Mass–metallicity relation for R2248 cluster members (filled circles) and the comparison sample of CLASH and zCOSMOS field galaxies (open circles) using the O3N2 metallicity calibration of Kewley et al. (2013). All cluster members and field galaxies with available measurements of the [OIII], Hβ, [NII], and Hα lines are considered. The magenta diamonds and curves represent the local SDSS relation, consistently derived using the O3N2 calibration, with its 1σ scatter. The red filled and blue open circles show the median (O/H) values of cluster and field galaxies, respectively. The black cross in the upper left corner represents the mean error of the (O/H) and mass derivation. The vertical dashed lines show the three different mass bins, for which the median (O/H) values were computed, with the “mass complete” sample represented by the intermediate bin (9.2 <  log(M/M_⊙) < 10.2). In this mass bin, the median (O/H) for the cluster galaxies is higher than the median (O/H) for the field galaxies by 0.065 dex, with a 1.8σ significance.