# Phase transitions under differential stress: Deviatoric stresses or pressure?

Sebastian Cionoiu, Lucie Tajcmanova, Evangelos Moulas, Holger Stünitz

► To cite this version:

Sebastian Cionoiu, Lucie Tajcmanova, Evangelos Moulas, Holger Stünitz. Phase transitions under dif- ferential stress: Deviatoric stresses or pressure?. 20th EGU General Assembly, 2018, Vienne, Austria. pp.18504. hal-03552809

HAL Id: hal-03552809

https://hal.science/hal-03552809

Submitted on 2 Feb 2022

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Phase transitions under differential stress: Deviatoric stresses or pressure?

Sebastian Cionoiu (1), Lucie Tajcmanova (1), Evangelos Moulas (2), Holger Stünitz (3,4)

(1) Department of Earth Sciences, ETH Zürich, Switzerland (sebastian-ci@erdw.ethz.ch), (2) Institut des Sciences de la Terre (ISTE), University of Lausanne, Switzerland, (3) Department of Geology, University of Tromsø, Norway, (4) Institut des Sciences de la Terre d'Orléans (ISTO), Université d'Orléans, France

We performed deformation experiments for the Calcite-Aragonite system in a (Griggs-Type) solid medium de-
formation apparatus, using different sample geometries. The confining pressure (σ₃) was maintained below the
Calcite-Aragonite transition while the first principal stress (σ₁) exceeded the transition pressure, changing with
sample strength. Thus, the bulk sample pressure, P₃D = (σ₁ + 2*σ₃)/3 was closely varying around the transition
pressure.

Light microscopy, Raman spectroscopy and electron backscatter diffraction (EBSD) show systematic, strongly heterogeneous patterns in the distribution of the phases, grain-sizes and deformation. These result from stress, strain and pressure variations within the sample. To better understand different mechanisms contributing to the phase transition and grain size reduction, the local distribution of first order parameters as pressure, stress and strain must be known.

We performed numerical modelling in order to quantify the stress, pressure and strain distribution within the deforming sample. The numerical results are compared to the stress distribution, as inferred from grain size, and finite strain. These show a good fit with modelling results using a viscous power law rheology. As the distribution of stress and strain in the numerical model matches the experiments, the modelled pressure is expected to depict the experimental conditions as well.

Finally, patterns of modelled pressure, stresses and strain are compared to experimentally produced phase-
transition patterns. From this comparison the confining stress (σ₃) can be excluded as being a driving force for
the phase transition. However, the discrimination between local σ₁ and local pressure (defined like P₃D) still needs
better quantification regarding experimental measurements.