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PRINCIPAL PUBLICATION

Exhumation-induced residual stress in undeformed ultra-high-pressure metamorphic rock, J.-B. Jacob et al., Earth Planet. Sci. Lett. 671, 119615 (2025); https:/doi.org/10.1016/j.epsl.2025.119615

REFERENCES

[1] J.-B. Jacob et al., J. Appl. Cryst. 57, 1823-1840 (2024).

setup combined with a microfocused X-ray beam enables non-destructive mapping of intra-crystalline orientation, strain, and stress fields down to the sub- micrometre scale [1].

The experiment

A garnet–quartz rock from the Italian Western Alps was selected for analysis. This rock experienced pressures of nearly 3 GPa – equivalent to about 100 km depth – during tectonic plate burial but displays almost no macroscopic deformation associated with exhumation, making it well suited for assessing the mechanical effects of decompression and cooling on residual stress development (Figure 84a-b). Using s3DXRD at beamline ID11, three-dimensional mapping of lattice orientation, strain, and stress at ~10 µm spatial resolution was conducted across an 800 µm- wide, 200 µm-thick volume (Figure 84c). Despite its intact appearance, the rock retained internal stress variations of up to one gigapascal (Figure 85). Stress concentrations were observed at specific microstructural sites, including around garnet edges, grain junctions, and mineral inclusions. These locations acted as mechanical amplifiers, where strain incompatibilities between neighbouring grains intensified local stresses.

In quartz, these stresses were sufficiently high to trigger local crystal plasticity, whereas garnet remained predominantly elastic. In the absence of significant macroscopic strain, the observed residual stresses were generated and retained through uneven elastic expansion of different constituent minerals during static decompression and cooling, combined with geometric incompatibilities at grain contacts.

This study demonstrates that exhumation from high- pressure conditions produces a complex and persistent internal stress field, even within seemingly undeformed rocks, suggesting that microscale stress heterogeneity is a fundamental feature of crustal rocks. Stress concentrations localise deformation, trigger micro- cracking, and influence chemical transformations by maintaining fluid pathways or facilitating reaction nucleation. Stress heterogeneity therefore represents a key yet often overlooked parameter governing large-scale strength, deformation mechanisms, and reactivity in Earth’s crust. By enabling non-destructive, high-resolution mapping of internal stress, s3DXRD provides a breakthrough approach for investigating the development and evolution of stress concentrations across scales in rocks and other polycrystalline materials.

Fig. 85: s3DXRD maps acquired on beamline ID11. a) Three-dimensional orientation map of garnet (top) and quartz (bottom), shown using inverse pole figure colours (z-axis). b) Local misorientation map across a two-

dimensional slice, quantified using the three-dimensional kernel median misorientation (3D-KMM). c) Equivalent von Mises stress across the same slice. Stress concentration sites are indicated by chevrons:

e: grain edges (indenter effect); b: sub-grain boundaries; c: confined grain (multi-anvil effect).