  We combine stellar orbits with the abundances of the heavy, $r$-process
element europium and the light, $\alpha$-element, silicon to separate in-situ
and accreted populations in the Milky Way across all metallicities. At high
orbital energy, the accretion-dominated halo shows elevated values of [Eu/Si],
while at lower energies, where many of the stars were born in-situ, the levels
of [Eu/Si] are lower. These systematically different levels of [Eu/Si] in the
MW and the accreted halo imply that the scatter in [Eu/$\alpha$] within a
single galaxy is smaller than previously thought. At the lowest metallicities,
we find that both accreted and in-situ populations trend down in [Eu/Si],
consistent with enrichment via neutron star mergers. Through compiling a large
dataset of abundances for 46 globular clusters (GCs), we show that differences
in [Eu/Si] extend to populations of in-situ/accreted GCs. We interpret this
consistency as evidence that in $r$-process elements, GCs trace the star
formation history of their hosts, motivating their use as sub-Gyr timers of
galactic evolution. Furthermore, fitting the trends in [Eu/Si] using a simple
galactic chemical evolution model, we find that differences in [Eu/Si] between
accreted and in-situ MW field stars cannot be explained through star formation
efficiency alone. Finally, we show that the use of [Eu/Si] as a chemical tag
between GCs and their host galaxies extends beyond the Local Group, to the halo
of M31 - potentially offering the opportunity to do Galactic Archaeology in an
external galaxy.
