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SUMMARY
Chemical geothermometers
by Stefán A rnórsson
Science Institute
University of Iceland, Reykjavík
The development of chemical geo-
thermometry in geothermal investigations is
reviewed. A summary is given of the as-
sumptions, approximations and limitations
involved in applying chemical geothermo-
meters to evaluate temperature conditions
in geothermal systems. The application of
chemical geothermometry to thermal waters
which have mixed with cold waters in the
upflow is also included.
Careful selection of drillhole data has
allowed correlation of silica concentrations
and Na/K ratios in thermal waters with
measured aquifer temperatures. The results
yield new temperature functions for the
silica and Na/K geothermometers in the
range 25-180°C and 25-250°C re-
spectively (equations lb, lc and 3b in table
II). With respect to silica the new tempera-
ture function compares well with the
chalcedony solubility data of Fournier
(1977). Above 150°C the new Na/K
temperature function compares well with
that of Ellis and White (see Truesdell, 1975)
and it compares well witli equilibrium
between solution, low albite and microcline
over the whole temperature range.
Il is shown that the Na/K geothermo-
meter is always superior to the Na-K-Ca
geothermometer.
It is necessary to correct the analysed
silica concentrations in the waters for the
fraction which is dissociated when the water
pH is greater than about 8.5 (referring to
measurement at 20°C). For very high pH
waters (greater than 10) it is recommended
to make this correction by making use of the
temperature dependent relation of Na + /H +
in geothermal waters (see equation 5 in table
II) and the analysed sodium concentration
rather than the measured pH (see equation
(11) in text and equation lc in table II).
138