Based on relative C02, NZ, and 'He contents, geothermal systems of the Taupo Volcanic Zone, TVZ, can be
subdivided into two major groups: arc-type systems, associated with andesitic magmatism along the eastern boundary
of the TVZ, and rift-type systems over its western parts. The highly increased N/He, COZ/'He and COz/Cl ratios
of the former are ascribed to entrainment of large proportions of volatiles of subducted, marine sedimentary origin.
The largely rhyolitic, rift-type magmas are assumed to have formed from volatile-depleted, residual melts.
Correlations of Cl and CO2 with heat contents suggest that formation of the two types of source magmas involves
mixing of two endmember components: a low-volatile melt with a heat/Cl ratio of 25 MJ/mol (0.7 MJ/g), and a low- Cl vapor with a heat/CO2 ratio of 5 MJ/mol. On the basis of these values, the heat associated with andesitic magmas
is found to be carried in close to equal parts by both vapor and melt; for the rhyolitic melts the proportion
contributed from the vapor is only about 2%. The heat/'He ratios of the rhyolitic source magmas for the rift-type
geothermal discharges from the TVZ of 0.120.05 MJ/pmol is within the range of 0.05 to 0.2 MJ/pmol of the upper
mantle. The higher heat/'He ratios for the arc-related magmas of 0.40.2 MJ/pmol are likely to be due to the high
proportions of heat carried in the form of a separate, low'He vapor phase derived largely from subducted sediments.