Multiple volcanic episodes of flood basalts generated by thermochemical mantle plumes

Recent geochronogical data commonly reveals more than one pulse of major eruptions of several flood basalts, spanning time scales of tens of millions of years. The long duration between eruptions suggests a deep and possibly common mechanism for this episodicity. We have shown that the entrainment by a thermal plume of a dense basal layer can generate secondary instabilities due to the interaction between thermal and compositional buoyancy forces. The time separation between the arrival at the base of the lithosphere of the original plume head and replenishment by subsequent instabilities ranges from less than 10 million to more than 100 million years. The dense layer contributes to strong lateral heterogeneity and intense mixing in the plume head and secondary instabilities that travel up the low viscosity plume conduit. Under certain conditions the plumes may remain as nearly steady features in the deep mantle for more than 70 million years, possibly providing an explanation for the low seismic velocity provinces in the lowermost mantle. The dynamics of mantle plumes can be significantly more complicated than that of purely thermal plumes. The intrinsic density contrast of the entrained material will also cause a much reduced topographic effect of the arrival of the plume below the lithosphere compared to that predicted from thermal plumes.