What was the first heavy metal album to hit no. 1?

A few years ago I caught Slayer on one of their "farewell" tours. It was sort of a mini-festival, with like 5 metal bands spanning multiple eras and subgenres.

And as much as I do like a couple of Slayer albums, I realized one thing: I am not that metal. Not at all.

But every now and then, there's a heavy metal album that just rocks my socks off (totally metal thing to say, right?). And very rarely, there's a metal album that breaks through and actually makes waves on the pop charts.

In fact, there are a select few metal albums that have even topped the charts. Which was one the first to do it?