Geochemistry of volcanic gases and hot springs of Satsuma-Iwojima, Japan: Following Matsuo

Abstract

The chemical and isotopic (D, ¹⁸O, ³⁴S) compositions were determined for 19 fumarolic gases, 11 hot spring waters and 3 meteoric waters collected from Satsuma-Iwojima volcanic island, Japan. The fumarolic gases discharge over a wide range of temperature (from 877 to 99°C), but are likely to be derived from a common parent gas with a deduced composition of (μmol/mol): H₂O:975000, CO₂:3800, total S:9800 (with an average oxidation state of 3.4), HCl:5800, HF:300, H₂:5000, CO:15, He:0.05, Ar:0.25, and N₂:60. The variation in gas composition from different fumaroles may largely be explained in terms of re-equilibration at lower temperature, addition of earlier deposited elemental S and mixing with meteoric waters at shallow depths. The chemical and isotopic composition of the highly acid hot spring discharges, pH 1.2 to 1.8, indicate that they are formed through absorption of Cl-depleted volcanic gas into local groundwaters in close to equal amounts.