Yup,it's official.Page & Plant's new album has genuine and intentional microtonal content.
In an interview I just saw on TV,in which they discussed briefly,their middle eastern/oriental explorations,Jimmy Page said of their new album,Walking into Clarksdale,(I quote from memory) "The Oriental stuff was done by a guy called Tim from Transglobal Underground (a British dance music outfit) who plays an Oriental keyboard that plays microtones."
Could this be one of those General Music "Arabian" keyboards ?
I also find that Robert Plant's vocal intonation is constantly veering off into non-12TET land and it seems to be an integral part of his approach.He did mention that he and Jimmy were fans of Eygptian singer Umm Kalthum (1904-75) of whom the Rough Guide to World Music says "indisputably the Arab world's greatest singer"
BTW that Rough Guide is one heck of a quick fix for gaining a wide general knowledge of regional/ethnic music.