Connectivity and fabric evolution with strain in eclogites : in-situ X-ray tomography under UHP conditions

Eclogites are a major lithology of the subducting oceanic crust, and the strength contrasts between and with lithologies such as blueschists, serpentinites and peridotites, at depths, is likely what commands the timing and style of HP rocks exhumation, within subduction zones (Agard et al., 2016). These contrasts also influence the roughness and stress at the interface between the subducting slab and the overlying mantle wedge (Agard et al., 2018), and therefore may play a role in the stress relaxation and intermediate depths earthquakes sequences. Deformation mechanisms of the main minerals of eclogite, pyroxene and garnet, have been studied individually under high pressure and temperature. The rheology of eclogites themselves has received some attention using high pressure experiments (e.g. Zhang and Green, 2007, Farla et al., 2017, Rogowitz et al, 2023, Molines et al., EGU25-5696). These works and numerical models (e.g. Yamato et al, 2019, Angiboust et al, 2024) underline the importance of the interplay between brittle vs. ductile mechanisms in eclogites rheology at experimental strain rates. The garnet vs. pyroxene volume fractions are expected to have a major effect on brittleness and strength, since the spatial contiguity of the strongest component, or connectivity of the weakest component, may lead to transitions in the control of the deformation.

Until now the effect of shear strain on phases connectivities under GPa pressures has not been quantified, while it is one path to achieve connections between strong or weak domains. Here, we will present results from torsion experiments on two-phase aggregates of garnet and pyroxene as a proxy for eclogites, with garnet fractions from 15% vol. to 85% vol. We use in-situ absorption contrast tomography at the PSICHE beamline (synchrotron SOLEIL), under pressures of 2 to 5 GPa and temperatures of 850°C, to characterize quantitatively the fabric/microstructure of the aggregates under increasing shear strain (up to ca. 5).

We will discuss these microstructural quantifications with respect to 1) recent in-situ mechanical measurements in the same aggregates compositions, by Molines et al. (EGU25-5696), and 2) similar in situ characterizations during torsion experiments of serpentine+olivine aggregates – hence a different strength contrast between phases – by Mandolini et al. (e.g. EGU25-13729).