Defining the composition of the deep mantle and the primordial He inventory of the Afar plume

Basaltic rocks generated by upwelling mantle plumes display a range of trace element and isotope compositions indicative of strong heterogeneity in deep material brought to Earth surface.  Helium isotopes are an unrivalled tracer of the deep mantle in plume-derived basalts.  It is frequently difficult to identify the composition of the deep mantle component as He isotopes rarely correlate with incompatible trace element and radiogenic isotope tracers. It is supposed that this is due to the high He concentration of the deep mantle compared to degassed/enriched mantle reservoirs dominating the He in mixtures, although this is far from widely accepted.  The modern Afar plume is natural laboratory for testing the prevailing paradigm.

The ³He/⁴He of basalt glasses from 26°N to 11°N along the Red Sea spreading axis increases systematically from 7.9 to 15 R_a. Strong along-rift relationships between ³He/⁴He and incompatible trace element ratios are consistent with a binary mixture between moderately enriched shallow asthenospheric mantle in the north and plume mantle evident in basalts from the Gulf of Tadjoura, Djibouti (the Ramad enriched component of Barrat et al. 1990).  The high-³He/⁴He basalts have trace element-isotopic compositions that are similar, but not identical, to the high ³He/⁴He (22 R_a) high Ti (HT2) flood basalts erupted during the initial phase of the Afar plume volcanism (Rogers et al. in press). This suggests that the deep mantle component in the modern Afar plume has a HIMU-like composition. From the hyperbolic ³He/⁴He-K/Th-Rb/La mixing relationships we determine that the upwelling deep mantle has 3-5 times higher He concentration than the asthenosphere mantle beneath the northern Red Sea.

Barrat et al. 1990.  Earth and Planetary Science Letters 101, 233-247.