Quartz geothermometer

Since the satiuation concentration is a function of pressme as well as temperature, the geothermometer temperature is dependent on pressmes. At 1000 bar, which is roughly equivalent to a depth below the seafloor of 1.6 km (750 bar) in a 2.5-km water depth (250 bar), the regression line intersects the solubility ciuwe of quartz at 345°C. Thus, the quartz geothermometer estimates a minimum temperature of 345°C for the undiluted hydrothermal fluid at the Galapagos vents. 

The quartz geothermometer (Fournier, 1977) requires that the dissolved silica concentration in an ascending hydrothermal solution becomes quenched in as the fluid approaches the surface. On the other hand, the phase transfer model for the rate of quartz deposition, described in this chapter, requires that the LEA be valid. The conditions for which each of these models is appropriate can be found by calculating values of Da, and Pe. Figure 8.1 shows the temperature range for the transition from LEA to quenched conditions. 

Several chemical geothermometers are in widespread use. The silica geothermometer (Fournier and Rowe, 1966) works because the solubilities of the various silica minerals (e.g., quartz and chalcedony, Si02) increase monotonically with temperature. The concentration of dissolved silica, therefore, defines a unique equilibrium temperature for each silica mineral. The Na-K (White, 1970) and Na-K-Ca (Fournier and Truesdell, 1973) geothermometers take advantage of the fact that the equilibrium points of cation exchange reactions among various minerals (principally, the feldspars) vary with temperature. 

Ghent E. D. (1976). Plagioclase-garnet-Al2Si05-quartz A potential geobarometer-geothermometer. Amer. Mineral, 61 710-714. 

Scotford DM (1975) A test of aluminum in quartz as a geothermometer. Am Mineral 60 139-142 Scott HG (1975) Phase relationship in the zirconia-yttria system. J Mater Sci 10 1827-1835 Seifert F, Czank M, Simons B, Schmahl W (1987) A conunensurate-inconunensurate phase transition in iron-bearing ermanites. Phys Chem Minerals 14 26-35 Shannon RD (1976) Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr A32 751-767... 

Agrinier P (1991) The natnral calibration of geothermometers Application to the quartz-rutile pair. 

Silica geothermometer is also useful for an estimate of reservoir temperature because silica concentration of geothermal water which strongly depends oti temperature due to the dependence of solubiUty of silica minerals (quartz, opal, chalcedony, amorphous silica) on temperature (Fig. 1.18). Temperature estimated... 

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