Early Earth differentiation processes investigated through the short-lived 146Sm-142Nd and 182Hf-182W isotope systems

The earliest history of Earth is difficult to capture due to the scarcity and high degrees of alteration of ancient terrains. Nevertheless, short-lived isotope systems can provide important constraints on early geological processes. The 182Hf-182W (t1/2 = 8.9 Ma) and 146Sm-142Nd (t1/2=103 Ma) systems are variably sensitive to differentiation processes that occurred during the first 50 Ma and 500 Ma of Earth's history, respectively. Eoarchean mantle-derived rocks from the Isua supracrustal belt (southwest Greenland) show well-resolved anomalies in both 182W (+5 to +21 ppm) and 142Nd (-10 to +15 ppm) compared to terrestrial standards. While there is evidence that W was mobilized in the crust accessed by the Isua suite, W and Nd isotopic anomalies are interpreted to primarily reflect Hadean processes that affected the mantle precursors of these rocks. Variations in 142Nd do not correlate with those of 182W, however, suggesting different mechanisms for the origin and retention of the isotopic anomalies present in these two systems. The 142Nd data, combined with 143Nd data for the long-lived 147Sm-143Nd system, are interpreted to reflect silicate crystal-liquid fractionation 4.3 Ga ago. Variations of 182W in the Isua lavas mantle source could also have been produced as a result of Hf/W fractionation caused by silicate differentiation during the lifetime of 182Hf, but these processes would have to somehow be decoupled from the processes that affected Sm/Nd. Alternatively, 182W variability could be the result of combined early Hf/W fractionations caused by metal-silicate segregation within discrete mantle domains and late accretion processes. Variations in 182W and 142Nd observed in the Isua supracrustal rocks are similar to those observed in the ancient rocks from Nuvvuagittuq Greenstone Belt, the Acasta Gneiss Complex, and the Saglek Block, suggesting that similar processes affected diverse early Earth rocks.