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

摘要

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 ~(26)Mg (~(26)Mg*) from the in-situ decay of ~(26)Al; the inferred initial ratio, (~(26)Al/ ~(27)Al)_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 (~(26)Al/~(27)Al)_0, ranging from 1.8 × 10 ~(-5) to 3.3 × 10~(-6). The ~(26)Al- ~(26)Mg systematics within the anorthite is consistent with thermal diffusion of Mg isotopes during metamorphism. We also show that the heterogeneous distribution of ~(26)Mg* in anorthite could have resulted from thermal diffusion of ~(26)Mg* over a 0.6-0.8 Ma time span. Mg diffusion thus may be responsible for the (~(26)Al/ ~(27)Al)_0 heterogeneity within anorthite in CAIs.