Nucleosynthetic isotope variations in chondritic meteorites and their relationship to bulk chemistry

Abstract

AbstractThe relationship of mass‐independent stable isotope anomalies with the chemistry of chondritic meteorites provides constraints on mixing and fractionation processes in the early solar nebula. The present study emphasizes the strong correlation of nucleosynthetic isotope variations among ordinary chondrites (OC), enstatite chondrites (EC), Earth, CI‐chondrites, and Ca, Al‐rich inclusions (CAI) in ε50Ti versus ε54Cr space. This correlation indicates variable contamination of chondritic reservoirs with material from a single source providing neutron‐rich nuclei such as 50Ti, 54Cr, and 62Ni. The well‐defined linear relationship of ε50Ti versus ε54Cr indicates that all reservoirs on the correlation line (“chondrite reference line”) started with a CI‐chondritic (solar) Cr/Ti ratio, irrespective of the present Cr/Ti ratio of the samples falling on the chondrite reference line. The isotope compositions of carbonaceous chondrites (CC) do not fit the chondrite reference line. Their isotope composition is consistent with a mixture of chondritic meteorites originally falling on the chondrite reference line and volatile element depleted CAIs. However, CC cannot result from addition of CAIs to OC or EC. Neither can OC and EC be produced by loss of refractory components from CI‐meteorites. Also, stable isotopes are inconsistent with OC being derived from EC, and vice versa, by a chemical fractionation process. The enrichment of the Earth in refractory lithophile elements is not the result of addition of a refractory component to a chondritic reservoir. It is rather the result of internal fractionation of a chondritic reservoir.