Helium isotope evidence for a deep-seated mantle plume involved in South Atlantic breakup

Earth history has been punctuated by episodes of short-lived (<10 m.y.), high-volume (>10(6) km(3)) magmatism. The origin of these events and their manifestations as large igneous provinces (LIPs) with associated continental flood basalts do not fit in the current plate-tectonic paradigm. Upper-mantle processes have been invoked for some LIPs, whereas the origin of others appears to be related to plumes rising from the deep mantle. The Parana-Etendeka LIP has remained enigmatic and highly contested in terms of plume versus upper-mantle models. Here, we provide evidence for a plume origin based on new isotopic (He, O, Sr, Nd, Pb) and trace-element data from olivine-rich dikes from Namibia. The composition of the dikes can be explained by mixing at shallow depths between a plume source with high He-3/He-4 (>26 R-A) and ambient asthenospheric mantle, before ascent through the thinning lithosphere.