The Moho transition zone (MTZ) of the Oman ophiolite commonly includes a number of gabbro sills surrounded by dunites. The petrology and geochemistry of these sills are investigated to provide constraints on how magma migrates from the subridge mantle to the oceanic crust. The gabbro sills have millimeter-scale to tens of centimeter-scale modal layering that closely resembles layering in lower crustal gabbros of the ophiolite. Variations in mineral compositions correlate with the modal layering, but there are no overall trends within the sills. The gabbroic sills and the layered gabbros have clear covariations among mineral compositions, which can be interpreted as a fractional crystallization path from a common parental magma. Together with constraints from mid-ocean ridge thermal evolution and crustal accretion dynamics, the petrological and geochemical observations on the gabbro sills indicate that they formed from small, open-system, melt-filled lenses within the MTZ. The thermal evolution of the MTZ melt lenses, buffered by the ambient mantle, is characterized by a slow cooling rate (<10-3 ¿C/yr) and a small temperature difference (~0.1--1 ¿C) within lenses. Internal origins for modal layering, such as gravity currents and oscillatory nucleation, are unlikely in such a thermal environment, and we propose that open-system evolution of the melt lenses is essential to produce the observed layering. The formation of the MTZ melt lenses may be a consequence of porous flow with low Peclet number entering a conductively cooling regime, where porosity becomes clogged by crystallized plagioclase. Preservation of fine-scale vertical variation in mineral composition, together with correlated compositions of different minerals, rules out diffuse porous flow as the primary mechanism of melt transport above these melt lenses. Instead, melt extraction must have been focused into porous channels or melt-filled fractures. Melt lenses drained by fractures would experience repetitious expulsion with continuous melt replenishment. Modal layering could develop through the expulsion cycles, probably via in situ crystallization at the margins of melt lenses. ¿ 1997 American Geophysical Union