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

Abstract

The composition of atmospheric helium is generally considered to be constant ( 3 He/ 4 He = 1.39 × 10 − 6 ) on a large spatial scale. However, local variations may arise in tectonically active areas due to focussed degassing of one of its two isotopes, for example degassing of mantle-derived 3 He or crustal-derived 4 He. If detected, such variations have the potential to trace open conduit degassing of magmatic bodies and/or diffusive emissions from volcanic and/or crustal sources. Here, we test the possibility of detecting such variations in the Afar region of north-eastern Ethiopia, which is located over a well-developed rift system. Special attention was paid to the Erta Ale volcanic edifice, where both magma lake activity and strong degassing occur. We conducted high-precision 3 He/ 4 He ratio measurements of air samples from this region using an analytical facility at the Centre de Recherches Pétrographiques et Géochimiques (CRPG), Nancy (France) that was specially designed for high-precision noble gas analyses. 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 R BB ). An additional air sample collected in a large tank (500 cm 3 ) in Afar in 2015 permitted replicate analysis (n = 8) and improved precision. The 3 He/ 4 He ratio of this additional sample was also identical to R BB within 0.19% (95% confidence interval, CI). However, a clear excess of 3 He (1.32 ± 0.64%, 95% CI, relative to R BB 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 3 He flux of 0.15 ± 0.09 mol/yr from the crater lava lake to be sustained. A similar 3 He flux of 0.12 ± 0.06 mol/yr is calculated from SO 2 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 CO 2 contents and the helium isotope ratios increased over time within the chamber, allowing us to evaluate the soil CO 2 and 3 He fluxes outside the lava lake area. These fluxes were found to be minor (~ 1%) compared to the lava lake flux. The CO 2 / 3 He ratio of (3.1 ± 0.7) × 10 9 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 3 He flux, we determined a CO 2 flux of 4.6 ± 3.0 × 10 8 mol/yr for the lava lake, which is about 10 5 times lower than the global volcanic subaerial CO 2 flux. Based on this pilot study, we suggest that 3 He excesses in air could provide another means with which to evaluate the fluxes of CO 2 and other volatile species in specific environments, such as highly active volcanic areas. • Air mixing quickly reduced 3 He excesses in air leading to a regional R A value. • Soil 3 He fluxes can be estimated via an accumulation chamber. • Lava lake degassing can lead to 3 He excesses in air of about 1%. • In few areas, 3 He excesses in air can be used to trace volcanic emissions.