  We calculate the stellar mass-metallicity relation at five epochs ranging to
z~2.3. We quantify evolution in the shape of the mass-metallicity relation as a
function of redshift; the mass-metallicity relation flattens at late times.
There is an empirical upper limit to the gas-phase oxygen abundance in
star-forming galaxies that is independent of redshift. From examination of the
mass-metallicity relation and its observed scatter we show that the flattening
at late times is a consequence of evolution in the stellar mass where galaxies
enrich to this empirical upper metallicity limit; there is also evolution in
the fraction of galaxies at a fixed stellar mass that enrich to this limit. The
stellar mass where metallicities begin to saturate is ~0.7 dex smaller in the
local universe than it is at z~0.8.
