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Oxygen, magnesium, and silicon isotopic abundances in Vigarano 1623‐5 were studied using secondary ion mass spectrometry to investigate correlations between isotopic and petrologic properties of this unique forsterite‐bearing FUN inclusion. Vigarano 1623‐5 displays large, correlated mass‐dependent fractionation effects, tightly linked to mineralogy within distinct petrologic units of the inclusion. The pyroxene‐rich and melilite‐rich interior parts of the inclusion display highly mass‐fractionated isotopic compositions of oxygen, magnesium, and silicon, consistent with Rayleigh distillation during evaporation of a melt with initial oxygen composition close to a solar composition. However, the chemical composition, enriched in magnesium and silicon, suggests a precursor already fractionated by prior melt evaporation. A discontinuous igneous rim was produced by a flash‐melting event followed by isotopic exchange in the rim melilite with planetary‐like oxygen, mechanical fragmentation, and reassembly with an accretionary rim of heterogeneous materials. Al‐rich minerals in 1623‐5 show evidence for having crystallized with live 26Al but at less than the "canonical" level of most CV calcium‐aluminum‐rich inclusions. However, well‐defined 26Al‐26Mg isochrons are not found and temporal implications are ambiguous. [ABSTRACT FROM AUTHOR]

Magnesium and silicon isotopic profiles across melilite grains in two type B1 and two type B2 calcium-aluminum-rich inclusions (CAIs) reveal differing but constant enrichments in heavy isotopes everywhere except ≤1000 μm from the CAI margins. There is no close correlation in the B1s or the B2s between isotopic composition and åkermanite content of the melilite, a measure of progressive igneous crystallization, yet such a correlation might be expected in a type B2: without a melilite mantle (as in B1s) to seal the interior off and prevent further evaporation, the melt would have maintained communication with the external gas. These observations indicate a model in which B1s and B2s solidified under differing conditions. The B2s solidified under lower hydrogen pressures ( [ABSTRACT FROM AUTHOR]

- Evaporation rates of K2O, Na2O, and FeO from chondrule-like liquids and the associated potassium isotopic fractionation of the evaporation residues were measured to help understand the processes and conditions that affected the chemical and isotopic compositions of olivine-rich type IA and type IIA chondrules from Semarkona. Both types of chondrules show evidence of having been significantly or totally molten. However, these chondrules do not have large or systematic potassium isotopic fractionation of the sort found in the laboratory evaporation experiments. The experimental results reported here provide new data regarding the evaporation kinetics of sodium and potassium from a chondrule-like melt and the potassium isotopic fractionation of evaporation residues run under various conditions ranging from high vacuum to pressures of one bar of H2+CO2, or H2, or helium. The lack of systematic isotopic fractionation of potassium in the type IIA and type IA chondrules compared with what is found in the vacuum and one-bar evaporation residues is interpreted as indicating that they evolved in a partially closed system where the residence time of the surrounding gas was sufficiently long for it to have become saturated in the evaporating species and for isotopic equilibration between the gas and the melt. A diffusion couple experiment juxtaposing chondrule-like melts with different potassium concentrations showed that the diffusivity of potassium is sufficiently fast at liquidus temperatures (DK > 2 × 10−4cm2 s−1 at 1650 °C) that diffusion-limited evaporation cannot explain why, despite their having been molten, the type IIA and type IA chondrules show no systematic potassium isotopic fractionation. [ABSTRACT FROM AUTHOR]