Quantitative models for the elemental and isotopic fractionations in the non-carbonaceous chondrites

Conel M. O’D. Alexander

Geochimica et Cosmochimica Acta

In Press, Accepted Manuscript, Available online 20 February 2019


“Two models that for the first time attempt to quantitatively reproduce the bulk elemental and isotopic fractionations amongst the non-carbonaceous chondrites (non-CCs) are explored here. In Model 1, a refractory component (CC-RI) that can explain the refractory element and isotopic variations amongst the carbonaceous chondrites (CCs) (Alexander, 2019) was subtracted from the initial non-CC reservoir. This is to explain why the best-fit line through the non-CC Al/Si/CI vs. Mg/Si/CI trend does not pass through CI. In Model 2, the best-fit to the bulk Al/Si/CI vs. Mg/Si/CI compositions is forced through CI. Otherwise, the two models are very similar and assume: (1) An initial reservoir (NCC-CI) that is CI-like in its elemental composition, except that it is reduced and anhydrous, but is isotopically distinct from CI; (2) an NCC-RI component that is enriched in both refractory and moderately volatile lithophiles, relative to Si; (3) two metal components, M1 that is refractory-siderophile-free and M2 that is refractory-siderophile-rich; (4) elements that are lost due to their volatility; (5) matrix that is compositionally the same as the NCC-CI reservoir; and (6) water that was accreted by the ordinary (OC) and Rumuruti (RC) chondrites, but not the enstatite chondrites (ECs). Least squares fits to the bulk non-CC compositions were used to estimate the relative abundances and elemental/isotopic compositions of the components. Both models are able to reproduce the bulk non-CC compositions to within the assumed uncertainties of the measurements. The bulk compositions of the OCs and RCs can be explained by fractionations of the same components, and should, therefore, be considered as members of the same chondrite class. Slightly different compositions for the EC-RI, metal and volatile components were required to reproduce the ECs. This is not very surprising given the much more reduced nature of the ECs. The compositions of the NCC-RI components do not resemble any recognized type of object in the non-CCs. Volatility played a minor role in shaping the compositions of the non-CCs and was confined primarily to the siderophiles and chalcophiles. The water accreted by the OCs and RCs had a significantly higher Δ17O than that accreted by the CCs (Alexander, 2019) and was mass fractionated in the opposite direction. The non-CI isotopic composition of the NCC-CI reservoir is consistent with the dichotomy that exists between the CCs and almost all other extraterrestrial materials (Warren, 2011). The components that were fractionated to produce the non-CC compositions do not seem to have played a role in producing the bulk composition of the Earth.”