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Atmospheric helium isotope composition as a tracer of volcanic emissions: A case study of Erta Ale volcano, Ethiopiaстатья
Статья опубликована в высокорейтинговом журнале
Информация о цитировании статьи получена из
Web of Science,
Scopus
Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 10 августа 2018 г.
- Авторы: Boucher Christine, Lan Tefang, Marty Bernard, Burnard Peter G., Fischer Tobias P., Ayalew Dereje, Mabry Jennifer, Maarten de Moor J., Zelenski Michael E., Zimmermann Laurent
- Журнал: Chemical Geology
- Том: 480
- Год издания: 2018
- Издательство: Elsevier BV
- Местоположение издательства: Netherlands
- Первая страница: 3
- Последняя страница: 11
- DOI: 10.1016/j.chemgeo.2017.05.011
- Аннотация: The composition of atmospheric helium is generally considered to be constant (3He/4He=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-derived3He or crustal-derived4He. 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-precision3He/4He 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 RBB). An additional air sample collected in a large tank (500cm3) in Afar in 2015 permitted replicate analysis (n=8) and improved precision. The3He/4He ratio of this additional sample was also identical to RBBwithin 0.19% (95% confidence interval, CI). However, a clear excess of3He (1.32±0.64%, 95% CI, relative to RBBbased 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 a3He flux of 0.15±0.09mol/yr from the crater lava lake to be sustained. A similar3He flux of 0.12±0.06mol/yr is calculated from SO2flux 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 CO2contents and the helium isotope ratios increased over time within the chamber, allowing us to evaluate the soil CO2and3He 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)×109of soil gases is comparable to that of the high temperature (1084°C) fumaroles sited in the north pit crater. Using this ratio and our estimated3He flux, we determined a CO2flux of 4.6±3.0×108mol/yr for the lava lake, which is about 105times lower than the global volcanic subaerial CO2flux. Based on this pilot study, we suggest that3He excesses in air could provide another means with which to evaluate the fluxes of CO2and other volatile species in specific environments, such as highly active volcanic areas.
- Добавил в систему: Зеленский Михаил Евгеньевич