1 Introduction It is difficult to identify carbon dioxide origin merely by C isotopic compositions in the C-H-O system near surface environments due to possible contamination of crustal components and ambiguous information provided by carbon isotopic compositions. However, analysis of MORB basalts, xenoliths, volcanic gas, hot-spring gas, etc., has shown that CO2 gas is the principal carrier phase of mantle-derived He (Gerlach, 1989; Javoy and Pineau, 1991; Trull et al., 1993). 3He is th…