Atmospheric helium isotope composition as a tracer of volcanic emissions: A case study of Erta Ale volcano, Ethiopia

Within the precision of our measurements (0.2-0.4%, 95% confidence interval), the helium isotopic compositions of air from both the Afar rift zone and the crater zone of Erta Ale are similar to the composition of air collected at Brabois Park in Villers-les-Nancy, France (labelled here as RBB). An additional air sample collected in a large tank (500 cm3) in Afar in 2015 permitted replicate analysis (n = 8) and improved precision. The 3He/4He ratio of this additional sample was also identical to RBB within 0.19% (95% confidence interval, CI). However, a clear excess of 3He (1.32 ± 0.64%, 95% CI, relative to RBB based on a weighted mean of two samples) was detected in air collected above the active lava lake located in the central pit crater of Erta Ale volcano. Such excess requires a 3He flux of 0.15 ± 0.09 mol/yr from the crater lava lake to be sustained. A similar 3He flux of 0.12 ± 0.06 mol/yr is calculated from SO2 flux measurements and fumerolic gas data obtained during the same field trip. At several sites in the rim of the Northern crater, we conducted soil flux measurements using an accumulation chamber. Both the CO2 contents and the helium isotope ratios increased over time within the chamber, allowing us to evaluate the soil CO2 and 3He fluxes outside the lava lake area. These fluxes were found to be minor (~ 1%) compared to the lava lake flux. The CO2/3He ratio of (3.1 ± 0.7) × 109 of soil gases is comparable to that of the high temperature (1084 °C) fumaroles sited in the north pit crater. Using this ratio and our estimated 3He flux, we determined a CO2 flux of 4.6 ± 3.0 × 108 mol/yr for the lava lake, which is about 105 times lower than the global volcanic subaerial CO2 flux. Based on this pilot study, we suggest that 3He excesses in air could provide another means with which to evaluate the fluxes of CO2 and other volatile species in specific environments, such as highly active volcanic areas.