Based on relative CO2, N2, and
3He 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 N2/3He,
CO2/3He and CO2/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 end member components: a low-volatile melt
with a heat/Cl ratio of 25 Ml/mol (0.7 MJ/g), and a low Cl vapor with a
heat/CO2 ratio of 5 Ml/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 Ml/pmol is
within the range of 0.05 to 0.2 Ml/pmol of the upper mantle. The higher
heat/3He ratios for the arc-related magmas of 0.40.2 Ml/pmol are
likely to be due to the high proportions of heat carried in the form of a
separate, low 3He vapor phase derived largely from subducted
sediments.