You can watch or rewatch the invited talk of Prof. Pierre Lanari entitled “Mapping equilibrium relationships in metamorphic rocks—petrological modelling beyond equilibrium phase diagrams” given during the online meeting 2021 of the Metamorphic Studies Group.
Lanari P., Duesterhoeft E. and Hermann J.
Equilibrium thermodynamics is a fundamental theoretical framework to model how a rock with a specific bulk composition responds to changes in pressure (P) and temperature (T). However, the underlying assumption that metamorphic minerals form and evolve at equilibrium conditions restricts the application of the models to relatively simple scenarios. Examples include isochemical models allowing phase relationships at equilibrium to be mapped out in the P–T space (e.g. phase diagrams), or non-isochemical models involving dynamic reactive bulk compositions (e.g. mineral or melt fractionation) along fixed P–T trajectories. By contrast, most of the metamorphic minerals in nature exhibit compositional zoning and patterns suggesting sluggish diffusion and/or partial re-equilibration associated to deformation and/or fluid-rock interaction. Where metastable relics are present, thermodynamic equilibrium was at best achieved locally during the evolution of the rock. This simple observation raises several questions about the general limits of the equilibrium models and potential biases on P–T estimates. Is the bulk rock composition of a hand-specimen sized sample representative of a reactive volume at any stage of the P–T path? What are the size and the geometry of the equilibrium volumes that have to be considered for accurate modeling?
To address these questions, we developed a modeling framework based on iterative thermodynamic models integrated with quantitative compositional mapping of thin section stitched into the software package BINGO-ANTIDOTE that is integrated in the mapping software XMAPTOOLS. The subroutine Bingo contains a scoring technique to quantitatively compare modeled and observed mineral assemblage, modes and compositions. One of the key features is that the local bulk composition (X) and the observations (modes and compositions) are taken in the same area. This mutual correspondence permits to build a fully quantitative comparison between model and observations as well as providing a statistical framework for evaluating the quality of the model. In addition, the subroutine Antidote includes mapping functions and a heuristic search method that can determine optimal P–T–X conditions. Bingo-Antidote is a powerful alternative to traditional modeling tools as the textural and compositional complexity of any sample can be taken into consideration when applying equilibrium models. The detailed investigation of a classical Grt–Bt–Ms–Ky–St–Pl–Qz metapelite from the Central Alps will be discussed. A series of Iterative thermodynamic models applied to local domains with different mineral assemblages revealed a detailed P–T history but also suggests that only 50 vol% of the rock volume was equilibrated at peak temperature conditions of 620 °C. Partial re-equilibration during prograde-to-peak metamorphism has several implications for the modelers as the reactive part of the rock controlling the mineral reaction evolves outside the P–T section mapped by traditional isochemical phase diagrams based on bulk rock composition.