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Scanning 3D X-ray diffraction reveals hidden stresses in high- pressure crustal rocks

• Understanding how rocks record the pressures and temperatures of Earth’s interior is critical for reconstructing tectonic processes and the long- term evolution of the crust and mantle. • Scanning 3D X-ray diffraction at beamline ID11 enabled non-destructive mapping of lattice orientation, strain, and stress in intact metamorphic rock volumes. • The measurements show that exhumation preserves highly heterogeneous internal stress fields, providing new constraints on rock strength, deformation behaviour, and the chemical reactivity of crustal and mantle materials.

The challenge

Through plate tectonic processes, slabs of oceanic crust are subducted into Earth’s mantle, where they experience pressures of several gigapascals – more than 10,000 times the atmospheric pressure. During their subsequent return toward the surface, for example, during the formation of a mountain chain, these rocks undergo decompression and cooling, which trigger chemical and mineral transformations that modify their

microstructure and mineral compositions. Although these mineralogical changes are well documented, the mechanical consequences of exhumation remain largely unexplored. As confining pressure is released, minerals expand at different rates, generating internal strain and stress within the polycrystalline microstructure that may remain “frozen in” after exhumation. Understanding how such residual stresses develop and persist in rocks is crucial for explaining the larger-scale strength, deformation behaviour, and reactivity of Earth’s crust and upper mantle.

Residual stress has traditionally been investigated using high-angular-resolution electron backscatter diffraction or X-ray Laue microdiffraction, both capable of detecting elastic strain variations as small as 0.01%. However, these techniques require destructive preparation of two-dimensional cross-sections, which can alter the pre-existing stress state. Raman spectroscopy can probe buried inclusions non-destructively but provides only partial strain information and no crystallographic data. In contrast, scanning three-dimensional X-ray diffraction (s3DXRD), developed and implemented at beamline ID11, overcomes these limitations. Exploiting the exceptional brilliance and coherence of the ESRF- EBS X-rays, this method captures the three-dimensional orientation and strain state of deeply embedded crystals in intact rock volumes. A tomographic acquisition

Fig. 84: Investigated material and

geological context. a) Geological setting:

ultrahigh-pressure rocks exhumed from

~100 km depth within a subduction channel. b) Microphotograph of the sample, consisting

of a macroscopically undeformed garnet– quartz assemblage. c) Segmented grain

map obtained by s3DXRD.