Abstract:
Relative to chondrites, Earth is poor in volatile elements. This depletion, however, is not accompanied by any stable isotope fractionation that would otherwise be expected during accretion. Thus, the physical mechanisms by which Earth accreted are yet to be reconciled with its chemical and isotopic composition. Here, we link N-body simulations with models for the compositional evolution of Earth. We show that the smooth, concave-down pattern of terrestrial volatile depletion reflects stochastic accretion of numerous, partially-vaporised precursor bodies whose elemental abundances were set by the temperature at the heliocentric distances at which they formed. All N-body simulations reproduce the terrestrial volatile depletion pattern when the disk temperature is set by an accretion rate of (1.08±0.17)×10-7 solar masses/yr, implying planetesimal formation within ~1 Myr, in accord with terrestrial Mn-Cr and Rb-Sr depletion ages. Subsequent impact events engender vaporisation, but atmospheric loss is only efficient during the early stages of accretion when it can readily escape the gravity of the growing Earth. Chemical- and isotopic signatures of evaporation are readily overprinted by that of volatile-rich, late-accreting material, during which time the proto-Earth is sufficiently large so as to limit atmospheric loss. Moderately- and especially highly volatile elements retain near-chondritic abundance ratios and isotope compositions, consistent with present-day observations.
Speaker: Dr. Paolo Sossi, Institute of Geochemistry and Petrology, ETH Zürich.
Speaker bio:
Paolo's interests lie in unravelling the high temperature characteristics (structure, oxidation, geochemistry) of melts, minerals and vapours. This is achieved by a combination of thermodynamic and petrologic modelling, spectroscopy, experimental petrology and stable isotope geochemistry. The overarching theme is to understand the variables that control planetary formation, differentiation, and melting and how the Earth differs from other terrestrial planets and meteorites.
Host: Shawn McGlynn, ELSI.
Date: Fri, 26 February, 2021, 15:30-16:30 JST (Fri, 26 February, 06:30-07:30 UTC)
Venue: Online