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. © 2010 The Meteoritical Society.