The stability of the basalt melt lens identified below fast spreading ridges has been tested by a thermal and mass balance model in order to investigate how the generation of the crust by basaltic melt can be affected by spreading rate. The model is constrained using field information from the Oman ophiolite, the main piece of evidence dealing with the nature and structure of the root zone of sheeted dikes which acts as a thermal boundary layer between the melt lens below, and the hydrothermally cooled sheeted dike unit above. For a mass balance calculation we first assume that the gabbro unit is entirely created by the melt issued from the lens. We examine how the heat generated by crystallization of this mass can be evacuated through the boundary layer on top of the melt lens. A steady-state situation is found for a given spreading rate and ratio of the Nusselt number to the field-deduced thickness of the boundary layer. If, as suggested by geophysical data, the melt lens exists only for spreading rates larger than 6 cm/yr, the boundary layer needs to be essentially conductive. With increasing spreading rates and increasing heat supply the system thermally adjusts itself until it must work in a new mode. Geological evidence suggests that this could be achieved by developing an additional feeding of the gabbro unit by a large number of sills being emplaced below the melt lens into the lower gabbros.¿ 1997 American Geophysical Union