Correlations Among CO2, Cl, 3He and Heat Discharged from Geothermal Systems of the Taupo Volcanic Zone, New Zealand

Organization: The Australasian Institute of Mining and Metallurgy
Pages: 6
Publication Date: Jan 1, 1995
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 0.12¦0.05 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 0.4¦0.2 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.
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