Grain-scale residual stress distribution associated with fluid-induced plagioclase breakdown

The magnitude and distribution of stress in Earth’s crust is difficult to quantify, but impacts deformation behavior, phase stability, and metamorphic reactions. Stress is influenced by a variety of factors including compositional heterogeneities, volume changes during ongoing reactions, and the influence of far-field stresses. During metamorphic reactions the stress distribution may be modified, but prevailing stresses may also impact reaction kinetics, or which reactions take place. We studied one of the most impactful reactions within the continental crust; the fluid-induced breakdown of plagioclase at high-pressure conditions. The samples are from former lower-crustal granulites exposed on Holsnøy, western Norway. They preserve a reaction front along which the dry granulite is transformed into an eclogite. Reaction progress is intimately linked to fluid ingress and there is no microstructural evidence of deformation. This lack of deformation indicates that the studied microstructures are entirely related to fluid-induced metamorphic reactions. We measured the residual stress associated with plagioclase breakdown by high-angular resolution electron backscatter diffraction and contrasted the results with compositional variations (scanning electron microscope and electron probe micro analyzer). (Scanning) transmission electron microscopy was conducted on selected sites to link this information with the associated dislocation configuration. We find that intragrain residual stress associated with the breakdown of plagioclase is on the order of hundreds of megapascals, and dominantly caused by the elastic interactions of dislocations. Before the reaction plagioclase contains few, randomly oriented dislocations. Compositional modification of plagioclase during the reaction (increasing albite content) leads to dislocations occurring more frequently in the more albitic part of the plagioclase. In that case, dislocations have a preferred orientation, but no significant long-range increase in dislocation density, i.e., increased organization. Our results thus suggest that as plagioclase breaks down, dislocations are mobilized to accommodate the variations in lattice parameters associated with hundreds-of-megapascal stress variations on the grain scale.