Analysis of crystal size distributions (CSD) of plagioclase in gabbros from the Oman ophiolite indicates that cooling rates in the lower crust beneath a medium- to fast-spreading oceanic ridge did not vary smoothly with depth. Cooling rates in the upper half of the gabbro section were approximately 1.5-2 times faster than in the lower half of the gabbro section during igneous crystallization and/or high temperature grain growth processes. Our results are inconsistent with thermal models for oceanic spreading ridges that approximate the effect of hydrothermal convection by using an enhanced thermal conductivity together with a conductive heat transfer mechanism. Instead, the Oman CSD data are consistent with recent seismic and modeling studies that suggest that hydrothermal convection leads to closely spaced, near-vertical isotherms in the upper half of the plutonic section within a few kilometers of the ridge axis. Copyright 2001 by the American Geophysical Union.