Comment on “Apatite 4He/3He and (U-Th)/He Evidence for an Ancient Grand Canyon” - Karlstrom, Karl; Lee, John; Kelley, Shari; Crow, Ryan; Young, Richard; Lucchitta, Ivo; Beard, L.; Dorsey, Rebecca; Ricketts, Jason; Dickinson, William; Crossey, Laura (Science, 2013)