Differences in elemental abundances between CI chondrites and the solar photosphere

CI chondrites have been a proxy for the solar system since the mid-20th century. The photospheric and CI chondrite abundances (P and CI, respectively) show a strong correlation. CI as a proxy is also justified by the (i) smoothness of their abundances plotted as a function of odd mass number and (ii) agreement within the error of P as determined spectroscopically. But our statistical assessment of spectroscopic studies and solar wind from the Genesis mission indicates that the small, ~10%–30%, differences (residuals) between CI and P depend on the 50% condensation temperature (Tc₅₀). So, if CI is to be used as a proxy for P, Cosmochemists may want to add a correction to individual elements. Our work is consistent with two published hypotheses: that (i) residuals are linear with Tc₅₀ and (ii) that elements having Tc₅₀ > 1343 K are depleted relative to those with 495 K < Tc₅₀ < 1343 K in CI. We discuss other interpretations which are also feasible. Understanding these small differences of the CI and P for different elements and their variation with Tc₅₀ can help constrain future models of solar system formation and the history of CI chondrites.