Mapping Mantle Mixing and the Extent of Superplume Influence Using He-Ne-Ar-CO2-N2 Isotopes: The Case of the East Africa Rift System

Abstract:

Earth’s mantle possesses distinct and variable volatile characteristics as sampled by magmatic activity in different tectonic environments. In general, trace element depleted mid-ocean ridge basalts, with low Sr and Pb isotope values (but high εNd and εHf), release mantle-derived noble gases characterised by ³He/⁴He ~8 ± 1R_A, (²¹Ne/²²Ne)_ex ~0.06 and ⁴⁰Ar/³⁶Ar ≥ 10,000 with CO₂ and N₂ having δ¹³C~-5‰ and δ¹⁵N ~-5‰, respectively. In contrast, enriched intraplate lavas possess higher ³He/⁴He (up to 50R_A), lower (²¹Ne/²²Ne)_ex ~0.035 and ⁴⁰Ar/³⁶Ar ≤ 10,000 with generally higher but variable δ¹³C and δ¹⁵N. These isotopic attributes of mantle-derived volatiles can be exploited to map the extent, and mixing characteristics, of enriched (plume) mantle with depleted asthenospheric mantle ± the effects of over-riding lithosphere and/or crust.

The East African Rift System (EARS) is superimposed upon two massive plateaux – the Ethiopia and Kenya domes – regarded as geophysical manifestations of a superplume source, a huge thermochemical anomaly originated at the core-mantle boundary and providing dynamic support for the plateaux. We present new volatile isotopic and relative abundance data (on the same samples) for geothermal fluids (He-CO₂-N₂), lavas (He-Ne-Ar) and xenoliths (He-Ne-Ar-CO₂-N₂) which provide an unprecedented overview of the distribution of mantle volatiles of the Ethiopia Dome, from the Red Sea via the Afar region and Main Ethiopian Rift (MER) to the Turkana Depression. Notably, peaks in geothermal fluid ³He/⁴He (16R_A) and δ¹⁵N (+6.5‰) are coincident within the MER but the maximum δ¹³C (-0.78‰) lies ~100 km to the south. Highs in ³He/⁴He (14R_A), δ¹³C (~-1‰) and δ¹⁵N (+3.4‰) for mafic crystals occur in the Afar region ~ 500km to the north. We assess the significance of the off-set in these volatile isotope signals, for sampling volatile heterogeneity in the plume source and/or the relative sensitivity of different volatiles to admixture of plume- with asthenospheric and lithospheric mantle. These results are contrasted with volatile data from the Kenya Dome where the plume signal is muted and the lithospheric mantle exerts a stronger control on geothermal/lava volatile characteristics.