A key to understanding the processes operating in the outer ‘rigid’ part of the Earth, the lithosphere, is to look at metamorphic rocks exhumed to the Earth’s surface. If properly interpreted, fabrics and microstructures in these rocks provide fundamental constraints on lithospheric evolution. However, ambiguities in the interpretation of complex rock fabrics and microstructures have led to conflicting models for the tectonic development and thermal evolution of the lithosphere. One of the reasons for inaccurate interpretations is a still inappropriate quantification of processes that lead to the rock microstructure development.

The project “Interplay between metamorphism and deformation in the Earth’s lithosphere” (2013-2018) is funded by the European Research Council within the ‘starting grant’ scheme and is conducted at the Earth Science Department, Eidgenössische Technische Hochschule (ETH) in Zürich, Switzerland. This ERC project aims to develop unconventional methods for rock microstructures to improve our understanding of processes in the Earth’s lithosphere.

Metamorphism is complex

The recrystallisation and phase transformations in solid rocks that occur with changing pressure and temperature within the lithosphere are referred to as metamorphism. Metamorphism imposes first order control on geodynamic processes. In fact, mineral reactions and transformations within the lithosphere, involving deformation, and fluid/melt flow, are responsible for mountain building, volcanic eruptions and triggering earthquakes (Fig. 1).

Recent work on mineral reactions and microstructures in metamorphic rocks has focused on forward chemical modelling of phase equilibria and on their description through chemical potential relationships which are believed to control the mass transfer in rocks. Currently, high resolution analytical devices have become more available to the Earth sciences, and reveal the three-dimensional size, shape and distribution of microstructural features down to the nanometre-scale. Interestingly, the smaller the scale considered, the more heterogeneous an apparently uniform rock sample is. This heterogeneity is not only characterised by variation in chemical composition but also in mechanical properties. This needs to be accounted for.

During mineral reaction, the overall mechanical state of the rock is very important. The rock strength may control the reaction progress with a certain volumetric change from 0-100%, which may result in the development of stress variation, and therefore pressure variation, on all scales. Such pressure variations in rocks strongly influence the fluid flow through the crust which can, in turn, significantly control the mechanical-chemical coupling rates and mechanisms of various processes in the Earth’s interior. Hence, considering the interplay of metamorphic reaction and mechanical properties is critical for correctly interpreting observations in metamorphic rocks.

MADE-IN-EARTH aims

The goal of our research is to investigate and interpret the mass transfer in small scale microstructures observed in metamorphic rocks with mechanically maintained pressure variations. These pressure variations are reflected in solid solutions with mechanically induced compositional variations which, contrary to the conventional wisdom, reflect overall chemical equilibrium. However, if such a phenomenon is to be quantified, the classical thermodynamic approach, which was developed for isobaric systems, cannot be applied. Therefore, despite the increasing number of more accurate and very good analytical data from metamorphic rocks, a methodology for quantitative understanding of the new observations is missing. This can lead to the incorrect use of petrology data in constraining geodynamic models.

Hence, it is clear that the lack of appropriate explanation of such small scale observations may negatively influence the understanding of large scale processes. The team focus on very small pieces of metamorphic rocks (millimetre down to nanometre scale), for which they develop theoretical methods for the quantification of systems with local pressure variations, and then validate these through numerical models to carefully selected key microstructural observations. This will significantly increase our understanding and allow for a quantitative and physically-based reconstruction of metamorphic processes in general.

Why is it important?

The development of the new quantification approach opens new horizons in understanding the phase transformations in the Earth’s lithosphere. Furthermore, the new data generated serve as a food for the next generation of geodynamic models as well as for societal aspects. In fact, the explicit formulation of mass transport for natural, complex chemical systems on a small scale will provide insights to problems also relevant in material sciences, e.g. rechargeable batteries, radioactive waste disposal and CO₂ storage programmes.

Highlights

Lucie Tajcmanova is the editor of a special issue on this topic in the Journal of Metamorphic Geology: “Deviations from Lithostatic Pressure during Metamorphism: fact or fiction?”; October 2015.

Lucie Tajcmanova
Assistant Professor for Metamorphic Petrology
ETH Zürich
+41 44 632 2977
lucie.tajcmanova@erdw.ethz.ch
http://www.petromodelling.ethz.ch/

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