  The chemical abundance patterns of gas and stars in galaxies are powerful
probes of galaxies' star formation histories and the astrophysics of galaxy
assembly but are challenging to measure with confidence in distant galaxies. In
this paper, we report the first measurements of the correlation between stellar
mass and multiple tracers of chemical enrichment (including O, N, and Fe) in
individual z~2-3 galaxies, using a sample of 195 star-forming galaxies from the
Keck Baryonic Structure Survey (KBSS). The galaxies' chemical abundances are
inferred using photoionization models capable of reconciling high-redshift
galaxies' observed extreme rest-UV and rest-optical spectroscopic properties.
We find that the stellar mass-O/H relation for our sample is relatively
shallow, with moderately large scatter, and is offset ~0.35 dex higher than the
corresponding stellar mass-Fe/H relation. The two relations have very similar
slopes, indicating a high level of alpha-enhancement -- with O/Fe approximately
2.2 times higher than solar O/Fe -- across two decades in stellar mass. The
stellar mass-N/H relation has the steepest slope and largest intrinsic scatter,
which likely results from the fact that many z~2 galaxies are observed near or
past the transition from "primary" to "secondary" N production and may reflect
uncertainties in the astrophysical origin of N. Together, these results suggest
that z~2 galaxies are old enough to have seen substantial enrichment from
intermediate mass stars, but are still young enough that Type Ia supernovae
have not had time to contribute significantly to their enrichment.
