Oxygen Isotope Geochemistry Of Geothermal Wairakite

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Journal Article: Oxygen Isotope Geochemistry Of Geothermal Wairakite

Oxygen and hydrogen isotopic compositions of wairakite were determined for drillcore samples from exploration wells in the active geothermal systems in Japan (Takigami and Kirishima) and in New Zealand (Wairakei and Ngatamariki) and for samples from some extinct Japanese hydrothermal systems. The Δ 18O values range from -0.4 to 7.8‰ for wairakite from the active systems, whereas they are generally higher (ranging from 8.0 to 14.1‰) for the mineral from the extinct systems, indicative of retrograde re-equilibration with local meteoric water at low temperatures. The ΔD values of channel water are lower than those of hydrothermal fluids or local meteoric water. The D/H fractionation between channel water and external water becomes more negative as temperature lowers. The monomineralic oxygen isotopic temperatures based on the framework oxygen-channel water fractionation of wairakite are in good agreement with the measured, present-day downhole temperatures and/or calculated geochemical temperatures for the Takigami and Wairakei geothermal fluids. At Takigami, wairakite and associated calcite are found to be in oxygen isotopic equilibrium at the measured fluid temperature of around 220°C. The Kirishima geothermal system is thought to be in a waning stage of geothermal activity as recorded by fluid inclusion temperatures. The wairakite isotopic temperatures are consistent with the above thermal history, reflecting precipitation of wairakite from the most recent, relatively cool water. The disequilibrium oxygen isotopic relationship between some calcite-wairakite pairs for the Kirishima geothermal system suggests that the two minerals were precipitated from the waters with different Δ 18O values at different stages in the evolution of the system, consistent with paragenetic relations of the minerals in core samples. Oxygen isotopic composition of wairakite can be used as a new, single-mineral isotopic geothermometer for active geothermal systems and be applied to elucidate their thermal evolution.

Masami Noto and Minoru Kusakabe

Published Journal 
Geochimica et Cosmochimica Acta, 1997



Masami Noto,Minoru Kusakabe. 1997. Oxygen Isotope Geochemistry Of Geothermal Wairakite. Geochimica et Cosmochimica Acta. (!) .