We present chemical abundances derived from high-resolution Magellan/MIKE spectra of the nine brightest known red giant members of the ultra-faint dwarf galaxy Reticulum II .
These stars span the full metallicity range of Ret II ( -3.5 < \mbox { [ Fe / H ] } < -2 ) .
Seven of the nine stars have extremely high levels of r -process material ( [ Eu/Fe ] \sim 1.7 ) , in contrast to the extremely low neutron-capture element abundances found in every other ultra-faint dwarf galaxy studied to date .
The other two stars are the most metal-poor stars in the system ( [ Fe/H ] < -3 ) , and they have neutron-capture element abundance limits similar to those in other ultra-faint dwarf galaxies .
We confirm that the relative abundances of Sr , Y , and Zr in these stars are similar to those found in r -process halo stars but \sim 0.5 dex lower than the solar r -process pattern .
If the universal r -process pattern extends to those elements , the stars in Ret II display the least contaminated known r -process pattern .
The abundances of lighter elements up to the iron peak are otherwise similar to abundances of stars in the halo and in other ultra-faint dwarf galaxies .
However , the scatter in abundance ratios is large enough to suggest that inhomogeneous metal mixing is required to explain the chemical evolution of this galaxy .
The presence of low amounts of neutron-capture elements in other ultra-faint dwarf galaxies may imply the existence of additional r -process sites besides the source of r -process elements in Ret II .
Galaxies like Ret II may be the original birth sites of r -process enhanced stars now found in the halo .