Knowledge of the fluid permeability within all parts of submarine hydrothermal systems is needed to understand hydrothermal circulation in ocean crust and the heat budget of mid-ocean ridges. If hydrothermal circulation extends into the gabbro layer of young ocean crust, and possibly to within meters of a crustal magma chamber, then the fluid permeability close to the interface between the magmatic and hydrothermal parts of the ridge system may originate by the growth of fractures induced by hydrothermal cooling. Similar incremental growth mechanisms for thermal contraction fractures located at the edges of lava flows, dikes, and crustal plutons are suggested by the occurrence of discrete microearthquakes during the solidification of basalt lava lakes and the presence of ''joint striations'' on the surfaces of some columnar joints in basalt. This similarity of fracture mechanisms approaching magma-solid interfaces may provide a means of estimating the spacing of thermal contraction fractures in submarine hydrothermal systems. Microearthquakes originating beneath the active hydrothermal vents at 21¿N East Pacific Rise have seismic moments of 1017 dyn cm and frequencies of 0.1--1.0 d-1. This scale of microearthquake activity is inconsistent with model seismic sources consisting of incrementally growing fractures spaced less than several meters apart and suggests that fluid permeability near the magma chamber boundary is 10-18 m2 or larger. The lower bound on fracture spacing and fluid permeability computed from the incremental fracture model is independent of any fluid transport model but is consistent with laboratory measurements of permeability and with the assumed permeabilities of recent numerical models of hydrothermal circulation. ¿ American Geophysical Union 1993