Robert Plant/Alison Krauss

The pairing of vocalists Plant and Krauss (he of Led Zep fame and miles-wide musical ears; she of crack-bluegrass outfit Union Station) creates serious sparks on this set of country, folk and rootsy rock/pop. “Killing the Blues,” a sweet country ballad complete with weeping steel guitar, finds them dueting hand-in-glove, their gentle inflection lovingly matched. Understated acoustic strings (banjo, mandolin, guitar) provide a lush bed for the gospel-folk of “Your Long Journey.” Elsewhere, as on “Fortune Teller,” T Bone Burnett’s production—all ambience, reverb and tremolo-laden guitar—overshadows the singers, who seem boxed-in by spare, repetitive melodies. There’s a ton of promise here, and some sublimely beautiful moments, which make the lesser cuts that much harder to take.