Iron-to-zinc relative abundance, [Fe/Zn], as a function of the logarithmic Zn ii column density. GRB host galaxy measurements are plotted as solid circles, and a sample of QSOs taken from Savaglio (2006) (open circles) are plotted for comparison. The dashed line shows the ordinary least-squares (OLS) bisector fit to the combined GRB and QSO data, which has a power-law index of  − 0.7. GRBs with limits on either Fe ii of Zn ii are plotted in grey with smaller symbols, as well as with up- and leftward pointing arrows, respectively.