Calcium-48 isotopic anomalies in bulk chondrites and achondrites: Evidence for a uniform isotopic reservoir in the inner protoplanetary disk

Abstract

Thermal ionization mass spectrometry (TIMS) was used to measure the calcium isotopic compositions of carbonaceous, ordinary, enstatite chondrites as well as eucrites and aubrites. We find that after correction for mass-fractionation by internal normalization to a fixed ⁴²Ca/⁴⁴Ca ratio, the ⁴³Ca/⁴⁴Ca and ⁴⁶Ca/⁴⁴Ca ratios are indistinguishable from terrestrial ratios. In contrast, the ⁴⁸Ca/⁴⁴Ca ratios show significant departure from the terrestrial composition (from -2 ε in eucrites to +4 ε in CO and CV chondrites). Isotopic anomalies in ε⁴⁸Ca correlate with ε⁵⁰Ti: εCa48=(1.09±0.11)×εTi50+(0.03±0.14). Further work is needed to identify the carrier phase of ⁴⁸Ca-⁵⁰Ti anomalies but we suggest that it could be perovskite and that the stellar site where these anomalies were created was also responsible for the nucleosynthesis of the bulk of the solar system inventory of these nuclides. The Earth has identical ⁴⁸Ca isotopic composition to enstatite chondrites (EH and EL) and aubrites. This adds to a long list of elements that display nucleosynthetic anomalies at a bulk planetary scale but show identical or very similar isotopic compositions between enstatite chondrites, aubrites, and Earth. This suggests that the inner protoplanetary disk was characterized by a uniform isotopic composition (IDUR for Inner Disk Uniform Reservoir), sampled by enstatite chondrites and aubrites, from which the Earth drew most of its constituents. The terrestrial isotopic composition for ¹⁷O, ⁴⁸Ca, ⁵⁰Ti, ⁶²Ni, and ⁹²Mo is well reproduced by a mixture of 91% enstatite, 7% ordinary, and 2% carbonaceous chondrites. The Earth was not simply made of enstatite chondrites but it formed from the same original material that was later modified by nebular and disk processes. The Moon-forming impactor probably came from the same region as the other embryos that made the Earth, explaining the strong isotopic similarity between lunar and terrestrial rocks.