An exploration of whether Earth can be built from chondritic components, not bulk chondrites
Conel M. O’D. Alexander
Geochimica et Cosmochimica Acta
Available online 17 December 2021
“Here, two of a range of possible models are explored that assume that: (i) two of the main chondritic components (chondrules and refractory inclusions) dominated the Earth’s building blocks, (ii) that their relative abundances differed from those of known chondrites, and (iii) that the elemental compositions of all components, as well as the isotopic compositions of refractory inclusions, resembled those of the components in carbonaceous chondrites.
In terms of the elemental abundances, the chondrules can explain the moderately volatile element fractionations in the bulk silicate Earth (BSE), except for a few elements like F and In, while the refractory inclusions explain the BSE’s refractory lithophile element enrichment. Accretion of a CM- or EL-like late veneer reproduces the S, Se, Te and highly siderophile element abundances in the BSE. The accretion of CI- or CM-like material prior to the late veneer, along with small amounts of cometary and implanted solar wind material, can explain the BSE elemental and isotopic abundances of the noble gases as well as the elemental abundances of most other highly volatile elements (e.g., H, C and N) provided that variable fractions of them can be sequestered into the core or into hidden mantle reservoirs. This CI- or CM-like material provides an upper limit for the contribution to the BSE from matrix that is much less than in any chondrites. In the models, Si, O and S are the dominant light elements in the core, in that order, and their abundances are constrained to be consistent with the first principles calculations of Umemoto and Hirose (2020). The core compositions of the models are also consistent with most geochemical constraints.
Satisfying all isotopic constraints is a challenge. The BSE Os isotopes are consistent with an EL dominated late veneer, but Ru isotopic evidence is best explained by the addition of CM-like material in the late veneer. Either CI- or CM-like material, in combination with small amounts of cometary and implanted solar wind material, can reproduce the BSE Ne, Ar and Kr isotopic compositions. CM-like, but not CI-like, material can roughly reproduce the BSE’s H and C isotopic compositions, but neither material can explain the BSE N isotopic composition. The BSE composition requires that the O isotopic compositions of the refractory inclusions (initially Δ17O≤-20 ‰) in Earth’s building blocks were reset in the nebula by interaction with high Δ 17O H2O, perhaps during chondrule formation. To plot on the inner Solar System ε54Cr vs. ε 50Ti (or ε 48Ca) trend, either the average Cr content of the chondrules was roughly half of that assumed here or most of the CI/CM-like and refractory-inclusion-rich materials were accreted late.”