Placing turntable on top of fireplace mantle?

Looks solid. You may need springs later.

The only major thing is corner placement on it's on can be problematic.

Looks like a perfect place for a turntable. Should be fine!

For me, a fireplace mantel is at least as stable as one of those Target wall shelves and likely to be much more so.  Plus, it is anchored into the chimney, the most structurally rigid part of most houses, certainly more rigid than a typical stud, even if the latter is weight bearing as it should be.  So, I have to ask why you think a mantelpiece might vibrate, unless the whole house is vibrating, which can occur, if you live near a major highway or if you live under an airport approach pattern, but which is usually a very transient phenomenon? Apart from that, there is nothing wrong with doing your recommended glass of water test; that would detect constant local vibration.  I just can't understand why that would happen, and if it does happen, there is likely to be no sanctified vibration-free location in the entire house.

I'll let you know as soon as I've reviewed the blueprints and the engineering reports.  

If one of the comments above was in response to my post............
because acoustical vibrations affect a tts performance more in a corner, especially if it is located behind speakers.