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Detailing the relationship between stress and reactions in metamorphic rocks has been controversial, and much of the debate has centered on theory. Here, we add to this discussion and make a major advance by showing in time-resolved synchrotron microtomography experiments that a reacting and deforming sample experiencing an elastic differential stress produces a fabric orthogonal to the largest principal stress. This fabric forms very early in the reaction and can be shown to be unrelated to strain. The consequences of this are significant because a non-hydrostatic stress state is a very common geological occurrence. Our data provide the basis for new interpretations of the classical, and enigmatic, serpentine fabrics of Val Malenco, Italy, and Cerro del Almirez, Spain, where we relate the reported fabrics to transient, and cyclical, differential stresses from magma intrusion and the earthquake cycle.

We examine the partial survival of high-temperature mantle microstructures throughout multi-stage hydration-and dehydration-mediated pseudomorphism at differing pressure-temperature-fluid conditions. Throughout the harzburgitic mantle section of the Leka Ophiolite Complex (Norway), finite domains of parallel olivine encompassed by mesh-textured olivine resembling 'perfectly cleaved' olivine grains were identified. Crystallographic orientation mapping, combined with micro-computed tomography, reveals that the parallel olivine grains are highly misoriented (up to 90 degrees) with no crystal-preferred orientation, despite remaining parallel in three dimensions. Parallel olivine grains exhibit free dislocations with low dislocation density, whereas within mesh-textured olivine dislocations are aligned into walls. MnO is enriched (up to 1.8 wt %) and NiO depleted (0.21 +/- 0.24 wt %) within parallel olivine grains compared with mesh-textured olivine (0.29 +/- 0.14 wt % MnO; 0.38 +/- 0.19 wt % NiO). Clinopyroxene lamellae that are crystal-plastically deformed occur sandwiched in lizardite layers between every parallel olivine grain or fully enclosed within olivine. Al2O3 and Cr2O3 concentrations of clinopyroxene lamellae (2.09 +/- 0.88 wt % Al2O3; 0.79 +/- 0.27 wt % Cr2O3) overlap with those of primary clinopyroxene grains (2.43 +/- 0.69 wt % Al2O3; 0.83 +/- 0.36 wt % Cr2O3) and are distinctly different from those of secondary diopside found within the parallel olivine domains. Intragranular serpentine inclusions (X-Mg = 0.95 +/- 0.01), displaying elevated Al2O3 (3.92 +/- 4.10 wt %) and Cr2O3 (0.78 +/- 0.82 wt %) concentrations, are exclusively found within parallel olivine grains. Lizardite (X-Mg = 0.92 +/- 0.02) within the domains originates from hydration of parallel olivine and compositionally overlaps with mesh-texture lizardite. Antigorite (X-Mg = 0.95 +/- 0.01) replaces both types of olivine grains. Whole-rock compositions indicate a harzburgitic composition; howev, AuthorCount:3