Recent Nd-142 isotope data indicate that the silicate Earth (its crust plus the mantle) has a samarium to neodymium elemental ratio (Sm/Nd) that is greater than that of the supposed chondritic building blocks of the planet. This elevated Sm/Nd has been ascribed either to a 'hidden' reservoir in the Earth(1,2) or to loss of an early-formed terrestrial crust by impact ablation(3). Since removal of crust by ablation would also remove the heatproducing elements potassium, uranium and thorium such removal would make it extremely difficult to balance terrestrial heat production with the observed heat flow(3). In the 'hidden' reservoir alternative, a complementary low-Sm/Nd layer is usually considered to reside unobserved in the silicate lower mantle. We have previously shown, however, that the core is a likely reservoir for some lithophile elements such as niobium(4). We therefore address the question of whether core formation could have fractionated Nd from Sm and also acted as a sink for heat-producing elements. We show here that addition of a reduced Mercury-like body (or, alternatively, an enstatitechondrite-like body) rich in sulfur to the early Earth would generate a superchondritic Sm/Nd in the mantle and an Nd-142/Nd-144 anomaly of approximately +14 parts per million relative to chondrite. In addition, the sulfur-rich core would partition uranium strongly and thorium slightly, supplying a substantial part of the 'missing' heat source for the geodynamo.