Local Bulk Composition Effects on Metamorphic Mineral Assemblages

Plate tectonic forcing leads to changes in the physical conditions that affect the lithosphere. In response to such changes, notably the local temperature ( T ) and pressure ( P ), rocks evolve dynamically. Processes mostly involve mineral transformations, i.e., solid-state reactions, but (hydrous) fluids are often involved, and partial melting may occur in the Earth’s middle and lower crust. While these chemical reactions reflect the tendency of natural systems to reduce their Gibbs free energy, metamorphic rocks commonly preserve textural and mineralogical relics, such as compositionally zoned minerals. Where relics are present, thermodynamic equilibrium clearly was not attained during the evolution of the rock. Petrochronology seeks to establish a temporal framework of petrologic evolution, and for this purpose it is essential to determine the P–T conditions prevailing at several stages. When analyzing a rock sample it is thus critical: 1. to recognize whether several stages of its evolution can be discerned, 2. to document the minerals that formed or were coexisting at each stage, and 3. to estimate at what physical conditions this happened. If (and only if) a chronometer then can be associated to one of these stages—or better yet several chronometers to different stages—then the power of petrochronology becomes realizable. This chapter is concerned with a basic dilemma that results directly from steps (b) and (c) above: P–T conditions are determined on the basis of mineral barometers and thermometers, which mostly rest on the assumption of chemical (or isotopic) equilibrium, yet the presence of relics is proof that thermodynamic equilibrium was not attained. One way out of the dilemma is to analyze reaction mechanisms and formulate a model based on non-equilibrium thermodynamics and kinetics (Lasaga 1998). While this can be fruitful for understanding fundamental aspects of metamorphic petrogenesis, there are more direct ways to address the limited scope needed for petrochronology. The alternative pursued …