A wellbore treatment fluid comprising: a base fluid; a viscosifier, wherein the viscosifier is a synthetic, cross-linked polymer having a thermal stability greater than 275° F.; a breaker, wherein the breaker decreases the viscosity of the treatment fluid at or above a breaker-activation temperature; and an activator, wherein the activator is a chelate complex, and wherein the activator activates the breaker to reduce the viscosity of the treatment fluid at a temperature less than the breaker-activation temperature. A method of treating a subterranean formation comprising: introducing the treatment fluid into the subterranean formation.