The complex evolution of elasticity during metamorphic transformation

Prograde metamorphic reactions that reduce solid volume are common in subduction zones and orogenic settings [1]. These reductions are often linked to irreversible deformation such as viscous compaction and deep mantle earthquakes [2]. Viscous compaction involves permanent closure of reaction-generated porosity and fluid release, making porosity transient property of metamorphic reactions [3]. The pore closure observation is often linked to the intuitive loss of the elastic strength of the rock leading to permanent strains [4, 5], but these assumptions are very rarely demonstrated experimentally. In most cases, the nature of field relationships and the design of experiments do not allow for such an assessment to be made. Time-resolved in situ experiments enable the observation of a sample volume undergoing metamorphic transformation to check such assumptions at every stage of the reaction (loading, heating, cooling and unloading).

In this contribution, we provide preliminary visual and quantitative strain mapping during the metamorphic reaction cycle of a rock sample at various stress states and reaction extent.

Using Mjolnir, an X-ray transparent miniature triaxial deformation rig, we performed a series of gypsum (Ca₂SO₄·2H₂O) dehydration experiments into bassanite (Ca₂SO₄·1/2H₂O) at constant confining pressure of 20 MPa, pore fluid pressure (5 MPa) and subjected to similar temperature paths (up to 125ºC).  We used a series of differential stresses (radial, hydrostatic and axial principal stresses) to explore how the rock volume responds mechanically while also displaying different reaction fabrics (see [6]). This dehydration reaction produces a solid volume reduction of ~30% [4] enabling the investigation of the evolution of elasticity by unloading fully and partially transformed samples.

Using synchrotron microtomography at the I13-2 beamline of the Diamond Light Source (MG34156), we performed high resolution imaging (1.625 microns/ voxel edge) during gradual unloading to observe and quantify the elastic behaviour both using the mechanical data from the Mjolnir rig [7], sample dimensions (using stitched images; [8]) and digital volume correlation (DVC) techniques (Avizo). 

Our results show the complexity of strain distribution and partial preservation in metamorphic rocks with:

• Significant elastic strain preservation during metamorphic reactions and its apparent minimisation during the ultimate stages of the reaction (textural “maturation” via pressure/solution).
• Complex strain distribution influenced by bassanite anisotropy, sample fabric, geometry, and stress state.

These experiments enable to visualise in 4D the grain-scale development of a complex porous network during the reaction. It opens pathways to document the emergence of poro-elasticity (initial solid has very low porosity) and then the release of the elastic strains. This dataset further demonstrates the importance and the complexity of elasticity in metamorphic systems, with complex displacement vector fields under relatively simple boundary conditions.

References:

[1] Brown & Johnson (2019). https://doi.org/ https://doi.org/10.2138/am-2019-6956

[2] van Keken & Wilson (2023). https://doi.org/10.1186/s40645-023-00573-z

[3] Putnis (2015). https://doi.org/10.2138/rmg.2015.80.01

[4] Leclère et al. (2018). https://doi.org/10.1016/j.epsl.2018.05.005

[5] Llana-Fúnez et al. (2012). https://doi.org/10.1007/s00410-012-0726-8

[6] Gilgannon et al. (2024). https://doi.org/10.1130/G51612.1

[7] Butler et al. (2020). https://doi.org/ 10.1107/S160057752001173X

[8] Turpin et al. in prep