An energy conservation equation is formulated that balances the heat generated by viscous dissipation in a peridotite simultaneously undergoing partial fusion and penetrative constant shear stress deformation with the heat removed by mobilization and ascent of basaltic magma from the region undergoing deformation. The solution of this parameterized energy equation gives the volume fraction of melt (ϑ) as a function of time (t) after the initiation of deformation. A stability analysis of the conservation equation shows that stable (ϑ<100%) or unstable (ϑ→∞) solutions exist depending on the magnitude of two dimensionless parameters K1/K2 and K3. For geologically reasonable values of K2/K2 and K3, the analysis indicates that peridotitic thermo-mechanical systems undergoing penetrative deformation at constant shear stress show a two-stage history. An early stage of growth where ϑ increases monotonically on a 2 to 3 m.y. time scale eventually is replaced by a steady s ate regime (constant ϑ). Typical values of ϑ lie in the range 3 to 5 volume percent; melting of peridotite to the extent of 20--30% appears to be precluded by this model.