Isotope geothermometers

Abstract

The best available isotope thermometers of the elements oxygen, sulfur, carbon and hydrogen are summarized, and some applications to metamorphic rocks, to sulfide ore deposits and to geothermal systems are discussed.

Whether or not isotope geothermometers are generally applicable depends whether or not isotope equilibrium is established, and if the existence of isotope equilibrium may be recognized. An inherent danger is the tendency to regard calculated temperatures as estimates of peak thermal conditions. However, the temperatures determined represent the last isotope equilibration, below which no further isotope exchange takes place. This temperature often coincides with fluid loss from the geological system.

Only those mineral pairs can be used as geothermometers where temperature calibrations exist. From the three different approaches a) theoretical calculation, b) calibration on an empirical basis, c) experimental determination, the latter seems to be the most promising, although considerable disagreement exists between some published calibration curves. However, with new sophisticated techniques on hand (CLAYTON et al., 1983a, 1983b) this difficulty may be overcome in the near future.