KINETIC CONTROLS ON THERMOBAROMETRY: THE COMPLIMENTARY STRENGTHS OF PSEUDOSECTIONS AND INDIVIDUAL REACTIONS

Determination of the depth and temperature at which metamorphic assemblages form (thermobarometry) is the tool for mapping the internal thermal structure of orogenic belts. Traditionally, thermodynamic analysis has been used to develop and employ thermobarometric methods. Kinetics has been considered only to evaluate the closure temperatures of thermobarometers. However, kinetic and textural controls determine reaction pathways and therefore, the nature of spatial distribution of domains of equilibrium. This in turn should guide the choice of locations of chemical analysis to determine mineral compositions that were in equilibrium at different stages of evolution of metamorphic rocks. An inability to account for these variations can result in incorrect P-T determinations and tectonic models. On the other hand, consideration of kinetic controls can reveal more details of the P-T evolution of metamorphic rocks. Simple model calculations that consider grain boundary transport can be used to demonstrate how a single closure temperature is inadequate to describe the temperatures recorded by element exchange geothermometers; it is possible to obtain a range of closure temperatures depending on kinetic and textural details. It can be seen that while pseudosection calculations benefit from the use of optimized thermodynamic databases, kinetic considerations are more difficult to implement and the choice of effective bulk compositions can sometimes be problematic. On the other hand, individual reactions can address the local nature of compositional variations, but may suffer from uncertainties of calibration. Combination of these two methods provides the most powerful tool for constraining the P-T conditions of metamorphism. Examples from particularly challenging migmatites from the Higher Himalayan Crystallines will be used to demonstrate this and to underscore some of the pitfalls of using either of these methods in isolation without consideration of kinetics.