ORIGIN OF CARBON IN FUMAROLIC GAS FROM ISLAND ARCS

We have measured He-3/He-4 ratios, delta(13)C-values and CO2/He-3 ratios of fumaroles and hot spring gases in three volcanic areas in Japan. The He-3/He-4 ratios, delta(13)C-values and CO2/He-3 ratios vary from 2.02 R(atm) to 7.55 R(atm) -9.7 parts per thousand to -2.7 parts per thousand 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 CO2 of volcanic and hydrothermal fluids from subduction zones. Taking compiled C isotopic data of limestone and organic sediment, assuming that the CO2/He-3 ratio and delta(13)C-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 deconvolve 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 CO2 produced by decarbonation of subducted marine limestone and slab carbonate.