Origin and implications of He-depleted Afar mantle plume

Abstract Mantle plumes originate in the deep Earth and where they reach the surface tend to produce abnormally hot basaltic lavas. Plume-derived lavas are characterised by enrichments in primordial volatiles that are best tracked by the isotopic composition of the noble gases He, Ne and Xe. In the prevailing paradigm the deep mantle is significantly less degassed thus plume-derived lavas should have higher concentration of primordial He compared to lavas from the more vigorously convecting shallow mantle. Demonstrating this has proved challenging and has led to alternative explanations that question the established Earth model. Here, we show that the 3He/4He ratio of submarine basaltic glasses from the Red Sea and Gulf of Tadjoura displays a coherent relationship with diagnostic trace element ratios and distance from the Afar plume axis. Contrary to the prevailing model the data require that the upwelling Afar mantle plume has 4-10 times less He than the nearby upper mantle despite maintaining high 3He/4He. This contradiction can be resolved in the Afar plume is itself a mix of He-rich, high-3He/4He deep mantle with He-depleted, low-3He/4He oceanic crust that has been subducted in the last 80 Myr. This model places strong constraints on the origin of the slab in the Afar mantle and where it is incorporated into the upwelling mantle. Further, it implies that He-depleted domains may be common in the upper mantle, it may explain why plume-derived lavas have lower He concentrations than mid-ocean ridge basalts and suggests that upwelling plumes with moderately high 3He/4He are unlikely to contain a geochemical signature of the deep mantle.