Characterizing the mantle transition zone (MTZ) is essential for understanding Earth’s evolution and dynamics. Petrological, geochemical, and geophysical evidence suggests that the MTZ may host subducted slabs containing sediments and volatiles, potentially forming enriched mantle 1 (EM1) reservoirs. However, the links between volatiles, subduction components, and mantle endmembers stored therein remain unclear. Here we present mercury isotope data for Cenozoic intraplate EM1-type basalts from Northeast Asia, derived from volatile-rich MTZ. These basalts exhibit negative mass-independent fractionation (Δ¹⁹⁹Hg, Δ²⁰¹Hg), and they also display mass-dependent fractionation (δ²⁰²Hg) that correlates with geochemical indicaters of mantle heterogeneity. The results indicate incorporation of recycled ancient terrigenous sediments with anomalous Hg signatures into the MTZ, later tapped during hydrous upwellings or entrained into ascending mantle plumes to form EM1-type basalts. Our findings demonstrate that distinct Hg-isotope anomalies can persist in the MTZ for over a billion years, recording long-term volatile recycling in Earth’s deep mantle.