Oxygen isotopic exchange equilibrium

Figure 1.29. Calculated changes in the S 0 values of volcanic rocks (5 0 = +7.0%c) as a result of equilibrium oxygen isotope exchange with waters of different initial compositions. The dotted areas represent the ranges of rocks in the zeolite and the sericite-chlorite zones (Green et al., 1983).

Fig. 2.17 cut across the 45° equilibrium lines at a steep angle as a result of the much faster oxygen isotope exchange of feldspar compared to that of quartz and pyroxene. If a low- 0 fluid such as meteoric or ocean water is involved in the exchange process, the slopes of the disequilibrium arrays can be regarded as isochrons where, with continued exchange through time the slopes become less steep and approach the 45° equilibrium line. These times represent the duration of a particular hydrothermal event. 

Halas S, Wolacewicz W (1982) The experimental study of oxygen isotope exchange reaction between dissolved bicarbonate and water. J Chem Phys 76 5470-5472 Halas S, Szaran J, Niezgoda H (1997) Experimental determination of carbon isotope equilibrium fractionation between dissolved carbonate and carbon dioxide. Geochim. Cosmochim. Acta 61 2691-2695... 

Muehlenbachs K, Kushiro I (1974) Oxygen isotopic exchange and equilibrium of silicates with CO2 or O2. Carnegie Inst Wash Yearb 73 232-236... 

Yang MH, Flyim CP (1994) Intrinsic diffusion properties of an oxide MgO. Phys Rev Lett 73 1809-1812 Yaqian Q, Jibao G (1993) Study of hydrogen isotope equilibrium and kinetic fractionation in the ilvaite-water system. Geochim Cosmochim Acta 57 3073-30082 Yeh HW, Savin SM (1976) The extent of oxygen isotope exchange between clay minerals and seawater. Geochim Cosmochim Acta 40 743-748... 

For geologic purposes, the dependence of the equilibrium constant K on temperature is the most important property (4). In principle, isotope fractionation factors for isotope exchange reactions are also slightly pressure-dependent because isotopic substitution makes a minute change in the molar volume of solids and liquids. Experimental studies up to 20kbar by Clayton et al. (1975) have shown that the pressure dependence for oxygen is, however, less than the limit of analytical detection. Thus, as far as it is known today, the pressure dependence seems with the exception of hydrogen to be of no importance for crustal and upper mantle environments (but see Polyakov and Kharlashina 1994). 

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