Noble gases confirm plume-related mantle degassing beneath Southern Africa

Abstract:

A striking feature of the African continent is the ~1 km elevation of the eastern and southern African plateaus, termed the African Superswell, this is also manifest by the shallow bathymetry of the southeastern Atlantic Ocean basin¹. These features have been attributed to recent thermal modification of the upper mantle with geophysical¹ and geochemical² studies indicating that the deeply rooted African superplume is the primary cause of this mantle anomaly, and contributes to Cenozoic rifting and volcanism of eastern Africa.

The Lesotho-KwaZulu-Natal region of south-eastern Africa displays numerous enigmatic geological phenomena given its location on a long-lived continent. These include active seismicity, anomalously high topography, recent small-scale volcanic activity, thermal springs, a high geothermal gradient and active CO₂ seeps.

The isotopic composition of the noble gases (He, Ne and Ar) are an established geochemical method of distinguishing between deep undegassed and shallow convective mantle sources. The presence of a noble gas signature of the deep mantle source associated with the ongoing CO₂ degassing would provide a measure of whether mantle upwelling is related to the deep-sourced African superplume or a shallow convection-driven process in the depleted upper mantle.

Here, we show that whilst ³He/⁴He are lower than a typical primordial mantle source of >8 R_A (where R_A = ³He/⁴He of air of 1.399 × 10⁻⁶), the Ne isotopic composition of the degassing mantle CO₂ requires a deep mantle source³, similar to that tapped by intraplate volcanism at Réunion or Kerguelen islands, rather than the convecting depleted upper mantle. This confirms the existence of the previously hypothesised Quathlamba mantle plume and illustrates that even modest plume induced lithospheric mantle melting, yet to result in significant extrusive volcanism, has incorporated a noble gas signature of the deep mantle source.

This provides the first geochemical verification of ongoing deep mantle upwelling in Southern Africa, corroborating existing geophysical evidence that small-scale mantle plumes are emanating from the top of the African superplume.

References

¹Adams & Nyblade, 2011. Geophys. J. Int. 186, 808–824

²Halldórsson et al., 2014. Geophys. Res. Lett. 41, 2304–2311

³Gilfillan et al., 2019. Nature Comms. 10, 5028