Late accretion, early mantle differentiation, and core-mantle interaction are processes that could have created subtle W-182 isotopic heterogeneities within Earth's mantle. Tungsten isotopic data for Kostomuksha komatiites dated at 2.8 billion years ago show a well-resolved W-182 excess relative to modern terrestrial samples, whereas data for Komati komatiites dated at 3.5 billion years ago show no such excess. Combined W-182, Os-186,Os-187, and Nd-142,Nd-143 isotopic data indicate that the mantle source of the Kostomuksha komatiites included material from a primordial reservoir that represents either a deep mantle region that underwent metal-silicate equilibration or a product of large-scale magmatic differentiation of the mantle. The preservation, until at least 2.8 billion years ago, of this reservoir-which likely formed within the first 30 million years of solar system history-indicates that the mantle may have never been well mixed.