How about a light source that's dim enough to not melt the ice but still worth placing to keep away mobs?
Any light level 11 or under would be safe for ice. So that is what your target light source would need to be. And mobs spawn under light level 7 so there you go you want 7 to 11 and rember all light sources drop 1 per block so you could just keep them fair enough away from the ice to make it safe.