<p>The isotopic imbalance between the accessible silicate Earth and chondritic meteorites implies a missing reservoir of unradiogenic lead. While prior models invoked sequestration in the core, the mechanisms capable of isolating lead from its radiogenic parents within the silicate Earth remain uncertain. Here we show, using first-principles structural prediction and melting-point calculations, that high-pressure Pb-S phases could stably host unradiogenic Pb deep within the mantle. PbS crystallises early and remains solid to core-mantle boundary conditions, potentially forming a long-lived reservoir isolated from U and Th. Under sulfur-rich conditions, PbS may react to form polysulfide phases, PbS₂ and PbS₃, whose contrasting melting behaviours could promote both storage and limited upward transport of Pb. These findings provide a plausible physical mechanism for the preservation and episodic release of ancient lead, which could account for unradiogenic signatures in the upper mantle. The results suggest a link between mantle redox and sulfur cycling to the isotopic evolution of the silicate Earth.</p>