Noble gases confirm plume-related mantle degassing beneath Southern Africa

Southern Africa is characterised by unusually elevated topography and abnormal heat flow. This can be explained by thermal perturbation of the mantle, but the origin of this is unclear. Geophysics has not detected a thermal anomaly in the upper mantle and there is no geochemical evidence of an asthenosphere mantle contribution to the Cenozoic volcanic record of the region. Here we show that natural CO2 seeps along the Ntlakwe-Bongwan fault within KwaZulu-Natal, South Africa, have C-He isotope systematics that support an origin from degassing mantle melts. Neon isotopes indicate that the melts originate from a deep mantle source that is similar to the mantle plume beneath Réunion, rather than the convecting upper mantle or sub-continental lithosphere. This confirms the existence of the Quathlamba mantle plume and importantly provides the first evidence in support of upwelling deep mantle beneath Southern Africa, helping to explain the regions elevation and abnormal heat flow.

A striking feature of the African continent is the~1 km elevation of the eastern and southern African plateaus. This has been termed the African Superswell 1 , and is also manifest by the shallow bathymetry of the southeastern Atlantic Ocean basin 2 . Seismic imaging beneath the African continent has revealed a large anomalous zone of low seismic velocity 3 , similar to that identified beneath the Pacific 4 . Termed Large Low Shear Wave Velocity Provinces (LLSVP) or superplumes, these are imaged to extend upwards from the core-mantle boundary 5 . Mantle flow induced by these velocity anomalies has been proposed to dynamically support elevated crustal regions 6 . The high topography of the eastern African plateau and unusual bathymetry of the southeastern Atlantic basin has been attributed to recent thermal modification of the upper mantle associated with the East African Rift System 7 . Recent geophysical 2 and geochemical 8 studies have indicated that the deeply rooted African superplume is the primary cause of this mantle anomaly, and is a major contribution to the Cenozoic rifting and volcanism of eastern Africa.
However, it is currently unclear if the anomalous topography of southern Africa is supported by a thermal perturbation in the lithospheric mantle 9 , the sub-lithospheric upper mantle 10 , the lower mantle 11 , or a combination of all three 2 . Previous seismic studies of the upper mantle structure beneath southern Africa have recorded only a small decrease in seismic velocities within the sub-lithosphere mantle, indicating that a thermal anomaly is unlikely 2 . Alternative hypotheses for the uplift of the region include; heating of the lithosphere by the tail of a Mesozoic plume that was stationary beneath the area for more than 25 million years 9 , or that it is the result of buoyancy from the African superplume present in the lower mantle 11 .
The Lesotho-KwaZulu-Natal region exhibits the highest relief in southern Africa 12 forming the southernmost part of the African Superswell. The region experiences active seismicity 13 and the sedimentary record of the Durban Basin and other Cretaceous basins surrounding southern Africa provide evidence for deposits sourced from distinct pulses of uplift and erosion in the Turonian, Oligocene, mid-Miocene and Pliocene 14 . Offshore, the anomalous bathymetry 15 and seamounts 13 of the Mozambique Basin, have been linked to active mantle upwelling associated with the hypothesised Quathlamba mantle plume 13 . Onshore, this could also explain the seismicity 16 , anomalous topography 12 , small-scale volcanic activity 17 , thermal springs 18 , elevated geothermal gradient 19 and active CO 2 seeps 13,20 of the region.
However, the nature of the upwelling mantle and whether it originates in the deep or shallow mantle is not understood, nor is the relationship to the underlying African superplume. The isotopic composition of the noble gases (He, Ne and Ar) are an established geochemical method of distinguishing between deep undegassed 21,22 and shallow convective mantle sources 23 . The presence of a noble gas signature of the deep mantle source associated with the ongoing CO 2 degassing would provide a measure of whether mantle upwelling is related to the deepsourced African superplume 24 as opposed to a shallow convection-driven process in the depleted upper mantle 8 .
Here, we show that whilst the 3 He/ 4 He are lower than a typical primordial mantle source of >8 R A (where R A is the 3 He/ 4 He of atmospheric air of 1.399 × 10 −6 ), the Ne isotopic composition of the degassing mantle CO 2 requires a deep mantle source, similar to that tapped by intraplate volcanism at Réunion 25 or Kerguelen islands 26 , rather than the convecting depleted upper mantle. This confirms the existence of the previously hypothesised Quathlamba mantle plume 13 and illustrates that even modest plume induced lithospheric mantle melting, which is yet to result in significant extrusive volcanism, has incorporated a noble gas signature of the deep mantle source. Our findings provide the first geochemical verification of ongoing deep mantle upwelling in Southern Africa and corroborates existing geophysical evidence that small-scale mantle plumes are emanating from the top of the African LLSVP in the region 24 .

Results
Natural CO 2 degassing in Lesotho-KwaZulu-Natal. CO 2 gas seeps are common in areas of active or recent magmatism, and are frequently associated with fault-related fluid migration from depth 27 . Natural CO 2 degassing is rare in South Africa; the natural cold CO 2 seeps along the Ntlakwe-Bongwan fault in southern KwaZulu-Natal are the largest concentration of such phenomena. The fault was identified during geological mapping between 1911 and 1916 28 with the seeps first described in 1923 29 (Fig. 1a-c). The fault is expressed at the surface over~80 km 30 and is defined by a~70 km wide arcuate zone of faulting that evolves southwards from an ENE-WSW to a north-south strike 31 . It is believed to be related to Gondwana rifting 32 which commenced~180 Ma and continues to the present (Fig. 1c).
The origin of the degassing CO 2 is enigmatic, with initial work proposing a link to dissolution of carbonate rocks at depth by acidic groundwater 33 . Later δ 13 C and δ 18 O measurements of the CO 2 indicated an origin from low temperature acidic groundwater reactions with carbonate rocks of similar composition to the Cambrian Matjies River Formation 20 of the Western Cape Province. Carbonates of the nearby Marble Delta Formation were ruled out as a source, as their δ 13 C was found to be distinct from the δ 13 C CO2 measured in the exsolving CO 2 20 , but this work did not take into account the potential fractionation of δ 13 C that would result from formation of a free-phase CO 2 during dissolution of carbonate rock. Mantle melting associated with the hypothesised Quathlamba mantle hotspot has also been proposed as both a potential CO 2 source, and a cause of a local thermal anomaly 13 .
Natural CO 2 can have multiple origins, including; shallow biogenic processes, carbonate hydrolysis, deep burial related mechanical breakdown or thermo-metamorphism of carbonates and degassing of magmatic bodies 34,35 . Whilst δ 13 C CO2 can often resolve these sources, CO 2 -rich natural gases frequently exhibit values that overlap with the range of carbon from magmatic source and carbonate breakdown 34 , making it challenging to resolve their origin 36 . Noble gas isotopes are powerful tracers of the origin of CO 2 , particularly in identifying mantle contributions 37 . Primordial isotopes, such as 3 He, originate in the Earth's mantle and gases from the depleted upper mantle define a narrow range of CO 2 / 3 He (of 1 to 10 × 10 9 ) 38-40 .
Combining δ 13 C CO2 with CO 2 and helium measurements to resolve CO 2 origin. Here we combine new noble gas analyses of Bongwan CO 2 and δ 13 C CO2 from six separate gas seeps, sampled from three locations along the Bongwan fault and associated splays (Supplementary Tables 1-2). δ 13 C CO2 range from −2.0 to −3.3‰ (V-PDB standard) in line with previous determinations 20 . 3 He/ 4 He ratios corrected for air ( 3 He/ 4 He c -see "Methods") within the samples range from 3.6 to 4.5 R A . These are considerably above the atmospheric ratio (1 R A ) indicating the presence of a significant amount of primordial 3 He. 4 He exhibits the widest range in concentration compared to the other noble gases, from 1.48 × 10 −9 (Umtamvuna Mound 2) to 9.62 × 10 −5 cm 3 (STP)cm −3 (Baker Farm). 20 Ne concentrations range from 4.01 × 10 −9 to 4.13 × 10 −8 cm 3 (STP)cm −3 , with 40 Ar ranging from 6.34 × 10 −6 to 7.83 × 10 −5 cm 3 (STP)cm −3 . As with 4 He concentrations, the lowest 20 Ne and 40 Ar values are exhibited by the CO 2 sampled from Umtamvuna Mound 2, and the highest values are from the Baker Farm sample. CO 2 from the Baker Farm and Mjaja seeps (A and B on Fig. 1c) exhibit CO 2 / 3 He of 1.88 and 6.78 × 10 9 , confirming a mantle origin (Fig. 2). The four seeps sampled at Umtamvuna exhibit considerably higher CO 2 / 3 He ratios. As 3 He is inert and insoluble 37 , and there is no significant 3 He in the crust 37 ( 3 He/ 4 He crust = 0.05 R A ) 41 , the variation in CO 2 / 3 He is predominantly linked to the addition of 3 He-poor CO 2 42 .
Combining CO 2 / 3 He with δ 13 C CO2 allows organic sediment and limestone-derived CO 2 to be distinguished from magmatic sources 38 (Fig. 2). The trend between CO 2 / 3 He and δ 13 C CO2 are consistent with the mixing of mantle-derived CO 2 with CO 2 derived from the overlying Marble Delta Formation carbonates at up to 70°C (see "Methods"). Based on the regional geothermal gradient of 30°C/km 19 and an average surface temperature of 14°C 43 , we estimate that mixing occurred at depths of less thañ 1900 m. Linking CO 2 / 3 He, CO 2 / 4 He and 3 He/ 4 He in a ternary plot allows CO 2 sources to be resolved 44,45 , permitting direct comparison of the relative proportions of CO 2 , 3 He, and 4 He, regardless of absolute concentrations (Fig. 3). Binary mixtures and loss or gain of a single component plot as straight lines on ternary plots. Figure 3 demonstrates that the Baker Farm gas requires the ingrowth/addition of 33 to 50 % radiogenic 4 He, derived from the lithosphere, to mantle magmas, relative to the depleted upper asthenosphere mantle (DM) (8 ± 1 R A ) 46 or subcontinental lithospheric mantle (SCLM) (6.1 ± 0.9 R A ) 47 , respectively. The remaining samples require the addition of 3 He-poor CO 2 . The modest 4 He required in this gas confirms a shallow crustal origin for this non-magmatic CO 2 (also see Supplementary Fig. 1). The low 4 He/ 20 Ne in these samples contrasts with the higher values measured in the Mjaja and Baker Farm gases and implies that the CO 2 has interacted with atmosphere-saturated groundwaters.
Recent work undertaken on CO 2 seeps in Australia, similar to those at Bongwan, has highlighted that equilibration of mantlesourced CO 2 with atmosphere-saturated groundwaters, followed by solubility controlled fractionation during exsolution of the CO 2 from the groundwater at CO 2 seeps, can result in depleted He concentrations and elevated CO 2 / 3 He from values which are originally within the magmatic source range 48 . Given the wide range of 4 He concentrations observed between the different seeps, it is likely that both mixing with crustal derived CO 2 and solubility fractionation, resulting in a relative loss in He, have acted to produce the observed CO 2 / 3 He ratios at Bongwan (Fig. 3).
Constraining the mantle source using Ne and Ar isotopes. The 3 He/ 4 He of the SCLM of the Karoo Large Igneous Province and the nearby volcanic ocean islands (Comores, Tristan da Cuna and Gough) are characterised by low 3 He/ 4 He (4.9-7.1 R A ) 49 . Hence, the mantle source in the region may have 3 He/ 4 He that is lower than MORB, but is most likely to be above the highest measured 3 He/ 4 He c of 4.27 R A . The CO 2 degassed at the Bongwan seeps has migrated from the mantle through the crust and will have incorporated radiogenic He from the Precambrian metamorphic basement and sedimentary cover. Radiogenic 4 He is required to account for CO 2 -He isotope systematics of the Baker Farm seep gas (Fig. 3) and can account for the 4 He/ 21 Ne* ratio of 1.23 × 10 7 in the sample, below the crustal ratio of 1.71 × 10 7 41 . Neon isotopes provide less ambiguous insights into the mantle source, enabling differentiation of depleted, convecting upper mantle (DM), the source of mid-ocean ridge basalts (MORB) and the primordial 20 Ne-enriched mantle that is sampled by intraplate magmatism, the source of ocean island basalts (OIB) 50 .
The Ne isotope composition of Baker Farm and Mjaja seeps show a clear mantle component, which is distinct from both atmospheric Ne and the mass fractionation line (Fig. 4). 40 Ar/ 36 Ar of the Mjaja and Baker Farm gases (550 ± 2 and 961 ± 4 respectively) are higher than the air value, consistent with a partial mantle origin of the non-atmospheric 40 Ar. This is supported by the 4 He/ 40 Ar* (1.55 and 1.77 respectively) which are close to the mantle value of 2 37   groundwater and/or He loss due to degassing of CO 2 from groundwater 48 , required to account for the elevated CO 2 / 3 He in these samples, compared to the more mantle-rich values of Mjaja and Baker Farm.
Importantly, the results of the high precision Ne analysis of Mjaja and Baker Farm seep gases do not plot on the MORB-air mixing line in Ne isotope space (Fig. 4). Instead they provide a clear indication that the mantle source of the CO 2 is more primordial than that of the convecting upper mantle (Fig. 4). The duplicate high precision determinations of the Baker Farm and Mjaja CO 2 samples overlaps with the trend defined by the Kerguelen 26 and Réunion hotspots 25 , implying the ultimate source of the upwelling mantle is deep. The low 3 He/ 4 He of the Bongwan gases relative to Kerguelen (12.3 ± 0.3 R A ) and Réunion (11.5-13.1 R A ) would require the incorporation of crustal radiogenic 4 He. However, the 3 He/ 4 He of the sub-lithospheric mantle source of the nearby Karoo Large Igneous Province is cited as 7.03 ± 0.23 R A 51 and the most proximal volcanic ocean islands (Comores, Tristan da Cuna and Gough) are characterised by low 3 He/ 4 He (4.9 to 7.1 R A ) 51 . Hence, the mantle source in the region may have a 3 He/ 4 He that is not elevated above the MORB range (8 ± 1 R A ), though is most likely to be above the highest measured 3 He/ 4 He c of 4.5 R A . It is also possible that the He and Ne systematics of the mantle under southern South Africa are decoupled, as has been observed in the Icelandic and the Colorado Plateau mantle sources 23,52,53 . This decoupling was attributed to either more compatible behaviour of He during low-degree partial melting or more extensive diffusive loss of He relative to the heavier noble gases. Incorporation of crustal-radiogenic 21 Ne to the Bongwan gases is also probable, but without constraint of the original Bongwan mantle 3 He/ 4 He this is impossible to determine.

Discussion
The Bongwan CO 2 seeps are located at the end of the hypothesised Quathlamba hotspot track. Hotspot migration has been proposed to explain chain of volcanic seamounts that track across the Mozambique Basin 13 , orientated in a direction that closely resembles reconstructions of the African plate movement (Fig. 1). Recently, anomalous 30 km elongate seamounts, have been identified within the Northern Natal Valley offshore of Durban 54 . The geospatial positioning of these could extend the East African Rift System southwards, but they are also within range and age of the proposed Quathlamba hotspot track (Fig. 1b). Furthermore, recent work has found that the seafloor adjacent to the Mozambican continental margin, and that of the central Mozambique Channel is 300 m and 1300 m shallower,   38,62 (mantle CO 2 / 3 He = 1-10 × 10 9 ; mantle δ 13 C = −9 to −4‰; Crustal CO 2 / 3 He = 1 × 10 12 -10 14 ; Limestone δ 13 C = 0 ± 2‰; Organic sediment δ 13 C = −30 ± 10‰). The range of δ 13 C values for South African carbonate sources from the Marble Delta 20 , Matjies River Formation 20 , and the Transvaal Supergroup 66 are provided. The extent of δ 13 C CO2 that would be produced by acid groundwater dissolution of Marble Delta Formation carbonate is also shown (grey box and black lines depicting the minimum δ 13 C CO2 that would result from dissolution of the formation carbonate between 10°C (left) and 100°C (right)) (see Methods). The trend between CO 2 / 3 He and δ 13 C CO2 are consistent with the mixing of mantle-derived CO 2 with CO 2 derived from the overlying Marble Delta Formation carbonates at up to 70°C (see "Methods"). δ 13 C CO2 was not measured from the sample collected at the Umtamvuna River Spring and hence this sample cannot be depicted on the plot ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-12944-6 respectively, than the conjugate basins in Antarctica, or than oceanic thermal subsidence models predict 15 . This has been attributed to the presence of thickened oceanic crust, linked to the passage of a mantle plume beneath the basin during the Paleogene 15 .
Plate movement reconstructions indicate that this hotspot moved under the continent approximately 10 million years ago (Fig. 1), coinciding with several periods of regional uplift 55 . The Ne isotope systematics of the CO 2-rich gas seeps provide the first geochemical evidence that small volumes of melting is occurring beneath the continental lithosphere in the region. This confirms the existence of the previously hypothesised Quathlamba mantle plume 13 and illustrates that even modest plume induced lithospheric mantle melting, which is yet to result in significant extrusive volcanism, has incorporated a noble gas signature of the deep mantle source. These findings provide the first geochemical verification of ongoing mantle upwelling in Southern Africa, confirming geophysical evidence that small-scale mantle plumes are emanating from the top of the African superplume 24 . Buoyant underplating of Southern Africa by the African superplume provides an explanation for the anomalous elevation, high heat flow, and how the Quathlamba mantle plume has incorporated deep-sourced mantle volatiles.

Methods
Fieldwork. The CO 2 seeps were identified in the field as bubble streams in pools of water, rivers and wellbores (Fig. 1, Supplementary Table 1). Gas samples were collected in September 2015 by placing a plastic funnel over the site of the CO 2 seep and flowing the gas through a 70 cm length of refrigeration grade copper tubing fitted with an exhaust hose to prevent turbulent back-mixing of air into the sample. The tubing was purged with the seeping gas for between 10 and 15 min before being clamped by a purpose built tube clamp at both ends to seal the copper tube with a cold-weld that is impervious to helium 36 . Tedlar sample bags were filled at each seep for stable isotope analyses. The Baker Farm borehole was sampled by sealing the well and using a soil gas probe to collect gas from as deep as possible within the well. A Geotechnical Instruments GA2000 portable gas analyser was then used to extract gas from the Baker Farm well, with the pump being connected downstream of the sampling apparatus. Further details on individual field sites and other surveys undertaken in the area are outlined in Supplementary Table 1 and in  previous work 27,56 .
Laboratory analysis. Bulk gas, stable isotope and noble gas analysis was undertaken at the Scottish Universities Environmental Research Centre (SUERC), using previously described techniques 57 . Bulk gas content as a percentage was determined using a Pfeiffer Vacuum QMS 200 quadrupole mass spectrometer with all seeps sampled exhibiting concentrations of >99% CO 2 . δ 13 C CO2 were measured using a VG SIRA II dual inlet isotope ratio mass spectrometer following established procedures 58 . Precision and reproducibility are typically better than ±0.2‰ for δ 13 C (Supplementary Table 2).
Noble gas analyses from all samples were performed on volumes of~10 cm 3 gas stored in copper tubes. Each sample was expanded to a titanium sublimation pump (900°C) and a series of SAES GP50 ZrAl getters (250°C) operating under ultrahigh vacuum, following established procedures [57][58][59][60] . The isotopic composition of He, Ne and Ar of all six samples was measured using a MAP 215-50 mass spectrometer using established techniques [57][58][59][60] (Supplementary Tables 2 and 3). Analytical errors are governed by the reproducibility of air calibrations, and for Ne, standard reproducibility was assessed using the best Gaussian fit to the probability density distribution of 21 Ne/ 22 Ne and 20 Ne/ 22 Ne ratios from 14 air calibrations, which is an objective way of filtering outliers. These samples were not corrected for any 20 NeH + contribution to 21 Ne.
High precision analysis of Ne isotopes within the Baker Farm and Mjaja samples was undertaken in multi-collection mode using a ThermoFisher ARGUS VI using the following procedures. Each copper tube sample was mounted on the ultra-high vacuum line attached to the MAP 215-50 mass spectrometer, and subjected to the same clean up procedure as outlined above, following which they were trapped in a 2 L stainless steel cylinder. Approximately 100 cm 3 of total gas was extracted from this cylinder to the ultra-high vacuum system attached to the ARGUS VI mass spectrometer as described in previous work 61 . The gas was exposed to another SAES GP50 ZrAl getter (held at 250°C) for 15 min and then a liquid nitrogen-cooled charcoal finger (held at −196°C) for 15 min to trap any remaining active gases along with Ar, Kr and Xe. Ne was then adsorbed on charcoal using an IceOxford cryopump (−243°C, 20 min) while He was pumped away. Pure Ne was released at −173°C and administered into the ARGUS VI low resolution mass spectrometer. The ARGUS clean-up and analysis procedure was undertaken twice for both the Baker Farm and Mjaja samples, and the results of the individual repeat measurements are plotted on both Fig. 4 and Supplementary  Fig. 3, and listed in Supplementary Table 4.
Analysis of Ne isotopes followed procedures described in previous work 61 . Ne isotopes were multi-collected ( 22 Ne + -H 2 , 21 Ne + -Axial, 20 Ne + -L2) on 10 12 Ω Faraday amplifiers. Isobaric interferences of 40 Ar 2+ and 44 CO 2 2+ were quantified using pre-determined singly/doubly charged ratios under measurement conditions and the in situ measurement of 40 Ar 2+ and 44 CO 2 2+ during analysis on the CDD detector. The contributions of 40 Ar 2+ to the corresponding 20 Ne peak were found to be~0.6% and 0.07% for Baker Farm and Mjaja samples, respectively. Contributions of 44 CO 2 2+ to 22 Ne for Baker Farm and Mjaja were found to be 0.2% and 0.04%, respectively. The contribution of 40 Ar + and 44 CO 2 + to the overall uncertainty of Ne isotopic ratios was found to be below 0.01%. Other interferences (H 2 18 O + , H 19 F + , 65 Cu 3+ ) were found to be negligible. The 20 NeH + contribution at m/z = 21 was determined using a pre-recorded calibration curve of 22 Ne -22 NeH at a constant hydrogen level, that exceeded the level of 22 Ne, where 20 Ne of each sample was measured 61 . The measurement of 22 NeH occurred at m/z = 23, corrected for 46 CO 2 2+ and blank. 20 NeH + correction at m/z = 21 was found to be 0.96% (Baker Farm) and 1.94% (Mjaja). The contribution of NeH correction toward the overall uncertainty is ± 0.02%. Mass fractionation was corrected by the repeated analysis of air prior to and after analysis (n = 9) with the reproducibility of 20 where A, B and C refer to three different components and A+B+C = 1.  40,68 , plotting between the Kerguelen and the Reunion mantle source 25 , implying an undegassed mantle origin for the Bongwan CO 2 . The uncertainty differences reflect different analytical procedures and the ARGUS samples have been corrected for the contributions to 21 Ne from NeH + , which is controlled by the amount of H 2 present in the mass spectrometer during analysis 61 (see "Methods") The predicted δ 13 C CO2 produced by the acid dissolution of the Marble Delta Formation between temperatures of 10 and 100°C, depicted on Fig. 2, was calculated from the measured Marble Delta Formation δ 13 C carb 20 using established fractionation factors between δ 13 C carb and gaseous CO 2 , calculated according to equation [4] 63 , where T is the temperature in Kelvin: 10