Variation of cooling rate with depth in lower crust formed at an oceanic spreading ridge: Plagioclase crystal size distributions in gabbros from the Oman ophiolite

[1] Analysis of crystal size distributions (CSD) of plagioclase in gabbros from the Oman ophiolite indicates that cooling rates in the lower crust ben eath a medium- to fast-spreading oceanic ridge did not vary smoothly with d epth. Cooling rates in the upper half of the gabbro section were approximat ely 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. I nstead, the Oman CSD data are consistent with recent seismic and modeling s tudies that suggest that hydrothermal convection leads to closely spaced, n ear-vertical isotherms in the upper half of the plutonic section within a f ew kilometers of the ridge axis.