In answering a question about the orbital path of Omega Centauri, I learned that it has a distinct chemical signature from the rest of the Milky Way. Basically, it is very rich in s-process elements, which I think are primarily produced in Asymptotic Giant Branch stars. It is not totally clear to me why that would be the case. Are AGB stars dominating metallicity in Omega Centauri, and if so, why? If it is not what, what is the cause?
After looking through a few papers, in particular Chemical Abundances and Kinematics in Globular Clusters and Local Group Dwarf Galaxies and Their Implications for Formation Theories of the Galactic Halo and references therein, I think I have a reasonable answer. Omega Centauri's chemical abundance seems to be most easily explained by it being an accreted dwarf spheroidal galaxy. The metallicities of its stars match up quite well with those of almost all of the Milky Way's dwarf spheroidal satellites.
The reason for the dwarf spheroidal galaxies having different metallicities is thought to be due to two factors: they form stars at lower efficiency than the Milky Way, and the material for star formation (that is, gas) is more easily driven out of dwarf spheroidals by galactic winds. Chemical Abundances for 855 Giants in the Globular Cluster Omega Centauri (NGC 5139) says that in this respect, Omega Centauri differs even from dwarf spheroidals, in that Type Ia supernovae played a minimal role in enriching the stars in the Omega Centauri system. Instead, its metallicity is dominated by the injection of elements produced by Type II supernovae early in the system's history, and "pollution" caused by intermediate mass stars in the asymptotic giant branch (AGB) phase, where certain elements found preferentially in Omega Centauri can form, and then are ejected because AGB stars are unstable.