ABSTRACT

Terrestrial gas samples from thermal springs located along and behind the volcanic arc of Japan have a variable chemical and isotopic composition, largely reflecting their different positions with respect to a convergent plate boundary. The CO₂ largely originates from subduction-related magmatism, though in N₂-rich gases the CO₂ may also have been derived from an organic sedimentary source. The chemical and isotopic compositions of the CO₂-CH₄ pair indicate that both isotopic and chemical equilibrium are commonly attained (and maintained) in the CO₂-rich gases, whereas CH₄ in the N₂-rich gases appears to be affected by bacterial oxidation. In some samples with low contents of CH₄, heavy δ¹³C_CH4 (–13 to –17‰) may result from quenching from magmatic conditions, though bacterial oxidation cannot be ruled out. In combination with the chemical composition of the thermal gases, noble gas isotopic signatures indicate two sources: a magmatic source containing a mantle component and gases from melting of the subducting slab; these are mixed with atmospheric gases near the surface.