Trace element distribution in coarse-grained Ca-Al-rich inclusions by femtosecond LA-ICP-MS: Implications for U-Pb chronology
arXiv:2611.12914 · doi:10.1016/j.gca.2026.10.033
Abstract
Calcium-aluminium-rich inclusions (CAIs) in chondrites are the first solids that formed in the Solar System, as established by U-Pb chronology. After initial condensation, coarse-grained CAIs experienced melting, resulting in igneous partitioning and spatial redistribution of trace elements among constituent minerals. Understanding these distributions is essential for identifying trace-element host phases and interpreting U-Pb ages. Rare earth elements (REEs) and the actinides Th and U are particularly informative tracers, yet actinide behavior in coarse-grained CAIs remains poorly constrained. Here, we present major- and trace-element distribution maps of six coarse-grained CAIs from CV3 (Vigarano-type) chondrites acquired by femtosecond laser ablation ICP-MS. In CAIs lacking perovskite or hibonite, our results show that Al,Ti-pyroxene is the dominant host of U, Th, and most REEs, with strong enrichments observed toward pyroxene grain edges, whereas light REEs reside in melilite. Trace-element zonation reflects igneous fractional crystallization and is overprinted by melt evaporation, which caused depletion of the most volatile REEs (Yb, Eu, Ce) and U in CAI outer regions and rims. Using Nd and Th distribution maps as proxies for Cm, we model the effects of 247Cm-235U decay on mineral-scale U isotopic compositions. Variable Cm-U partitioning among coexisting minerals is predicted to generate resolvable inter-mineral 235U/238U variations, with the largest excesses in melilite. These variations can produce apparent U-Pb age offsets of ~0.1-1.0 Myr when U isotopic compositions and radiogenic Pb are derived from isotopically distinct mineral reservoirs, but generally remain small (< 0.15 Myr) when using bulk U isotopic compositions. Our results demonstrate that fractional crystallization and melt evaporation exert a primary control on U distribution and isotope systematics in igneous CAIs.
arXiv commentAccepted for publication in Geochimica et Cosmochimica Acta