Origins of mass-dependent and mass-independent Ca isotope variations in meteoritic components and meteorites

The Ca isotope composition of meteorites and their components may vary due to mass-dependent and/or -independent isotope effects. In order to evaluate the origin of these effects, five amoeboid olivine aggregates (AOAs), three calcium aluminum inclusions (CAIs), five chondrules (C), a dark inclusion from Allende (CV3), two dark fragments from North West Africa 753 (NWA 753; R3.9), and a whole rock sample of Orgueil (CI1) were analyzed. This is the first coupled mass-dependent and -independent Ca isotope dataset to include AOAs, a dark inclusion, and dark fragments. Where sample masses permit, Ca isotope data are reported with corresponding petrographic analyses and rare earth element (REE) relative abundance patterns. The CAIs and AOAs are enriched in light Ca isotopes (delta Ca-44/40 -5.32 to +0.72, where delta Ca-44/40 is reported relative to SRM 915a). Samples CAI 5 and AOA 1 have anomalous Group II REE patterns. These REE and delta Ca-44/40 data suggest that the CAI 5 and AOA 1 compositions were set via kinetic isotope fractionation during condensation and evaporation. The remaining samples show mass-dependent Ca isotope variations which cluster between delta Ca-44/40 +0.53 and +1.59, some of which are coupled with unfractionated REE abundance patterns. These meteoritic components likely formed through the coaccretion of the evaporative residue and condensate following Group II CAI formation or their chemical and isotopic signatures were decoupled (e.g., via nebular or parent-body alteration). The whole rock sample of Orgueil has a delta Ca-44/40 +0.67 +/- 0.18 which is in agreement with most published data. Parent-body alteration, terrestrial alteration, and variable sampling of Ca-rich meteoritic components can have an effect on delta Ca-44/40 compositions in whole rock meteorites. Samples AOA 1, CAI 5, C 2, and C 4 display mass-independent Ca-48/44 anomalies (epsilon Ca-48/44 + 6 to + 12) which are resolved from the standard composition. Other samples measured for these effects (AOA 5, CAI 1, CAI 2, C 3, D 1, D 2, D 3) possess the same Ca-48/44 isotope composition as the standard within analytical uncertainty. These data indicate a heterogeneous distribution of Ca-48 in the early solar nebula during formation of CAIs, AOAs, and chondrules. In a epsilon Ca-48/44 vs. delta Ca-44/40 plot, no strong correlation is evident which suggests that the thermal processing event which caused a heterogeneous distribution of epsilon Ca-48/44 in the solar nebula is unlikely to be directly related to the thermal processing event that caused coupled REE and Ca mass-dependent isotopic fractionation in meteoritic components. (C) 2018 Elsevier Ltd. All rights reserved.