S4 THE ORIGIN OF GASES RELEASED FROM ANDESITIC CENTERS IN NEW ZEALAND

抄録

The range of chemical components at present analysed in New Zealand volcanic gas discharges include : H_2O, CO_2,SO_2,H_2S, HC1,HF, NH_3,He, Ne, Ar, O_2,N_2,CH_4,CO and higher hydrocarbons to C_8,together with the isotopic components ^2H-H_2O, ^2H-H_2,^2H-CH_4,^<13>C-CO_2,^<13>C-CH_4,^<34>S-SO_2,^<34>S-H_2S, ^<34>S-SO^<2->_4 and ^3He. Of these, the chemistry of the reactive species CO_2,CO, CH_4,SO_2,H_2S, NH_3 and H_2 points to the existence of two distinct equilibration environments. The kinetically "slow" species CH_4 and NH_3 attain equilibrium and are largely formed within the hydrothermal envelope surrounding the volcanic conduits. This environment is characterised by the presence of an acid brine, its salinity increasing with temperature. There, the redox potential is governed by that of di- and tri-valent iron in the rock matrix. It is sufficiently low to lead to virtually complete conversion of originally magmatic SO_2 to H_2S.