In situ oxygen, magnesium, and silicon isotopic compositions of the FUN inclusion Vigarano 1623-5

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 Al-26 but at less than the canonical level of most CV calcium-aluminum-rich inclusions. However, well-defined Al-26-Mg-26 isochrons are not found and temporal implications are ambiguous.

Automated summary

Oxygen, magnesium, and silicon isotopic abundances in Vigarano 1623-5 were studied to investigate correlations with petrology. The inclusion displayed mass-dependent fractionation effects, consistent with Rayleigh distillation during melt evaporation. However, the enriched chemical composition suggests prior fractionation. A discontinuous igneous rim and isotopic exchange events were observed, indicating mechanical fragmentation and reassembly. Evidence of live Al-26 was found in Al-rich minerals, but its implications are unclear.