Annealing fractured single quartz in the presence of pore fluid at 200 MPa (2 kb) pressure at 400¿ and 600¿C for various periods resulted in the reorganization of intially planar cracks into arrays of spherical and tubular fluid inclusions, a process termed crack healing. Samples heated at 600¿C for several days with no added pore fluid showed no optically measurable crack healing. The amount of healing in samples with added pore fluids is a function of the temperature, time, initial concentration of silica in the pore fluid, and the initial crack dimensions. Several lines of evidence indicate that cracks heal as silica is locally transported by diffusion along the crack surface and/or through the pore fluid. Extensive crack healing occurs at temperatures as low as 400 ¿C in times as short as 1--2 days. Estimation of the thermal activation parameters for crack healing suggest that microcracks in quartz, will have geologically short lifetimes at temperatures of 200 ¿C or greater.