Spectroscopic Confirmation of a Protocluster at $z=3.37$ with a High
  Fraction of Quiescent Galaxies
  We report the discovery of MAGAZ3NE J095924+022537, a
spectroscopically-confirmed protocluster at $z = 3.3665^{+0.0009}_{-0.0012}$
around a spectroscopically-confirmed $UVJ$-quiescent ultra-massive galaxy (UMG;
$M_{\star}=2.34^{+0.23}_{-0.34}\times10^{11} {\rm M}_\odot$) in the COSMOS
UltraVISTA field. We present a total of 38 protocluster members (14
spectroscopic and 24 photometric), including the UMG. Notably, and in marked
contrast to protoclusters previously reported at this epoch which have been
found to contain predominantly star-forming members, we measure an elevated
fraction of quiescent galaxies relative to the coeval field
($73.3^{+26.7}_{-16.9}\%$ versus $11.6^{+7.1}_{-4.9}\%$ for galaxies with
stellar mass $M_{\star} \geq 10^{11} {\rm M}_\odot$). This high quenched
fraction provides a striking and important counterexample to the seeming
ubiquitousness of star-forming galaxies in protoclusters at $z>2$ and suggests,
rather, that protoclusters exist in a diversity of evolutionary states in the
early Universe. We discuss the possibility that we might be observing either
"early mass quenching" or non-classical "environmental quenching." We also
present the discovery of MAGAZ3NE J100028+023349, a second
spectroscopically-confirmed protocluster, at a very similar redshift of $z =
3.3801^{+0.0213}_{-0.0281}$. We present a total of 20 protocluster members, 12
of which are photometric and 8 spectroscopic including a post-starburst UMG
($M_{\star}=2.95^{+0.21}_{-0.20}\times10^{11} {\rm M}_\odot$). Protoclusters
MAGAZ3NE J0959 and MAGAZ3NE J1000 are separated by 18 arcminutes on the sky (35
comoving Mpc), in good agreement with predictions from simulations for the size
of "Coma"-type cluster progenitors at this epoch. It is highly likely that the
two UMGs are the progenitors of Brightest Cluster Galaxies (BCGs) seen in
massive virialized clusters at lower redshift.
