Geothermometers

In order to obtain reliable formation temperatures based on electrum-sphalerite geothermometer, the following conditions have to be satisfied. 

Stannite is the most common tin sulfide mineral in the ore deposits associated with tin mineralization. This mineral sometimes contains appreciable amounts of zinc, together with iron. Several workers have suggested that the zinc and iron contents of stannite are related to temperature. With respect to the study of the phase relationships in the pseudobinary stannite-kesterite system. Springer (1972) proposed zincic stannite as a possible geothermometer mainly based on the chemical compositions of the two exsolved phases (stannite and kesterite). Nekrasov et al. (1979) and Nakamura and Shima (1982) experimentally determined the temperature dependency of iron and zinc partitioning between stannite and sphalerite. 

Figure 1.177. Comparison between the stannite-sphalerite geothermometer after Nekrasov et al. (1979) and one after Nakamura and Shima (1982). Crossbars indicate experimental uncertainties (Shimizu and Shikazono, 1985).

Both geothermometers are in agreement being close 380°C (Fig. 1.177). However, at the lower and higher temperatures the difference between the temperatures estimated from equations (1-82) and (1-83) becomes larger. 

Figure 1.178 represents a comparison between the stannite-sphalerite temperatures and homogenization temperatures of fluid inclusions or sulfur isotope temperatures. It can be seen in Fig. 1.178 that Nakamura and Shima s geothermometer would be rather consistent with the temperature estimated based on the fluid inclusions or sulfur isotope studies. It is notable that almost all stannite-sphalerite temperatures are within 30°C of average homogenization temperatures and sulfur isotope temperatures. 

Casadevall, T. and Ohmoto, H. (1977) Sunnyside mine. Eureka mining district. Sun Juan County Geochemistry of gold and base-metal ore deposition in a volcanic environment. Econ. GeoL, 72, 1285-1320. Cathelineau, M. and Nieva, D. (1985) A chlorite geothermometer. The Los Azufres (Mexico) geothermal system. Contr. Mineral. Petrol., 91, 235-244. 

D Amore, F. and Panichi, C. (1980) Evaluation of deep temperatures of hydrothermal systems by a new gas geothermometer. Geochim. Cosmochim. Acta, 44, 2021-2032. 

Fournier, R.O. and Truesdell, A.H. (1973) An empirical Na-K-Ca geothermometer for natural waters. Geochim. Cosmochim. Acta, 37, 1255-1275. 

Reed, M.H. and Spycher, F. (1984) Calculation of pH and mineral equilibria in hydrothermal waters with application to geothermometer and studies of boiling and dilution. Geochim. Cosmochim. Acta, 46, 513-528. 

The Na/Li ratio of geothermal waters decreases with increasing temperature and has been used as a geothermometer (Fig. 2.10) (Fouillac and Michard, 1981), suggesting that this ratio is controlled by feldspar-solution equilibrium (Shikazono, 1978a). 

Shikazono (1976) attempted to interpret this Na-K-Ca geothermometer based on thermodynamic equilibrium calculation. 

The concentrations of elements in hydrothermal solution depend not only on the compositions of rocks, but also on temperature. It is well known that Si02 concentration in hydrothermal solution increases with increasing of temperature and it can be used as a geothermometer. 

Shikazono, N. (1976) Thermodynamic interpretation of Na-K-Ca geothermometer in the natural water system. Geochem. J., 10, 47-50. 

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. 

To invoke our geothermometer, we need to recombine the vapor and fluid phases and then heat the mixture to determine saturation indices as functions of temperature. We could do this in two steps, first titrating the vapor phase into the liquid and then picking up the results as the starting point for a polythermal path. We will employ a small trick, however, to accomplish these steps in a single reaction path. The trick is to add the vapor phase quickly during the first part of the reaction path but use the cutoff option to prevent mass transfer over the remainder of the path. The commands to set the mass transfer are... 

Fournier, R. O., 1977, Chemical geothermometers and mixing models for geothermal systems. Geothermics 5,41-50. 

Fournier, R. O and R. W. Potter II, 1979, Magnesium correction to the Na-K-Ca chemical geothermometer. Geochimica et Cosmochimica Acta 43,1543-1550. 

Paces, T., 1975, A systematic deviation from Na-K-Ca geothermometer below 75 °C and above 10-4 atm Pco2- Geochimica Cosmochimica Acta 39, 541— 544. 

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. 

Other chemicals behave conservatively during the low-temperature mixing process and, thus, can also be used as geothermometers. As shown in Figure 19.14b, the hydrothermal system is a sink for magnesium because of incorporation into silicate rocks (Eq. 19.1). Extrapolation of this trend to zero magnesiiun ion concentration also yields... 

Geothermometers Minerals whose chemical composition can be used to determine their temperature of crystallization. 

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