1996PASP..108..900C

We review the literature to derive estimates for the abundances of the four "alpha" elements' abundances relative to iron [O/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe], as a function of [Fe/H] and age, in the globular clusters. Magnesium is under-studied, and is depleted in some metal-poor cluster giants, and hence is not useful for study. While [O/Fe] values within clusters vary, due to mixing of ON cycle material into their photospheres, consideration of only those stars with low sodium abundances, [Na/Fe] ≤ 0.00, shows that the unmixed stars in clusters have uniformly high [O/Fe] values, from [Fe/H] = -2.24 to -0.58. The "mean alpha" values from two of the other three elements, silicon and titanium, also do not appear to vary. [Ca/Fe] does appear to drop as [Fe/H] increases or age decreases, but this may indicate a problem with the use of neutral calcium lines. Thus, there does not appear to be any sign of Type Ia supernovae contributions to the globular clusters, despite wide ranges in [Fe/H] and age. If the three globular cluster classes, "old halo", "young halo", and "disk", are related to one another, if our relative cluster ages are correct, and if the Type Ia supernovae model is the correct explanation of the declining [alpha/Fe] ratios observed among field stars, then its timescale must be _much longer than 10^9 years. If the timescale is that short, then at a minimum, it is clear that the "old halo" and "disk" globular clusters do not share a common chemical history, despite both showing prograde Galactic rotation. In that case, one of the classes presumably formed farther from the Galaxy and was later accreted by it.