Impurity source mechanisms which appear during the heating of the Alcator C tokamak by injection of lower hybrid frequency waves are described. Silicon is a dominant impurity in these experiments where SiC was the limiter surface material. At low values of injected power, the silicon source rate is dominated by physical sputtering. As the amount of injected power is raised, evaporation caused by the tail electron heat flux to the limiter becomes the primary source for impurities entering the plasma. Measurements of silicon line emission from the plasma as well as other central and edge parameters are presented as functions of injected radiofrequency power.