The late Earth's accretion: Processes and materials

Abstract

In accord with the standard Earth accretion scenario, the late accretion supervened the last collision with a massive proto-planet, segregation of the core, and (partial) solidification of the magma ocean. These processes took place $\approx 40$ Ma after Sun formation or somewhat later. Traces of the processes and respective materials have been preserved as specific elemental and isotopic abundances in the earth's mantle. Three groups of chemical elements, showing rather different behavior, allow the principal processes and materials to be restored: (i) involatile siderophile elements and the $^{182}$Hf–$^{182}$W and $^{190}$Pt–$^{187}$Re–$^{186,187}$Os isotopic systematics highlight the time scale of core formation, the late veneer materials, and post-core-formation interactions between the mantle and the core; (ii) involatile lithophile incompatible elements and the $^{147}$Sm–$^{143}$Nd isotopic systematics indicate the early differentiation of the silicate Earth; $^{176}$Lu–$^{176}$Hf one recorded the early crustal processes; (iii) highly volatile elements, noble gases, and the $^{238}$U–$^{235}$U–$^{232}$Th–He–Ne and $^{244}$Pu–$^{238}$U–$^{129}$I–Xe systematics trace the accreting materials and the rate of mantle mixing and degassing. Recently proposed interpretations of this last systematics appear to be precarious and are particularly discussed in this contribution. During the late accretion a terrestrial regolith, including chondritic and solar-wind-irradiated materials, was rapidly accumulating on the surface of the early thick basaltic crust, enriched in incompatible elements. This early crust had not been preserved. Its overturn(s) into the mantle during several 100th Ma after Sun formation and (partial) isolation from the mantle convection allow all principal observations, related to the informative systematics mentioned above, to be satisfied, providing the transfer of the crust&regolith "cake" was not accompanying by fractionation and degassing, in contrast to present-day slab subduction.

Received 30 October 2017; accepted 12 January 2018; published 4 February 2018.