The origin and implications of primordial helium depletion in the Afar mantle plume

Mantle plumes are responsible for the Earth’s largest volcanic provinces. In the prevailing paradigm, the deep mantle is less degassed than convecting shallow mantle, implying that plume-derived lavas have higher concentrations of primordial volatiles such as helium (He). Demonstrating this has led to explanations that question the established Earth model. Here, we show that the 3 He/ 4 He of basalts from the Red Sea display coherent relationships with trace elements, allowing the helium concentration of the Afar plume to be calculated. Contrary to the prevailing model it appears the helium concentration of the Afar plume is 10-25% of the upper mantle. This contradiction is resolved if the plume material itself is a mixture of helium-rich high- 3 He/ 4 He deep mantle with helium-depleted low- 3 He/ 4 He recently subducted oceanic crust. This implies that helium-depleted domains may exist in convecting mantle and that moderately high 3 He/ 4 He plumes likely do not contain a notable contribution of the deep mantle.