Mavis Staples has already carved out her spot in the annals of American pop music. As lead singer of the gospel-soul legend the Staple Singers, Mavis breathed the fire into classics like “I’ll Take You There” and “Respect Yourself.” She’s already been inducted into the Rock and Roll Hall of Fame, won a Grammy Lifetime Achievement Award, and been named one of the 100 greatest singers of all time by Rolling Stone. Why then is she here in 2011, performing in crusty Gowanus, Brooklyn, singing music she wrote with a alt rock legend almost 20 years her junior? This August, at age 71, Staples released You Are Not Alone, a collaboration with Wilco’s Jeff Tweedy, proving that even a proven diva has room to experiment. “Wonderful Savior” is an acapella gospel church-stomp, and it serves as proof that Staples’ fabled vocal chords still in excellent form.