Schematic illustration of the expected characteristics of the star-forming [O/Fe]–sSFR relation of galaxies. Young, high-redshift main sequence galaxies (with high sSFR) occupy the upper right part of the relation. Their chemical enrichment is dominated by prompt sources (CCSNe), which are expected to eject material with supersolar oxygen-to-iron abundance ratios. Below a certain sSFR (when the characteristic timescale τ_SF = 1/sSFR becomes comparable to the minimum SN Ia delay), SNe Ia begin to contribute significantly to the iron enrichment. As a result, old, low-redshift main sequence galaxies occupy the lower left part of the relation, with [O/Fe] approaching the solar ratio (orange dashed line). The exact shape of the relation depends on a number of uncertain factors (e.g. relative oxygen and iron yields, and rates and delays of different supernovae).