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Oxygen isotopic composition of fissure-grown quartz, adularia, and calcite from Broadlands geothermal field, New Zealand, with an appendix on quartz-K-feldspar-calcite-muscovite oxygen isotope geothermometers

1975; Yale University; Volume: 275; Issue: 7 Linguagem: Inglês

10.2475/ajs.275.7.785

1945-452X

Peter Blattner,

earthquake and tectonic studies

Oxygen isotope analyses of well developed, fissure grown quartz, adularia, and calcite crystals are related to downhole temperatures and other data from the Ohaki-Broadlands geothermal field. The bore No. BR 7 profile shows how oxygen isotope analyses of geothermal minerals provide quantitative evidence on subsurface hydrology, particularly with regard to zones of boiling, and on the past stability of a geothermal system. The lower part of this profile permits correlation of a temperature of 275 degrees C with 10 ^3^ ln\[(O18/O16)quartz/(O18/O16)K-feldspar\] = 3.3. Based on the constraints gained from Broadlands, on the 600 degrees C quartz-K-feldspar fractionation from Blattner and Bird (1974), and on earlier data of O9Neil and Taylor (1967, 1969) and O9Neil, Clayton, and Mayeda (1969), quartz-feldspar, quartz-calcite, and quartz-muscovite oxygen isotope geothermometers may be calibrated independently of isotopic analyses of solutions. The corresponding new equations are: 10 ^3^ ln alpha (quartz-K-feldspar) = 0.82+0.74 10 ^6^ K (super -2) 10 ^3^ ln alpha (quartz-muscovite) = 1.30+1.27 10 ^6^ K (super -2) 10 ^3^ ln alpha (quartz-calcite) = 0.37+0.88 10 ^6^ K (super -2) \[NBS-28 = 9.8/PDB = 30.36\]. The constant of the quartz-calcite equation depends directly on the scales of reporting used for silicates and carbonates respectively.