Influence of pore fluid pressure and differential stress on gypsum dehydration and rock texture revealed by 4D synchrotron X-ray tomography

Tectonic-scale features happening at convergent plates are ultimately the outcome of microscopic, grain scale processes. In collision zones, prograde metamorphism occurs by gradual increase of pressure and temperature [1; 2]. Among the most important prograde mineral reactions are dehydration reactions, which are characterized by solid volume reduction, porosity creation, fluid release and high pore fluid pressures [3]. Most models linking dehydration and mechanical instabilities [4-6] involve feedback loops between coupled chemical, hydraulic and mechanical processes. Feedbacks control pore fluid pressure build-up and drainage, and provide efficient pathways for the transport of chemical components. Gypsum dehydration is crucial in the formation of detachment faults thin-skinned tectonics [7]. It is also used as a proxy for serpentine dehydration and the generation of intermediate depth seismic events/aseismic slip activity [8].

We performed a set of experimental gypsum dehydrations both at the TOMCAT microtomography beamline at the Swiss Light Source, and in the laboratory. Using a modified version of the Mjolnir triaxial rig [9] that allowed control of pore fluid pressure in the synchrotron microtomography setup enabled us to document how differential stress (∆σ) and pore fluid pressure (P_f) influence the dehydration of Volterra alabaster gypsum to bassanite at a constant confining pressure and temperature in 4D.

We derived data on mineral phase transformation and formation of pore networks by applying a deep-learning algorithm in ORS Dragonfly® software, which reduced data processing times, minimized interpretation biases, and allowed analysing larger volumes. The results exhibit an extremely high accuracy compared to standard procedures. The analysis of phase proportions (gypsum, bassanite and porosity) of segmented volumes correlates very well to theoretical predictions indicating a correct segmentation from the algorithms and self-consistency of the generated datasets. Comparing results obtained  at various ∆σ and P_f to the light of mechanical data and additional in-house experiments allows us to better interpret their effect on reaction duration, magnitude and textural evolution of the rock. Transient phenomena as well as individual grain transformation and growth are now traceable in a fully automated way.

Our data further our understanding of gypsum dehydration: We found that ∆σ greatly influences the assemblage of the bassanite needles, which tend to grow nearly vertical at ∆σ ≅ 0. Increasing ∆σ significantly increases sample compaction. On the contrary, increasing P_f decreases the bulk deformation and slows down the reaction. As pores grow around bassanite needles, the control of the orientation of needles by differential stress can influence the overall pore network and thus introduce anisotropies during transient and final stages of the reaction. Our data confirm that ∆σ and P_f greatly influence transient and final rock texture, which has implications on drainage during nappe emplacements.

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