3. Radionuclide Produced Isotopic Variations in Mantle Rocks

Evidence from short- and long-lived radioisotope systems indicates that Earth was largely built from volatile-depleted planetesimals and planetary embryos during runaway accretion that occurred within 1–10 Ma of formation of the first solar system solids at 4.567 ± 0.001 Ga. Both long- and short-lived radionuclides leave isotopic signatures in mantle rocks that bear on when and how the silicate Earth formed and differentiated. The longstanding view that the Hadean mantle was compositionally undepleted appeared to be contradicted by differences between mantle and chondrite Nd isotopes which suggested a very early enriched terrestrial reservoir. Although recent work indicates this difference reflects differing irradiation histories of Earth-forming-materials and meteorites, and thus has little bearing on the timing of silicate differentiation, both terrestrial and lunar Lu–Hf zircon data appear to require global silicate differentiation by 4.50 ± 0.02 Ga billion years. Tungsten isotopic data from mantle rocks provides evidence of core formation by about 4.53 Ga and either very early isotopic isolation of silicate reservoirs or disturbance by a late chondritic veneer. Despite evidence that Moon formed substantially from proto-Earth material between about 4.53 and 4.50 Ga, the exact mechanism by which this occurred remains controversial. The once widely accepted model of collision of a Mars-sized body has lost support in light of contradictory evidence in the form of indistinguishable isotopic compositions of volatile and refractory elements between Earth and Moon. Models that appear to transcend this problem (hit-and-run collision, synestia, successive smaller collisions, magma ocean heating, etc.) are currently being evaluated. Although geochemical evidence requiring an early terrestrial magma ocean is almost entirely lacking, the sources of thermal energy available during accretion make such an appearance appear inevitable. If solidification proceeded from the bottom up, vigorous convection would have caused the lower mantle to rapidly crystallize with the upper mantle becoming largely solidified within several million years. The high abundance of highly siderophile elements in the upper mantle is strong evidence that Earth added at least half a percent of its present mass following core formation but prior to the Mesoarchean. Preservation of mantle isotopic anomalies throughout the Hadean-Archean seem unlikely to reflect sluggish mantle convention in a stagnant lid tectonic regime during that period as the plate tectonic era is associated with a large range of isolated mantle isotopic domains.