Abstract
We have measured 3He 4He ratios, δ 13C-values and CO 2 3He ratios of fumaroles and hot spring gases in three volcanic areas in Japan. The 3He 4He ratios, δ 13C-values and CO 2 3He ratios vary from 02.05 R atm −9.7‰ to −2.7‰ and from 7.74·10 9 to 1.18·10 11, respectively. Based on these data and those in the literature, we present the first quantitative estimates of the upper mantle, sediment and limestone including slab components in CO 2 of volcanic and hydrothermal fluids from subduction zones. Taking compiled C isotopic data of limestone and organic sediment assuming that the CO 2 3He ratio and δ 13C-value of mid-ocean ridge basalt (MORB) are representative of the upper-mantle component, and that later isotopic and elemental fractionations are minimal, it is possible to deconvole each fraction of C on the basis of simple mixing equations. Up to 20% of the carbon in high-temperature volcanic gases in subduction zones is derived from a MORB-type source, while the major fraction of the gases is attributable to CO 2 produced by decarbonation of subducted marine limestone and slab carbonate.
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