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 RA (where RA = 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- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMH213...07G
- Keywords:
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- 0454 Isotopic composition and chemistry;
- BIOGEOSCIENCES;
- 1020 Composition of the continental crust;
- GEOCHEMISTRY;
- 1022 Composition of the hydrosphere;
- GEOCHEMISTRY;
- 1034 Hydrothermal systems;
- GEOCHEMISTRY