A plate tectonic origin of kimberlites on a cooling Earth

Abstract

International audienceDuring the past 20 years it has become fashionable to link global kimberlite magmatism to large igneous provinces, hotspot tracks, and mantle plumes. We reappraise the evidence used to propose these connections and find that compelling cases of cause-and-effect relationships between thermal anomalies in the deep mantle and kimberlite eruptions on thick continental lithospheres are rare if not absent. A new integrated analysis of emplacement ages, petrologic phase equilibria, and Nd-Hf-W isotopic compositions of kimberlites from Africa through time suggests that these CO2-H2O-rich high-Mg magmas represent low-degree partial melting products of rather 'normal' fertile peridotite beneath the thickest portions of relatively 'cold' continental lithospheres. Near the LAB beneath cratonic regions, volatile-fluxed incipient mantle melting dominates over a major melting regime; only the latter leads to production of large basaltic magma volumes. Importantly, upper mantle melting by volatile fluxing gained significance only after 2 Ga, when the ambient mantle potential temperature had dropped to 2 conditions directly beneath cratons explain the strong link to kimberlite melt formation after 2 Ga. We acknowledge that global kimberlite magmatism between 250 and 50 Ma appears to be attracted to the surface projection of the western margin of the African LLSVP. However, a new geodynamic reconstruction, in which we combine African plate velocities and kimberlite eruption incidents, demonstrates a link between plate tectonic motions and volatile-rich mantle-derived magmatism. This diverse approach suggests that global kimberlite magmatism, as recorded at Earth's surface, does not necessarily represent plume-related melting events, but rather melt drainage events through thick continental lithospheres that have been repeatedly under significant tectonic stresses while moving on a cooling Earth (Tappe et al., 2018). Tappe et al., 2018, Geodynamics of kimberlites on a cooling Earth. Earth and Planetary Science Letters 484, 1-14