Thermal metamorphic history of a Ca,Al‐rich inclusion constrained by high spatial resolution Mg isotopic measurements with NanoSIMS 50L

Abstract– Mg isotope data were collected by NanoSIMS with high‐precision and high‐spatial resolution from a coarse‐grained type B Ca‐, Al‐rich inclusion (CAI), EK1‐6‐3, in the Allende CV3 chondrite to evaluate the time scale of parent body thermal metamorphism. The CAI melilite and fassaite contain excesses of ²⁶Mg (²⁶Mg*) from the in‐situ decay of ²⁶Al; the inferred initial ratio, (²⁶Al/²⁷Al)₀ = (5.8 ± 2.4) × 10^?5, is consistent with many previously reported coarse‐grained CAIs from CV chondrites (e.g., MacPherson et al. 1995 ). However, the anorthite has heterogeneous (²⁶Al/²⁷Al)₀, ranging from 1.8 × 10^?5 to 3.3 × 10^?6. The ²⁶Al‐²⁶Mg systematics within the anorthite is consistent with thermal diffusion of Mg isotopes during metamorphism. We also show that the heterogeneous distribution of ²⁶Mg* in anorthite could have resulted from thermal diffusion of ²⁶Mg* over a 0.6–0.8 Ma time span. Mg diffusion thus may be responsible for the (²⁶Al/²⁷Al)₀ heterogeneity within anorthite in CAIs.