Our group is working on various multidisciplinary topics which help to improve our understanding the processes in the Earth interior. Our projects involve field work, laboratory-based measurements and thermodynamic and numerical modeling in order to provide quantitative and physically-based tools for interpreting common microstructures in metamorphic rocks. In this section, you can have a look at current projects and methods we use to solve the scientific tasks.
More information about ERC starting grant (2013-2018): https://cordis.europa.eu/project/rcn/109320/factsheet/en
- Equilibrium under pressure variations
- Asymmetric chemical zoning in garnet
- Preservation of enigmatic chemical zoning in minerals
- Deformation experiments with Griggs rig apparatus
- Coupling diffusion and deformation
- Raman spectroscopy and geo-speedometry
Equilibrium under pressure variations
Observations in natural rocks, as well as theoretical predictions indicate that pressure can be spatially heterogeneous on the grain-scale (e.g. see review of Moulas et al., 2013). It has been recently documented that grain-scale pressure variations can be responsible for maintaining chemical zoning in mineral solid solutions under equilibrium conditions even at high temperature (Tajčmanová et al., 2014).
Therefore, we develop new thermodynamic formulations to derive pressure variations from petrographic observations. The new unconventional geobarometric method can now be applied to a binary system (Tajcmanova et al. 2014). For more details on this method and its significance for diffusion modelling see our invited review article in Lithos (Tajcmanova et al. 2015).
Complementary to this, we also develop tools which couple phase equilibria and mechanical modelling for systems under pressure variations.
Contour diagram of chemical potential in plagioclase used to derive pressure gradient from compositional zoning (for details see Tajcmanova et al, 2014).
Asymmetric chemical zoning in garnet
Numerical modelling is carried to simulate garnet growth along a prograde P-T path under grain-scale fluid pressure variation. The numerical model uses Perple_X for thermodynamic equilibrium calculation and Brute-Force method to test the P-T path. The calculated zonation pattern can be fitted to the measured garnet zonation to evaluate the magnitude of grain scale pressure variation and P-T path. Insights is given to evaluate how local pressure gradient influences the pattern of asymmetric zonation and how it improves the retrived P-T path.
Calculated garnet zonation under ca. 1kbar pressure variation
Preservation of enigmatic chemical zoning in minerals
A research project which integrates petrographic and field observations with numerical simulation to test theoretical predictions of mechanically controlled multi-component chemical zoning in metamorphic minerals.
Chemical zoning in garnet from intermediate granulite (T=950°C and 16-14 kbar) from Eastern margin of the Bohemian Massif.
Deciphering the pressure (P) – temperature (T) history of a rock is fundamental for understanding of geodynamic processes and the evolution of mountain belts. Chemical zoning in minerals reflects variations in (P) and (T) along the path which the rock experienced. Although theory and experiments predict chemical zoning to be rapidly obliterated by diffusion at high temperature, an increasing amount of examples from high grade terrains shows the preservation of zoning.
It has been recently documented that grain-scale pressure variations can be responsible for maintaining chemical zoning in binary mineral solid solutions under equilibrium conditions (at T>800°C). We systematically evaluate the effect of mechanically maintained pressure variations on chemical zoning in multicomponent systems in one of the most common metamorphic minerals in the Earth’s crust - garnet.
Garnet is collected from two regional metamorphic terrains - the Western Gneiss Region in Norway and Bohemian Massif in the Czech Republic. The enigmatic preservation of chemical zoning in minerals observed in these high-grade rocks therefore presents the optimal testing ground to assess the potential of equilibrium based pressure-controlled chemical zoning. Data collection includes petrographic study of thin sections (optical microscopy/SEM), quantitative mineral chemical analyses (EMPA) including elemental X-ray mapping, and whole rock chemistry (XRF). Our newly developed thermodynamic software will be used and extended to compare observations from nature to theoretical predictions of equilibrium under external force in multicomponent systems.
Deformation experiments with Griggs rig apparatus
The research within this project will systematically evaluate the nonhydrostatic-stress effect on a precipitation during solid transformation in metamorphic rocks. The described goal requires a multidisciplinary approach including a combination of deformation experiments with material science analytical techniques and theoretical quantification approaches. The project is focused on implementation of Raman spectroscopic techniques for direct measurements of residual pressures as well as transmission electron microscopy for high resolution strain-analysis in order to bring an alternative interpretation of the transformations under nonhydrostatic conditions. Such an approach will provide a quantitative and physically-based guideline for interpreting common microstructures in metamorphic rocks. Results will therefore have a strong impact on our understanding of solid state transformations in geological materials as well as on understanding the phase transformations under nonhydrostatic compression in materials in general.
Schematic classical isothermal free energy-pressure diagram portraying calcite and aragonite gibbs free energy curves. Arg=aragonite, Clc=calcite, Clc_strained=strained calcite, σ1=the maximum stress.
Coupling diffusion and deformation
Compositional zoning in metamorphic minerals have been generally recognized as an important geological feature to decipher the metamorphic history. Two important processes occur in a chemically zoned mineral in a metamorphic rock: chemical diffusion and formation. This project attempts to couple these two processes, and specifically focuses on the interplay between the grain-scale pressure variation and the preserved chemical zonation. The governing physical theory is developed in analogy to poroelasticity and thermoelasticity by choosing appropriate conservation equations for mass, concentration, momentum, and is complemented by the fundamental equilibrium thermodynamics. Numerical modelling (Finite Difference Method) is performed to quantitatively investigate the generation of local pressure variation due to chemical diffusion and the feedback effect of the pressure variation on preserved chemical zonation.
The concentration profiles (left and middle) as function of time for the coupled model (this project) and decoupled model (classical Fick’s diffusion), and the profile of generated pressure variation (right) due to chemical diffusion (from Zhong et al., 2017).
Raman spectroscopy and geo-speedometry
Recently, with my former PhD student Xin Zhong, we showed that the duration of exhumation can be reconstructed using a simple combination of laser Raman spectroscopic data from mineral inclusions with mechanical solutions for viscous relaxation of the host. More specifically, tectonics and surface erosion lead to the exhumation of rocks from the Earth’s interior to the surface. This process is characterized by three main variables: pressure, temperature and time. Among them, the duration of exhumation in different geological processes has the largest variation spanning orders of magnitude (from hours to billion years). In fact, constraining especially the duration of rapid magma ascent is often challenging. The inclusion-chronometer that we developed, can be used for fast geological events such as kimberlite magma ascent as well as for decompression lasting several million years for high-pressure metamorphic rocks. This is the first precise timing information harvested on microstructures from a pure mechanical perspective, and thus it is completely independent from radiometric dating which can only be applied to a limited number of mineral phases. This brings an unprecedented geological value to the tiny mineral inclusions as timekeepers that contributes to a better understanding on the large-scale tectonic history and thus has significant implications for a new generation of geodynamic models. This new method needs testing and an improvement of Raman data for various mineral inclusions.
Moreover, by combining the direct stress measurements by Raman spectroscopy and mechanical solutions for stress relaxation in a mineral, we were able to develop a unique method that allows stress relaxation to be calculated directly from natural samples (Zhong et al., 2018).
Relation between duration of exhumation (