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

摘要

Calculations of the energetics of terrestrial accretion indicate that the Earth was extensively molten in its early history(1). Examination of early Archaean rocks from West Greenland (3.6 - 3.8 Gyr old) using short-lived Sm-146 - Nd-142 chronometry indicates that an episode of mantle differentiation took place close to the end of accretion (4.46 +/- 0.11 Gyr ago)(2-4). This has produced a chemically depleted mantle with an Sm/Nd ratio higher than the chondritic value. In contrast, application of Lu-176 - Hf-176 systematics to 3.6 - 3.8-Gyr-old zircons from West Greenland indicates derivation from a mantle source with a chondritic Lu/Hf ratio(5-7). Although an early Sm/Nd fractionation could be explained by basaltic crust formation(8), magma ocean crystallization(2) 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 processes(5). 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.