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Thermal metamorphic history of a Ca, Al‐rich inclusion constrained by high spatial resolution Mg isotopic measurements with NanoSIMS 50L
Author(s) -
ITO Motoo,
MESSENGER Scott
Publication year - 2010
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2010.01038.x
Subject(s) - anorthite , melilite , geology , chondrite , parent body , mineralogy , geochemistry , isotope , allende meteorite , metamorphic rock , meteorite , paleontology , astrobiology , physics , spinel , quantum mechanics
– 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.