Sumit Chakraborty

Abstract

Compositions of minerals function as recorders of temperature, pressure or other intensive thermodynamic variables (e.g. fO₂, fH₂O) that were attained during the evolution of rocks if two sets of conditions are fulfilled: (i) the minerals were participants in a reaction that is sensitive to the relevant thermodynamic parameter, and their compositions are unique functions of these variables (defined, for example, by the coexistence of certain other minerals as part of an equilbrium assemblage), (ii) the kinetics of subsequent readjustments were such as to permit the preservation of the compositions that were attained at that particular stage of evolution of the rock. Several approaches are currently used to consider thermodynamic equilibria (e.g. individually calibrated reactions, calculation of phase equilbria by minimization of free energy in an assumed equilibrating bulk composition). The kinetic resetting of compositions are considered largely in the context of diffusive reequilibration driven by chemical gradients. The role of interfacial energy and textural maturation has not been considered in as much detail, but recent experimental and theoretical studies show that this quantity plays an important role through various grain recycling processes, and through the interaction with stress and presence of fluids. This has major implications for the interpretation of temperatures, pressures, dates, closure temperatures, as well as cooling / exhumation rates that are recorded in minerals. These aspects will be discussed using some observations on experimental and natural systems, and some theoretical models.