Metal-Rich Globular Clusters: an Unaccounted Factor Responsible for   Their Formation?
  Presently unaccounted but quite probable "chemical factor" may be responsible for the formation of old metal-rich globular clusters (MRGCs) in spheroids, as well as of their conterparts, young (intermediate-age) massive star clusters (MSCs) in irregulars. Their formation presumably occurs at the same stage of the host galaxies' chemical evolution and is related to the essentially increased SF activity in the hosts around the same metallicity, Zsol/3 ([Fe/H]~-0.5). It is achieved very soon in massive spheroids, later in lower-mass spheroids, and (much) more later in irregulars. 
Valery V. Kravtsov (Universidad Catolica del Norte, Antofagasta,   Chile; Sternberg Astronomical Institute, Moscow, Russia)