Comparison Mg, Al, Si, and Ca abundances for the same stars as functions of effective temperature for three studies (this work, first column; uncalibrated APOGEE DR14, second column; and Mészáros et al. 2015, third column). Blue triangles indicate upper limits. To clear up the diagrams with intrinsic clusters abundances variations, only stars of the first population ([Al/Fe] < 0.2) are shown. For clarity sake, the results for two more metal-rich clusters M 71 and M 107 have been disregarded in this figure.