One important aspect of kinetic studies concerns the amount of overstepping of the equilibrium temperature in metamorphic reactions. Overstepping is promoted by the reaction rate being more sluggish than the heating rate. This might be apparent in contact aureoles, where the heating rate is typically rapid. In this case, reaction paths may vary significantly from those deduced by local equilibrium buffering. To further clarify the effect of reaction kinetics on the development of metamorphic reaction paths, overstepping can be estimated by combining results of kinetic laboratory experiments with heating rates, obtained by thermal evolution models. Although a great deal of uncertainty exists in the extrapolation of the rates of experimental processes to natural systems, such calculations provide an insight into the extent of non-equilibrium during metamorphic reactions. The Ballachulish location was chosen, since this is probably the most thoroughly studied model case for contact metamorphism (see Voll et al., 1991 for review). In the aureole, several thin sheets of Appin limestone are interbedded with pelites and quartzites.
The calculation is based on new kinetic data for the reaction
3 dolomite + 4 quartz + 1 H2 O ¤ 1 talc + 3 calcite + 3 CO2
(Widmer and Metz, 1996), which is the outermost isograde reaction in the Appin Limestones of the Ballachulish aureole. Heating rates were obtained from a conductivity model (Buntebarth, 1991), constrained by thermal profiles from geothermometry and isograde patterns. The reaction-path calculation procedure used was similar to that outlined by Lasaga (1989). The fluid flow rate was assumed to be zero, considering stable isotope studies, which do not support the infiltration of externally derived fluid into the calcareous rocks.
The results of the calculations can be summarised as follows: At locations where the maximum temperature exceeds the equilibrium temperature of the talc-calcite reaction only to some degree, i.e. in the vicinity of the isograde, the reaction path follows closely the XCO2 - T - equilibrium curve. In this case internally buffering of pore fluids is evident and local equilibrium is likely.
In contrast, at locations where the equilibrium temperature of the talc-calcite reaction is exceeded by a great amount, i.e. close to the contact, the reaction path crosses the equilibrium curve nearly vertical. The increase of XCO2 in the fluid is slow, and the fluid composition can not keep up with the temperature change caused by rapid heating. Therefore the actual temperature is different from the equilibrium temperature, resulting in a non-equilibrium reaction-path.
The occurrence of completely different reaction paths, depending on the distance from the contact, is compatible with observed XCO2 - evolution paths and microtextural interpretations by Masch and Heuss-Aßbichler (1991). In accordance with these authors we do not support the concept of progressive metamorphism, whereby each high grade rock has evolved through the sequence of low- and middle-grade assemblages seen laterally distributed in the aureole.
Buntebarth, G., In Equilibrium and kinetics in contact metamorphism (Voll et al., eds.), 379-402 (1991).
Lasaga, A.C., Earth Planet. Sci. Lett. 94, 417-424 (1989).
Masch, L. & Heuss-Aßbichler, S., In Equilibrium and kinetics in contact metamorphism (Voll et al., eds.), 211-227 (1991).
Voll, G., Töpel, J., Pattison, D.R.M. & Seifert, E., (eds.) Equilibrium and kinetics in contact metamorphism, Springer Verlag, Berlin Heidelberg, 484 pp. (1991).
Widmer, J. & Metz, P., in prep. (1996).