Subcritical cracking and sealing along cleavage planes and subgrain boundaries in plagioclase from the Mado...

Subcritical cracking propagates fractures via stress-assisted chemical reactions at crack tips under stresses below the critical threshold. Although this mechanism is increasingly recognized as a driver of time-dependent damage accumulation and permeability evolution in the Earth's crust, direct natural constraints on the lower oceanic crust remain limited. Here, we document plagioclase microstructural and compositional features consistent with subcritical cracking in a deformed oxide gabbro from the Mado Megamullion oceanic core complex, Shikoku Basin, Philippine Sea. We combine co-registered scanning electron microscopy, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy mapping with MATLAB MTEX Toolbox processing to link plagioclase crystallographic orientation, misorientation, and anorthite content (An = Ca/(Ca + Na + K)). Anorthite content maps show heterogeneous low-An domains within plagioclase porphyroclasts. These low-An domains are locally subparallel to the trace of the (001) cleavage plane and locally contain subgrain boundaries. We interpret the low-An domains as possible crack-seal-related features formed by stress-corrosion-assisted subcritical cracking along cleavage planes and subgrain boundaries, followed by host-controlled epitaxial precipitation of sodic plagioclase. These grain-scale crack-seal features may indicate that permeability creation by cracking can be followed by permeability reduction through sealing in gabbros from oceanic core complexes at slow-spreading ridges.