Host-inclusion mineral systems as a new probe for in situ mineral rheology at non-ambient conditions

The understanding of geodynamic processes such as earthquakes and mountain building requires a deep knowledge of mineral and rock deformation mechanisms (e.g. Karato, 2013). The most used approach to study mineral and rock rheology is by means of experimental investigations. However, they can be significantly challenged by both apparatus corrections and grain-boundary interactions that result in inhomogeneous stress states within deforming samples. Moreover, few experimental data are available for single crystals under tensile stress even if this is a quite common environment in the crust at all scales (e.g. Fossen, 2010). Finally, most of the data that we have on mineral and rock rheology comes from gem quality, often synthetic, crystals, but they are far to represent the bulk of the crust.

In this contribution, a novel approach that aims to overcome some of these difficulties is presented. The rheology of minerals can be explored using natural host-inclusion mineral systems instead of an experimental deformation apparatus on synthetic products. Host-inclusion systems are the simplest natural “rock samples” occurring on Earth because they consist of two mineral grains and one grain boundary. Moreover, because of the contrast in the thermal expansion and compressibility coefficients between the host and the inclusion, host-inclusion mineral couples are pre-stressed under most pressure and temperature conditions. Therefore, by applying pressure and/or temperature to such systems in the laboratory, it is possible to generate tensile and compressive stresses in the host mineral which can be measured in situ using Raman spectroscopy without applying apparatus corrections (e.g. Campomenosi et al. 2024). Finally, mineral flow laws along with the role of grain boundaries can be investigated from the host deformation experiments coupled with numerical simulation modelling (e.g. Zhong et al. 2024).  This new methodology can improve our quantitative understanding of mineral strength under different stress state at non-ambient conditions, providing a significant step forward in the quantification of larger scale geodynamic processes.

 

References

Campomenosi, N., Angel, R. J., Mihailova, B., & Alvaro, M. (2024). Mineral host inclusion systems are a window into the solid-state rheology of the Earth. Communications Earth & Environment, 5(1), 660.

Fossen, H. (2010). Structural Geology. Cambridge University Press, 480 pp.

Karato, S. I. (2013). Rheological properties of minerals and rocks. Physics and Chemistry of the Deep Earth, 94-144.

Zhong, X., Wallis, D., Kingsbery, P., & John, T. (2024). The effect of aqueous fluid on viscous relaxation of garnet and modification of inclusion pressures after entrapment. Earth and Planetary Science Letters, 636, 118713.