The coupled 182W-142Nd record of early terrestrial mantle differentiation

New Sm-Nd, Lu-Hf, Hf-W, and Re-Os isotope data, in combination with highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, and Pd) and W abundances, are reported for the 3.55 Ga Schapenburg komatiites, South Africa. The Schapenburg komatiites define a Re-Os isochron with an age of 355087 Ma and initial Os-187=+3.70.2 (2SD). The absolute HSE abundances in the mantle source of the Schapenburg komatiite system are estimated to be only 295% of those in the present-day bulk silicate Earth (BSE). The komatiites were derived from mantle enriched in the decay products of the long-lived Sm-147 and Lu-176 nuclides (initial Nd-143=+2.40.1, Hf-176=+5.7 +/- 0.3, 2SD). By contrast, the komatiites are depleted, relative to the modern mantle, in Nd-142 and W-182 (W-182=-8.4 +/- 4.5, Nd-142=-4.9 +/- 2.8, 2SD). These results constitute the first observation in terrestrial rocks of coupled depletions in Nd-142 and W-182. Such isotopic depletions require derivation of the komatiites from a mantle domain that formed within the first approximate to 30 Ma of Solar System history and was initially geochemically enriched in highly incompatible trace elements as a result of crystal-liquid fractionation in an early magma ocean. This mantle domain further must have experienced subsequent melt depletion, after Hf-182 had gone extinct, to account for the observed initial excesses in Nd-143 and Hf-176. The survival of early-formed W-182 and Nd-142 anomalies in the mantle until at least 3.55 Ga indicates that the products of early planetary differentiation survived both later planetary accretion and convective mantle mixing during the Hadean. This work moreover renders unlikely that variable late accretion, by itself, can account for all of the observed W isotope variations in Archean rocks.