This review describes the stable isotope character of the Earth's mantle, the 2900 km thick silicate layer between the core and crust that comprises 67 % of the Earth's mass and 84% of the Earth's total volume, and summarizes the stable isotope evidence for its heterogeneity. The introduction and first section describe the petrological and geochemical character of the mantle, the geochemical processes operating therein, types and significance available mantle samples (lithospheric mantle fragments and mantle-derived volcanic rocks). The second section discusses insights about the geochemical character of the mantle provided by variations in the isotopic composition of the five traditional volatile elements (H, C, O, N and S) which is measured by IR-MS techniques. As a consequence of significant relative mass differences, the isotopes of H, C, O, N and S may experience larger fractionations than for heavier elements. Kinetic fractionations and gas/solid exchange reactions may play an important role in determining the isotope composition of solids in the presence of gaseous species (H2, CO2, CH4, N2, SO2, H2S), thereby enlarging isotope fractionations and causing heterogeneity in the mantle, documented for instance in the variable C and N isotope composition of diamonds. The next section discusses stable isotopes another 22 elements (Li, B, Mg, Si, Cl, K, Ca, Ti, V, Cr, Fe, Ni, Cu, Zn, Se, Zr, Mo, Ba, W, Hg, Tl and U). It was not until the introduction of the MC-ICP-MS technique that the very small variations for these non-traditional isotope systems in mantle materials could be precisely determined for most of these elements. The existing database acquired largely over the past 20 years clearly demonstrates that that mantle isotope heterogeneity is a product of the partial melting, metasomatism, magmatic differentiation and crustal recycling processes operating individually or in tandem. Respective contributions from the different fractionation and mixing processes vary from element to element, for instance, non-equilibrium diffusion-driven isotope fractionation may play an important role for elements like Li and K. Of special importance for discerning mantle heterogeneity are the fluid-mobile trace elements B, Cl, K, Ba, Mo and Tl which trace surface materials subducted from the Earth's surface into the mantle. Mantle metasomatism including the formation of new minerals (silicates, sulfides, carbonates) contributes to isotopic heterogeneity for mobile elements. For well-studied elements, stable isotope compositions of the Bulk Silicate Earth (BSE) and major mantle components (reservoirs) are constrained.
