Evidence for reaction-induced microfracturing in granulite facies migmatites

Water-undersaturated partial melting at granulite facies conditions, follow ed by accumulation and upward migration of the resulting granitic melt, is one of the principle causes of crustal differentiation. Microfracturing cau sed by small, positive volume changes associated with water-undersaturated melting reactions may facilitate rapid extraction of melts from melting sit es. We have successfully imaged annealed microfractures in residual quartz from three granulite facies migmatites. Annealed microfractures have been d iscovered in (1) quartz inclusions in peritectic garnet; (2) quartz inclusi ons in K-feldspar; and (3) quartz occurring as entrained grains. In the lat ter, oscillatory zoned quartz has overgrown fractures in the residual core, indicating that fracturing occurred prior to melt crystallization and that postanatectic volume changes are not responsible for the observed fracturi ng. Fractures are generally <5 mu m wide, are parallel sided, and have low sinuosity, Crack densities are two to three times higher than experimental studies involving muscovite melting and approach theoretical interconnectiv ity thresholds for percolation, implying that reaction-induced microfractur ing is a viable mechanism contributing to melt interconnectivity during ana texis under granulite facies conditions.