Context : A fundamental element of galaxy formation is the accretion of mass through mergers of satellites or gas .
Recent dynamical analysis based on Gaia data have revealed major accretion events in Milky Way ’ s history .
Nevertheless , our understanding of the primordial Galaxy is hindered because the bona fide identification of the most metal-poor and correspondently oldest accreted stars remains challenging .

Aims : Galactic Archaeology needs a new accretion diagnostic to understand primordial stellar populations .
Contrary to \alpha -elements , neutron-capture elements present unexplained large abundance spreads for low metallicity stars , that could result from a mixture of formation sites .

Methods : We have analysed the abundances of yttrium , europium , magnesium and iron in Milky Way satellite galaxies , field halo stars and globular clusters .
The chemical information has been complemented with orbital parameters based on Gaia data .
In particular , the orbit ’ s average inclination has been considered .

Results : The [ Y/Eu ] abundance behaviour with respect to the [ Mg/Fe ] turnovers for satellite galaxies of different masses reveals that higher luminosity systems , for which the [ Mg/Fe ] abundance declines at higher metallicities , present enhanced [ Y/Eu ] abundances , particularly in the [ Fe/H ] regime between -2.25 dex and -1.25 dex .
In addition , the analysis has uncovered a chemo-dynamical correlation for both globular clusters and field stars of the Galactic halo , accounting for about half of the [ Y/Eu ] abundance spread .
In particular , [ Y/Eu ] under-abundances typical of protracted chemical evolutions , are preferentially observed in polar-like orbits , pointing to a possible anisotropy in the accretion processes .

Conclusions : Our results strongly suggest that the observed [ Y/Eu ] abundance spread in the Milky Way halo could result from a mixture of systems with different masses .
They also highlight that both nature and nurture are relevant to the Milky Way ’ s formation , since its primordial epochs , opening new pathways for chemical diagnostics of our Galaxy building up .