Evidence for reaction-induced microfracturing in granulite facies migmatites

Abstract

Water-undersaturated partial melting at granulite facies conditions, followed by accumulation and upward migration of the resulting granitic melt, is one of the principle causes of crustal differentiation. Microfracturing caused by small, positive volume changes associated with water-undersaturated melting reactions may facilitate rapid extraction of melts from melting sites. We have successfully imaged annealed microfractures in residual quartz from three granulite facies migmatites. Annealed microfractures have been discovered in (1) quartz inclusions in peritectic garnet; (2) quartz inclusions in K-feldspar; and (3) quartz occurring as entrained grains. In the latter, 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 fracturing. Fractures are generally <5 µm 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 interconnectivity thresholds for percolation, implying that reaction-induced microfracturing is a viable mechanism contributing to melt interconnectivity during anatexis under granulite facies conditions.