherzolite and dunite. The retrieved mantle lithospheric compositions have elevated Mg# (∼90–93) compared to asthenospheric mantle (Mg# ∼89), consistent with their having undergone differing degrees of melt extraction at mean pressures of 3–4 GPa. There are no indications for systematic differences in mantle composition or thermal structure with craton age or location. Instead, we find that the potential temperature of the asthenosphere beneath cratons is roughly 50°C cooler than the surrounding ambient mantle. This suggests that the insulating (i.e., heating) effect of continents may not be as prominent as implied by dynamical studies simulating the exchange of heat and material across the mantle.