Carbon isotopes of methane and carbon dioxide in hydrothermal gases of Japan

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 CO2largely 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-CH4pair 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 δ13cch4(—13to — 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. © 1993, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.