ABSTRACT

N₂- and CO₂-rich bubble gases collected from five springs and four wells in Paleozoic metamorphic rock and Cretaceous granite areas of Central Japan contain up to 22.9% methane. δD and δ¹³C values are between -100 and -180‰, and between -24 and -51‰, respectively, for the methane. A linear correlation between δD and δ¹³C values observed is almost identical with that for the TT(m) type thermogenic methane. The methane in the bubble gases is likely to be originated by thermal decomposition of organic matter in deep-seated metasedimentary rocks. The contribution of volcanic, magmatic and mantle-derived methanes to the bubble gases is probably insignificant, whereas ³He/⁴He values given by previous works indicate the presence of gas components derived from deep-seated sources, such as the magma, lower crust and mantle, in the gases.