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A study of Mg and K isotopes in Allende CAIs: Implications to the time scale for the multiple heating processes
Author(s) -
Ito Motoo,
Nagasawa Hiroshi,
Yurimoto Hisayoshi
Publication year - 2006
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.1945-5100.2006.tb00457.x
Subject(s) - melilite , isochron , allende meteorite , isochron dating , pyroxene , chemistry , isotope , geology , mineralogy , analytical chemistry (journal) , chondrite , meteorite , olivine , geochemistry , nuclear physics , physics , spinel , paleontology , chromatography , astronomy
— The measurements of magnesium and potassium isotopic compositions of refractory minerals in Allende calcium‐aluminum‐rich inclusions (CAIs), 7R‐19–1, HN3–1, and EGG3 were taken by secondary ion mass spectrometry (SIMS). The 7R‐19–1 contains 16 O‐rich and 16 O‐poor melilite grains and define a single isochron corresponding to an initial 26 Al/ 27 Al ratio of (6.6 ± 1.3) × 10 −5 . The Al‐Mg isochron, O isotope measurements and petrography of melilite in 7R‐19–1 indicate that 16 O‐poor melilite crystallized within 0.4 Myr after crystallization of 16 O‐rich melilite, suggesting that oxygen isotopic composition of the CAI‐forming region changed from 16 O‐rich to 16 O‐poor within this time interval. The 16 O‐poor melilite is highly depleted in K compared to the adjacent 16 O‐rich melilite, indicating evaporation during remelting of 7R‐19–1. We determined the isochron for 41 Ca‐ 41 K isotopic systematics in EGG3 pyroxene with (4.1 ± 2.0) × 10 −9 (2s̀) as an initial ratio of 41 Ca/ 40 Ca, which is at least two times smaller than the previous result (Sahijipal et al. 2000). The ratio of 41 Ca/ 40 Ca in the EGG3 pyroxene grain agrees within error with the value obtained by Hutcheon et al. (1984). No evidence for the presence of 41 K excess (decay product of a short‐lived radionuclide 41 Ca) was found in 7R‐19–1 and HN3–1. We infer that the CAI had at least an order of magnitude lower than canonical 41 Ca/ 40 Ca ratio at the time of the CAI formation.