Understanding the effect of mineralogy on rock deformation style: 3D-evolution under extreme conditions
Effect of secondary phase fraction on rheology and rock fabrics in subduction zones rocks
Modeling the earth evolution throughout geological times, understand dynamics of subduction zones at regional scales from earthquakes to convection, requires to quantify rock viscosity at the interface between the subducting plate and de mantel wedge. Minerals that constitute rocks have different elastic and plastic properties, and the deformation mechanisms in rocks and their rheology change with strong vs. weak mineral proportions. MADISON’s main objective was to study experimentally the impact of these mechanical contrasts on rocks fabrics and rheology. The results can then be integrated or compared with numerical models of polyphase aggregates, rheological models, or serve for interpretation of natural microstructures.
In-situ, synchrotron study of fabric from X-ray absorption contrast tomography and stress partitioning from X-ray diffraction
2 phase aggregates proxies for the rocks of interest with strong or weak mechanical contrast were studied (serpentine-olivine et garnet-pyroxene), with different volume fractions of weak phase in the aggregate and various total strains, under high pressure and high temperatures (3-5 GPa, 350 to 850°C).
The distribution of stress and the fabric evolution were obtained with in-situ synchrotron measurements. Absorption contrast X-ray tomography allows observing the distribution of the two phases, characterize fabric, and quantify the connectivity of the secondary phase. X-ray diffraction allowed quantifying the stresses distribution between the phases and deduce the modalities of this distribution (strong phase bearing the load vs. weak phase controlling the deformation).
The starting material, the microstructures and the deformation mechanisms, post mortem, were characterized by scanning and transmission electron microscopy at the University of Lille and Institut Chevreul electron microscopy platform.
Main outputs
One main result is on synthetic olivine + serpentine aggregates representing serpentinized peridotites. In this aggregates with a strong mechanical contrast, we show a critical threshold between 10 and 20% vol. fraction, for the fabric evolution (Mandolini et al, submitted) and for the stresses distribution (Hilairet et al. 2024). This threshold separates behaviors and fabrics dominated by the strength of olivine, from those controlled by serpentine rheology, even potentially for modest deformation amounts (e.g. 20%).
The project started the development and the application of torsional rock deformation above 2 GPa, coupled to synchrotron tomography (Mandolini et al. 2023). Also included were knowledge and know-how transmission though lectures, practicals for engineers and students, and internships and doctoral supervision. The HP lab in Lille acquired a Rotational Paris-Edinburgh (RoPEc) press in order to further develop ex-situ torsional deformation experiments.