Trace element distribution in coarse-grained Ca-Al-rich inclusions by femtosecond LA-ICP-MS: implications for U-Pb chronologyOPEN ACCESS
Merel E. Onderwater, Elishevah M.M.E. van Kooten, Glenn J. MacPherson, Alexander N. Krot, Martin Bizzarro
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 24 October 2026
“Calcium-aluminium-rich inclusions (CAIs) in chondritic meteorites are the first solids that formed in the Solar System, as established by U-Pb chronology. After initial condensation, coarse-grained CAIs (Compact Type A, Type B, Type C) 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. Further, the decay of short-lived 247Cm to 235U (t1/2 = 15.6 Myr) complicates U-Pb age determination of bulk CAIs, but its impact at the mineral scale remains elusive.
Here, we present major- and trace-element distribution maps of six coarse-grained CAIs mainly composed of melilite, Al,Ti-pyroxene, anorthite and spinel (±forsterite) 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 and provide new constraints on the interpretation and robustness of high-precision U-Pb chronology of the earliest Solar System solids.”
