| Summary: | Basalts from high flux intra-plate volcanism (Iceland, Hawaii, Samoa) are characterised by 3He/4He that are significantly higher than those from the upper mantle sampled at mid-ocean ridges. The prevailing paradigm requires that a largely undegassed deep Earth is enriched in primordial noble gases (3He, 20Ne) relative to degassed convecting upper mantle. However, the He concentration of high 3He/4He oceanic basalts are generally lower than mid-ocean ridge basalts (MORB). This so called ‘He paradox’ has gained infamy and is used to argue against the conventional model of Earth structure and the existence of mantle plumes. While the paradox can be resolved by disequilibrium degassing of magmas it highlights the difficulty in reconstructing the primordial volatile inventory of the deep Earth from partially degassed oceanic basalts. Basalts from 26 to 11°N on the Red Sea spreading axis reveal a systematic southward increase in 3He/4He that tops out at 16 Ra in the Gulf of Tadjoura (GoT), Djibouti, bear the Afar triple junction. The GoT 3He/4He overlaps the highest values measured in sub-aerial basalts from Afar and Main Ethiopian Rift and is arguably located over modern Afar plume. The along-rift 3He/4He variation is mirrored by a systematic change in incompatible trace element ratios that appear to define two-component mixing between E-MORB and HIMU. Despite some complexity, hyperbolic mixing relationships are apparent in 3He/4He-K/Th-Rb/La space. Using established trace element concentrations in these mantle components we can calculate the concentration of He in the Afar plume mantle. Surprisingly it appears that the upwelling plume mantle has 3-8 times less He than the convecting asthenospheric mantle despite the high 3He/4He (and primordial Ne isotope composition). This contradicts the prevailing orthodoxy but can simply be explained if the Afar mantle plume is itself a mixture of primordial He-rich deep mantle with a proportionally dominant mass of Hepoor low 3He/4He HIMU mantle. This is consistent with the ...
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