The infiltration of fluids into continental lithospheric mantle is a key mechanism for controlling abrupt changes in the chemical and physical properties of the lithospheric root(1,2), as well as diamond formation(3), yet the origin and composition of the fluids involved are still poorly constrained. Such fluids are trapped within diamonds when they form(4-7) and so diamonds provide a unique means of directly characterizing the fluids that percolate through the deep continental lithospheric mantle. Here we show a clear chemical evolutionary trend, identifying saline fluids as parental to silicic and carbonatitic deep mantle melts, in diamonds from the Northwest Territories, Canada. Fluid-rock interaction along with in situ melting cause compositional transitions, as the saline fluids traverse mixed peridotite-eclogite lithosphere. Moreover, the chemistry of the parental saline fluids-especially their strontium isotopic compositions-and the timing of host diamond formation suggest that a subducting Mesozoic plate under western North America is the source of the fluids. Our results imply a strong association between subduction, mantle metasomatism and fluid-rich diamond formation, emphasizing the importance of subduction-derived fluids in affecting the composition of the deep lithospheric mantle.