The "influences" of earlier periods tend to reside purely in the solos, and arrangements tend to get built underneath from the cycle of fifths and suchlike. Of course, bands have been deliberately trying to move away from this recently, with a lot of attention being paid to "old" modes and "jazz structures" - but it's still quite prevalaent.

 

I'm not convinced that Scarlatti created the riff, and I'm even less convinced that he influenced any metal musicians until maybe recently, when the fascination with really exploring composers actually kicked in (and this is only with a limited set of bands - most still go for the generic approach).

 

"The Planets", however, is a clear source of inspiration for Black Sabbath, and even clearer on Diamond Head, whose influence in turn is legendary.

 

"Carmina" hasn't really directly influenced anything in metal that I know of (unless you count Magma as metal), but it's hard not to hear it in much of the more "symphonic" black metal bands - and Ozzy always used it as his intro piece at gigs.

 

The other piece that influenced practically the whole of metal is Ron Grainer's theme tune to Doctor Who, as realised by Delia Derbyshire in 1963. Iron Maiden build a whole career on variations of that riff. Come to think of it, so did Sabbath - in fact, "Children of the Grave" is pretty much the Dr Who theme unmodified