Trace-element fractionation in Hadean mantle generated by melt segregation from a magma ocean

Calculations of the energetics of terrestrial accretion indicate that the Earth was extensively molten in its early history1. Examination of early Archaean rocks from West Greenland (3.6–3.8 Gyr old) using short-lived 146Sm–142Nd chronometry indicates that an episode of mantle differentiation took place close to the end of accretion (4.46 ± 0.11 Gyr ago)2,3,4. This has produced a chemically depleted mantle with an Sm/Nd ratio higher than the chondritic value. In contrast, application of 176Lu–176Hf systematics to 3.6–3.8-Gyr-old zircons from West Greenland indicates derivation from a mantle source with a chondritic Lu/Hf ratio5,6,7. Although an early Sm/Nd fractionation could be explained by basaltic crust formation8, magma ocean crystallization2 or formation of continental crust, the absence of coeval Lu/Hf fractionation is in sharp contrast with the well-known covariant behaviour of Sm/Nd and Lu/Hf ratios in crustal formation processes5. Here we show using mineral–melt partitioning data for high-pressure mantle minerals that the observed Nd and Hf signatures could have been produced by segregation of melt from a crystallizing magma ocean at upper-mantle pressures early in Earth's history. This residual melt would have risen buoyantly and ultimately formed the earliest terrestrial protocrust.