He, N and C isotopes and fluxes in Aira caldera: Comparative study of hydrothermal activity in Sakurajima volcano and Wakamiko crater,Kyushu, Japan

摘要

We investigate the degassing activity of an active submarine crater, Wakamiko, and an active sub-aerial volcano, Sakurajima, both located in Aira caldera, southern Kyushu, Japan. We provide 3~He/4~He, δ~(13)C-CO_2 and δ_(15)N data for 15 hot springs, wells and bubbling gas from Sakurajima volcano, along with 3~He/4~He from sea-water at four different sites for both Kagoshima bay and Wakamiko crater. We find a common magmatic 3~He/ 4~He ratio for Sakurajima and Wakamiko, 7.2 ± 0.8 Ra, which is consistent with 1) a mixing between air-saturated water (ASW) and MORB-type He, and 2) a common magmatic source located in the center of Aira caldera. Corrected 3~He/4~He, δ_(13)C-CO_2 and CH_4/~3He data for Sakurajima are correlated with the distance from the volcanic vent (Showa crater), which we attribute to crustal contamination and biogenic reaction. The low δ_(13)C-CO_2 values (-10.1 ± 0.2‰ to -13.7 ± 0.3‰) observed at Sakurajima may result from the addition of carbon from organic matter from basement rocks in magmatic source. After correction for air-derived nitrogen, we find δ_(15)N_c values range between — 1.7‰ and + 43%° which indicates that magmatic N is dominated by a sedimentary-derived component (up to 65.8%). We calculate Wakamiko fluxes of 4He (975 ± 228 mol/y), 3He (0.011 ± 0.003 mol/y), CO_2 (184 ± 43 t/d), and heat (195 ± 22 MW). Our helium and heat fluxes are the first in situ fluxes ever reported for Wakamiko crater. All these Wakamiko fluxes are at least one order of magnitude lower than those observed for Sakurajima (CO_2: 1800 t/d; 3~He: 0.71 mol/y; heat: 2100 MW); degassing at Sakurajima volcano is much stronger than that at Wakamiko crater. The variation of Sakurajima CO_2 flux with time, source (Minamidake or Showa crater) and eruptive activity, appears not to significantly affect the CO_2 flux at Wakamiko crater, which is much more stable (132-307 t/d) during the last 30 years. This indicates that there is no link between Sakurajima and Wakamiko degassing activity, despite having the same magmatic source.