CLEAR I: Ages and Metallicities of Quiescent Galaxies at $\mathbf{1.0 <
  z < 1.8}$ Derived from Deep Hubble Space Telescope Grism Data
  We use deep \textit{Hubble Space Telescope} spectroscopy to constrain the
metallicities and (\editone{light-weighted}) ages of massive ($\log
M_\ast/M_\odot\gtrsim10$) galaxies selected to have quiescent stellar
populations at $1.0<z<1.8$. The data include 12--orbit depth coverage with the
WFC3/G102 grism covering $\sim$ $8,000<\lambda<11,500$~\AA\, at a spectral
resolution of $R\sim 210$ taken as part of the CANDELS Lyman-$\alpha$ Emission
at Reionization (CLEAR) survey. At $1.0<z<1.8$, the spectra cover important
stellar population features in the rest-frame optical. We simulate a suite of
stellar population models at the grism resolution, fit these to the data for
each galaxy, and derive posterior likelihood distributions for metallicity and
age. We stack the posteriors for subgroups of galaxies in different redshift
ranges that include different combinations of stellar absorption features. Our
results give \editone{light-weighted ages of $t_{z \sim 1.1}= 3.2\pm 0.7$~Gyr,
$t_{z \sim 1.2}= 2.2\pm 0.6$~Gyr, $t_{z\sim1.3}= 3.1\pm 0.6$~Gyr, and
$t_{z\sim1.6}= 2.0 \pm 0.6$~Gyr, \editone{for galaxies at $z\sim 1.1$, 1.2,
1.3, and 1.6. This} implies that most of the massive quiescent galaxies at
$1<z<1.8$ had formed $>68$\% of their stellar mass by a redshift of $z>2$}. The
posteriors give metallicities of \editone{$Z_{z\sim1.1}=1.16 \pm
0.29$~$Z_\odot$, $Z_{z\sim1.2}=1.05 \pm 0.34$~$Z_\odot$, $Z_{z\sim1.3}=1.00 \pm
0.31$~$Z_\odot$, and $Z_{z\sim1.6}=0.95 \pm 0.39$~$Z_\odot$}. This is evidence
that massive galaxies had enriched rapidly to approximately Solar metallicities
as early as $z\sim3$.
