LECTURES ON GEOCHEMICAL !N1 ERPRETATION OF ...

- 44 -RA/K GEOTHERHOKETERSThe BalK geothe~omete~ is gene~ally thought to take longer to reachequilibrium at a given temperature than other commonly used geothcrmometers.Therefore, the WalK ratio is commonly used to estimate possiblehighest temperatures in deeper parts of a system where waters reside forrelatively long periods of time, and other geothermometers are used toestimate lower temperatures that occur in shallower reservoirs where watersreside for relatively short periods of time. This may be an appropriateuse of the geothermometer when applied to some well waters. but is adangerous practice when applied to hot-spring waters.Figure 1 shows log (Ha/K) versus lOOO/T for base exchange betweenalbite and adularia, and between albite and microcline, calculated usingequilibrium constant equations published by Arnorsson (1982). Positionsof empirical Ha/K geothermometers suggested by various authors also areshown in Figure 1. Three of these geothermometers (Truesdell, 1916;Tonani, 1980; Arnorsson, 1983) give essentially the same result while theHa/K geothermometers of Hieva (1981), Fournier (1979) and Giggenbach (1988)give significantly higher temperatures for reservoir waters below about300°C. Hote that all of these empirical Ha/K geothermometers lie betweenthe albite- microcline and albite-adularia curves. The data points thatwe~ used in the determination of these empirical geothermometers(different data sets for the different geothermometers) also lie in abroad band between the albite- microcline and albite- adularia curves, andshow the g~atest amount of scatter at lower temperatures. All of theempirical geothermometers give about the same result at about 300·C.A considerable range in structural states of alkali feldspars maypersist metastably at low temperatu~s, and base exchange involvingdifferent kinds of X- feldspar can explain the entire range of HalK valuesexhibited by the data and the various empirical Ha/K geothermometers.Also, other minerals may participate in cation exchange reactions involvingHa and K, such as Ha- and K- montmorillonites,