Fig. 2 | Nature Communications

Fig. 2

From: Noble gases confirm plume-related mantle degassing beneath Southern Africa

Fig. 2

CO2/3He against δ13C for the Bongwan CO2 samples. Individual samples are plotted as red circles along with labels outlining their associated air-corrected 3He/4Hec in RA (where RA is the atmospheric 3He/4He of 1.399 × 10−6) and their location corresponding to Fig. 1 (A – Baker Farm, B – Mjaja and C – Umtamvuna). Mixing lines (black lines and grey shading) are shown for CO2 derived from the mantle, limestone and organic sediments38. End-member compositions cover the range of values38,62 (mantle CO2/3He = 1–10 × 109; mantle δ13C = −9 to −4‰; Crustal CO2/3He = 1 × 1012–1014; Limestone δ13C = 0 ± 2‰; Organic sediment δ13C = −30 ± 10‰). The range of δ13C values for South African carbonate sources from the Marble Delta20, Matjies River Formation20, and the Transvaal Supergroup66 are provided. The extent of δ13CCO2 that would be produced by acid groundwater dissolution of Marble Delta Formation carbonate is also shown (grey box and black lines depicting the minimum δ13CCO2 that would result from dissolution of the formation carbonate between 10 °C (left) and 100 °C (right)) (see Methods). The trend between CO2/3He and δ13CCO2 are consistent with the mixing of mantle-derived CO2 with CO2 derived from the overlying Marble Delta Formation carbonates at up to 70 °C (see “Methods”). δ13CCO2 was not measured from the sample collected at the Umtamvuna River Spring and hence this sample cannot be depicted on the plot

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