We have addressed quartz precipitation resulting from cooling of silica-saturated, high-temperature hydrothermal solutions ascending through crack-controlled permeable regions beneath an ocean ridge crest. The results of this work indicate that for systems with high initial permeabilities, such as black-smoker-like systems, fluid channels can exist for several decades without serious sealing as a result of quartz precipitation, provided the kinetics of silica precipitation are considered. In lower-permeability systems, even though kinetics of precipitation remain important, reduction of permeability as a result of quartz precipitation can occur on a time-scale of a decade, leading to a decline in heat output and temperature of fluid as it encounters lower-temperature regions. In addition, we show that kinetics considerations result in the preferential sealing of thin cracks. This result is consistent with seismic velocity data, which suggest that the increase in seismic velocity with depth and age in the oceanic crust may result from the preferential closing of thinner cracks. These results also suggest that precipitation of quartz should occur near the dike/lava transition zone or in mineralized zones where large temperature gradients may exist as a result of mixing between cool seawater and hot hydrothermal fluids. The results of this model are thus consistent with the observed increase in quartz veins in such regions beneath the TAG mound and in ophiolites. ¿ 2000 American Geophysical Union