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Fluids and geophysics: influence of mineral reactions on petrophysical properties

Examining how mineral reactions change rheological and petrophysical rock properties is crucial to understand the dynamics of the solid Earth. Using field mapping of exhumed deformed rocks, detailed analysis of the rock-forming mineral assemblages on various scales, experiments involving reaction and deformation, and numerical simulations we develop the understanding of processes such as subduction, mountain building, and earthquakes.

Influence on petrophysics

Petrophysical properties are directly connected to the minerals which form the rock. Mafic rocks in the Earth’s crust containing minerals such as pyroxene, amphibole, and feldspar, have a lower density than, e.g., an eclogite, which contains garnet and omphacite. The geometric arrangement of the minerals, their abundances, and the structure in rocks influences many other bulk rock petrophysical properties such as compressibility, shear moduli, porosity, and permeability. These properties directly affect the seismic signals which are obtained by seismologists from geophysical measurements. For a detailed insight into geological processes at depth it is therefore crucial to understand how mineral reactions affect the seismic signal so that geophysical data can be interpreted correctly. By direct petrophysical measurements on minerals and rocks found in exhumed rocks and using averaging techniques by acknowledging the entire field relations, we may develop a more complete understanding of how small scale petrophysical properties from a natural observation influence the seismic signal that may be recorded at the Earth’s surface. By doing so, we may also better understand what happens at depth beneath an orogen.

Further reading: Zertani et al. (2019a); Zertani et al. (2019b); Zertani et al. (2020);

Influence on Rheology

Deformation on earth occurs in a brittle, ductile, or transitional manner controlled by rheological properties of the rocks. These properties depend on temperature, pressure, water and/or fluid supply, metamorphic mineral reactions, and the petrophysical characteristics of the rock mineral assemblage. In addition, these parameters in turn influence the deformation mechanisms, e.g., diffusion, creep mechanisms, and grain-size reduction occurring in different depths from crust to the deep mantle and define the behaviour of a rock under influence of an applied strain. To decode the evolution of a solid or elastic rock body, which is exposed to external forces causing strain, such as seismic waves, plays a key role to understand the processes within subduction zones, such as earthquakes or the formation of shear zones. We study examples that illustrate how fluid infiltration coupled with strain accumulation influence the rock strength and control the evolution of an eclogite-facies shear zone network.

Further reading: John et al. (2009); Kaatz et al. (2021)

Finite element modelling

FEM model of real mapped structures by Sascha Zertani

Detailed observations of the minerals, structures, and processes in rocks from natural outcrops from the meter up to the kilometer scale can be used in numerical models to obtain the elastic response of a single rock, outcrop, or entire geological unit. Combining geological mapping with direct petrophysical measurements such as seismic velocities and density, a quantitative estimate of the average bulk petrophysical properties can be made by using the stress calculated over the entire model domain. An example is the study from Zertani et al. (2020), where detailed maps of exhumed lower crust from the Bergen Arcs were subjected to a numerical calculation. The calculated stress of the heterogeneous material from different numerical deformation experiments was combined to find the effect of eclogitization on the P-wave anisotropy. This gave new insights into ongoing metamorphic processes at depth beneath the Himalayan mountain range.