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Binary mergers (NSMs) of double neutron star (and black hole-neutron star)systems are suggested to be major sites of r-process elements in the Galaxy byrecent hydrodynamical and nucleosynthesis studies. It has been pointed out,however, that the estimated long lifetimes of neutron star binaries are inconflict with the presence of r-process-enhanced halo stars at metallicities aslow as [Fe/H] ~ -3. To resolve this problem, we examine the role of NSMs in theearly Galactic chemical evolution on the assumption that the Galactic halo wasformed from merging sub-halos. We present simple models for the chemicalevolution of sub-halos with total final stellar masses between 10^4 M_solar and2 x 10^8 M_solar. Typical lifetimes of compact binaries are assumed to be 100Myr (for 95% of their population) and 1 Myr (for 5%), according to recentbinary population synthesis studies. The resulting metallcities of sub-halosand their ensemble are consistent with the observed mass-metallicity relationof dwarf galaxies in the Local Group, and the metallicity distribution of theGalactic halo, respectively. We find that the r-process abundance ratios [r/Fe]start increasing at [Fe/H] <= -3 if the star formation efficiencies are smallerfor less massive sub-halos. In addition, the sub-solar [r/Fe] values (observedas [Ba/Fe] ~ -1.5 for [Fe/H] < -3) are explained by the contribution from theshort-lived (~1 Myr) binaries. Our results indicate that NSMs may have asubstantial contribution to the r-process element abundances throughout theGalactic history.

Neutron Star Mergers as the Origin of r-Process Elements in the Galactic Halo Based on the Sub-halo Clustering Scenario