We present the first comprehensive abundance study of the massive , outer halo globular cluster ( GC ) M 75 ( NGC 6864 ) .
This unique system shows a very extended trimodal horizontal branch ( HB ) , but no other clues for multiple populations have been detected in its colour-magnitude diagram ( CMD ) .
Based on high-resolution spectroscopic observations of 16 red giant stars , we derived the abundances of a large variety of \alpha , p-capture , iron-peak , and n-capture elements .
We found that the cluster is metal-rich ( [ Fe/H ] = -1.16 \pm 0.02 dex , [ \alpha /Fe ] = +0.30 \pm 0.02 dex ) , and shows a marginal spread in [ Fe/H ] of 0.07 dex , typical of most GCs of similar luminosity .
We detected significant variations of O , Na , and Al among our sample , suggesting three different populations .
Additionally , the two most Na-rich stars are also significantly Ba-enhanced , indicating a fourth population of stars .
Curiously , most stars in M 75 ( excluding the two Ba-rich stars ) show a predominant r-process enrichment pattern , which is unusual at the cluster ’ s high metallicity .
We compare the abundance properties of M 75 and NGC 1851 ( a GC very similar to M 75 in terms of age , metallicity , and HB morphology ) and draw conclusions on M 75 ’ s possible formation scenarios .