Isotope Geothermometers

In general, experimental calibrations of isotope geothermometers have been performed between 250 and 800°C. The upper temperature limit is usually determined by the stability of the mineral being studied or by limitations of the experimental apparatus, whereas the lower temperature limit is determined by the decreasing rate of exchange. 

On the basis of these systematic tendencies of emichment found in nature, significant temperature information can be obtained np to temperatures of 1,000°C, and even higher, if calibration cnrves can be worked ont for the varions mineral pairs. The pnbhshed hterature contains many calibrations of oxygen isotope geothermometers, most are determined by laboratory experiments, althongh some are based on theoretical calcnlations. 

Figure 11a illustrates the difference between the actual temperature, measured at the well head (Twh), and the temperature determined from the isotopic fractionation (TIS) between H2 and H20. This isotope geothermometer seems to work well for this system, at least in the sense that it predicts subsurface temperatures actually encountered in deep drill holes (Panichi et al. 1979). Before 1979 (full symbols), the AT... 

Blattner P, Braithwaite WR, Glover RB (1983) New evidence on magnetite oxygen isotope geothermometers at 175°C and 112°C in Wairakei steam pipelines (New Zealand). Isotope Geosci 1 195-204... 

Kotzer TG, Kyser TK, King RW, Kerrich R (1993) An empirical oxygen- and hydrogen-isotope geothermometer for qrrartz-tourmahne and torrmraline-water. Geochim Cosmochim Acta 57 3421-3426... 

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. 

Method (c) is based on the idea that the calculated/orman on temperature of a rock (calculated from other geothermometers) serves as a calibration to the measured isotopic fractionations, assuming that all minerals were at equilibrium. However, because there is evidence that equilibrium is not always attained or retained in nature, such empirical calibrations should be regarded with caution. 

Carbon isotope fractionations under equilibrium conditions are important not only at low-temperature, but also at high temperatures within the system carbonate, CO2, graphite, and CH4. Of these, the calcite-graphite fractionation has become a useful geothermometer (e.g.. Valley and O Neil 1981 Scheele and Hoefs 1992 Kitchen and Valley 1995) (see discussion on p. 227). 

Bechtel A, Hoernes S (1990) Oxygen isotope fractionation between oxygen of different sites in illite minerals a potential geothermometer, Contr Miner Petrol 104 463 70... 

Kyser TK, O Neil JR, Carmichael ISE (1986) Reply to Possible non-equilibrium oxygen isotope effects in mantle nodules, an alternative to the Kyser-O NeU-Carmichael geothermometer. 

Bolognesi, L., Noto, P. Panichi, C. 1990. More mileage for isotope gas geothermometers the case of the Larderello geothermal field. Geothermal Resources Council Transactions, 14, 857-861. 

One of the most important uses of oxygen isotopes in carbonates is as geothermometers. Isotope fractionation is pressure independent (Clayton et al., 1975), giving it an advantage over other geothermometers which are generally both temperature and pressure dependent. The equation for calculating the temperature at which calcite precipitates from water is ... 

In principle, isotopic analyses of natural samples can also be used to calibrate fractionation factors. Effective use of the natural sample method, however, requires that several stringent criteria be met. (1) The phases being calibrated first attained isotopic equilibrium at some temperature, and subsequently retained their equilibrium isotopic compositions, (2) equilibration temperatures in the samples of interest are well determined by independent methods, and (3) the geothermometers used to determine... 

Graham CM, Valley JW, Eiler JM, Wada H (1998) Timescales and mechanisms of fluid infiltration in a marble An ion microprobe study. Contrib Mineral Petrol 132 371-389 Gregory RT, Criss RE (1986) Isotopic exchange in open and closed systems. Rev Mineral 16 91-127 Gregory RT, Taylor HP (1986a) Possible non-equihbrium oxygen isotope effects in mantle nodules, an alternative to the Kyser-O Neil-Camuchael geothermometer. Contrib Mineral Petrol 93 ... 

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