  We present the elemental abundances and ages of 19 massive quiescent galaxies
at $z\sim1.4$ and $z\sim2.1$ from the Keck Heavy Metal Survey. The ultra-deep
LRIS and MOSFIRE spectra were modeled using a full-spectrum stellar population
fitting code with variable abundance patterns. The galaxies have iron
abundances between [Fe/H] = -0.5 and -0.1 dex, with typical values of $-0.2$
[$-0.3$] at $z\sim1.4$ [$z\sim2.1$]. We also find a tentative
$\log\sigma_v$-[Fe/H] relation at $z\sim1.4$. The magnesium-to-iron ratios span
[Mg/Fe] = 0.1--0.6 dex, with typical values of $0.3$ [$0.5$] dex at $z\sim1.4$
[$z\sim2.1$]. The ages imply formation redshifts of $z_{\rm form}=2-8$.
Compared to quiescent galaxies at lower redshifts, we find [Fe/H] was $\sim0.2$
dex lower at $z=1.4-2.1$. We find no evolution in [Mg/Fe] out to $z\sim1.4$,
though the $z\sim2.1$ galaxies are $0.2$ dex enhanced compared to $z=0-0.7$. A
comparison of these results to a chemical evolution model indicates that
galaxies at higher redshift form at progressively earlier epochs and over
shorter star-formation timescales, with the $z\sim2.1$ galaxies forming the
bulk of their stars over 150 Myr at $z_{\rm form}\sim4$. This evolution cannot
be solely attributed to an increased number of quiescent galaxies at later
times; several Heavy Metal galaxies have extreme chemical properties not found
in massive galaxies at $z\sim0.0-0.7$. Thus, the chemical properties of
individual galaxies must evolve over time. Minor mergers also cannot fully
account for this evolution as they cannot increase [Fe/H], particularly in
galaxy centers. Consequently, the build-up of massive quiescent galaxies since
$z\sim2.1$ may require further mechanisms such as major mergers and/or central
star formation.
