Fig. 4

²⁰Ne/²²Ne plotted against ²¹Ne/²²Ne for the Bongwan CO₂ samples. Lower precision analysis performed on the MAP 215-50 mass spectrometer are plotted as red diamonds, higher precision analysis undertaken on the ARGUS VI mass spectrometer are depicted as red circles, with air plotted as a black square. 1σ errors associated with each measurement are provided along with mixing lines between air⁶⁷, continental crust⁴¹, MORB⁴⁶ and solar end members⁶² shown as solid black lines. The Kerguelen, Reunion, Loihi-Kilauea, and Iceland hotspots are depicted as thin dashed lines⁴⁶ with the mass fractionation line (MFL) shown as a thick dashed line. The Baker Farm (BF) and Mjaja (M) gases plot above the established air-MORB mixing line^40,68, plotting between the Kerguelen and the Reunion mantle source²⁵, implying an undegassed mantle origin for the Bongwan CO₂. The uncertainty differences reflect different analytical procedures and the ARGUS samples have been corrected for the contributions to ²¹Ne from NeH⁺, which is controlled by the amount of H₂ present in the mass spectrometer during analysis⁶¹ (see “Methods”)