Skip to main content

Interplay between metamorphism and deformation in the Earth’s lithosphere

Final Report Summary - MADE-IN-EARTH (Interplay between metamorphism and deformation in the Earth’s lithosphere)

Recrystallization 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.

During mineral reactions or phase transformations, 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 to 100% which may result in the development of stress, 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 correct interpreting observations in metamorphic rocks.

The goal of the MADE-IN-EARTH research project was to investigate the effect of mechanically-induced pressure variations on the mass transfer in rock microstructures.
With my team, we have focused on very small pieces of metamorphic rocks (millimetre down to nanometre scale). We developed the theoretical methods for quantification of systems with local pressure variations and validate it through numerical models to carefully selected key microstructural observations.

Using those tools we discovered:
1/ The mechanically-induced pressure variation on the grain scale leads to a redistribution of the chemical components and to a development of “enigmatic” (i.e. from conventional point of view “unusual”) chemical zoning. We call those microstructures as mechanically-controlled. Due to the advances in analytics, such pressure variations can now be even directly measured in some samples by High angular resolution electron backscatter diffraction, spectroscopic methods or micro-diffraction.

2/ The presence of the pressure variations precludes the use of lithostatic pressure assumption, i.e. a conversion of pressure to depth. Because we would have to solve a following question: Which pressure would we take from the microstructure to convert to depth?

3/ The initial chemical diffusion in a grain, due to the mechanical coupling connected with the volume differences, generates pressure variations within the grain. This inhibits the chemical diffusion and the chemical zoning can be preserved on geologically relevant time scales. The development of this coupled quantification tool opened up a completely new world for diffusion modelling where the role of mechanics has never been considered. The conclusion of this research is that the mechanical effect can significantly influence the diffusion rate, which might have severe consequences also for the experiments that infer diffusion coefficients.

4/ Mineral inclusions can be used as a new chronometer. The time information can be reconstructed using a simple combination of laser Raman spectroscopic data from mineral inclusions with mechanical solutions for viscous relaxation of the host.

5/ Our tools are scale independent, therefore can also be applied within a large scale context.

Why is it important?
The development of the new quantification approach opened new horizons in understanding the phase transformations and mineral reactions 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, explicit formulation of mass transport for natural, complex chemical system on a small scale also provides insights relevant to problems in environmental, energy and societal applications.