Ohaaki reservoir chemistry

Characteristics of an arc-type hydrothermal system in the Taupo Volcanic Zone, New Zealand

B. W. Christenson, E. K. Mroczek, B. M. Kennedy, Matthijs Van Soest, M. K. Stewart, G. Lyon

Research output: Contribution to journalArticle

35 Citations (Scopus)

Abstract

Situated along the eastern margin of the Taupo Volcanic Zone (TVZ), the high-gas and high-enthalpy production fluids from the Ohaaki geothermal field are chemically similar to other arc-type volcanic systems found along this part of the TVZ. Two thermal upwellings can be distinguished within the field, each apparently emanating from fault structures in the Mesozoic basement greywackes. Although both plumes have similar salinities and appear to be fully equilibrated with the reservoir rocks containing them, fluids associated with the eastern upflow zone are enriched in B and F compared to those in the western upflow zone. δ2H and δ18O signatures for water suggest that up to 20% of the water in the discharges may derive from a magmatic source, and a δ2H-Cl trend established by the East-Bank fluids indicate end-member mixing between a high-Cl fluid and Cl-deficient steam containing as much as 40% arc-type vapour. Gas signatures in fluids from the two production fields show compositional differences that cannot be explained by physical reservoir processes. The eastern fluids have higher N2/Ar and CO2/He ratios, and consistently higher CH4-CO2 13C fractionation temperatures than those in the western upflow zone. In addition, stark differences in 3He/4He and 40Ar/36Ar ratios for the two production fields suggest two-component mixing for the respective source components, with the eastern fluids having a significantly larger radiogenic gas content than those to the west. Collectively, these signatures suggest the presence of a relatively young and shallowly seated degassing intrusive beneath the eastern upflow zone, perhaps as shallow as 4 km beneath the surface.

Original languageEnglish (US)
Pages (from-to)53-82
Number of pages30
JournalJournal of Volcanology and Geothermal Research
Volume115
Issue number1-2
DOIs
StatePublished - Jun 15 2002
Externally publishedYes

Fingerprint

hydrothermal systems
New Zealand
hydrothermal system
volcanology
arcs
chemistry
Fluids
fluid
fluids
signatures
Radiogenic gases
Gases
gases
gas
Geothermal fields
Water
degassing
Degassing
Steam
upwelling water

Keywords

  • Fluid chemistry
  • Geothermal
  • Magmatic degassing
  • Magmatic heat sources
  • Noble gas chemistry

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

Cite this

Ohaaki reservoir chemistry : Characteristics of an arc-type hydrothermal system in the Taupo Volcanic Zone, New Zealand. / Christenson, B. W.; Mroczek, E. K.; Kennedy, B. M.; Van Soest, Matthijs; Stewart, M. K.; Lyon, G.

In: Journal of Volcanology and Geothermal Research, Vol. 115, No. 1-2, 15.06.2002, p. 53-82.

Research output: Contribution to journalArticle

Christenson, B. W. ; Mroczek, E. K. ; Kennedy, B. M. ; Van Soest, Matthijs ; Stewart, M. K. ; Lyon, G. / Ohaaki reservoir chemistry : Characteristics of an arc-type hydrothermal system in the Taupo Volcanic Zone, New Zealand. In: Journal of Volcanology and Geothermal Research. 2002 ; Vol. 115, No. 1-2. pp. 53-82.
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