A study of Mg and K isotopes in Allende CAIs: Implications to the time scale for the multiple heating processes

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 O-16-rich and O-16-poor melilite grains and define a single isochron corresponding to an initial 26A1/27AI ratio of (6.6 +/- 1.3) X 10(-5). The Al-Mg isochron, O isotope measurements and petrography of melilite in 7R-19-1 indicate that O-16-poor melilite crystallized within 0.4 Myr after crystallization of O-16-rich melilite, suggesting that oxygen isotopic composition of the CAI-forming region changed from O-16-rich to O-16-poor within this time interval. The O-16-poor melilite is highly depleted in K compared to the adjacent O-16-rich melilite, indicating evaporation during remelting of 7R-19-1. We determined the isochron for Ca-41-K-41 isotopic systematics in EGG3 pyroxene with (4.1 +/- 2.0) X 10(-9) (2 sigma) as an initial ratio of Ca-41/Ca-40, which is at least two times smaller than the previous result (Sahijipal et al. 2000). The ratio of Ca-41/Ca-40 in the EGG3 pyroxene grain agrees within error with the value obtained by Hutcheon et al. (1984). No evidence for the presence of K-41 excess (decay product of a short-lived radionuclide Ca-41) was found in 7R-19-1 and HN3-1. We infer that the CAI had at least an order of magnitude lower than canonical Ca-41/Ca-40 ratio at the time of the CAI formation.