Ca isotopes in refractory inclusions

Abstract

We report measurements of the absolute isotope abundance of Ca in Ca-Al-rich inclusions from the Allende and Leoville meteorites. Improved high precision measurements are reported also for 46Ca. We find that nonlinear isotope effects in Ca are extremely rare in these inclusions. The absence of nonlinear effects in Ca, except for the effects in FUN inclusions, is in sharp contrast to the endemic effects in Ti. One fine-grained inclusion shows an excess of 46Ca of (7 ± 1)%., which is consistent with addition of only 46Ca or of an exotic (∗) component with 46Ca ∗ ~ 48Ca ∗. FUN inclusion EK-1-4-1 shows a small 46Ca excess of (3.3 ± 1.0)%.; this confirms that the exotic Ca components in EK-1-4-1 were even more deficient in 46Ca relative to 48Ca than is the case for normal Ca. The Ca in the Ca-Al-rich inclusions shows mass dependent isotope fractionation effects (as deduced from the absolute 40Ca 44Ca ) which have a range from −3.8 to +6.7%. per mass unit difference. This range is a factor of 20 wider than the range previously established for bulk meteorites and for terrestrial and lunar samples. Ca and Mg isotope fractionation effects in the Ca-Al-rich inclusions are common and attributed to kinetic isotope effects which imply the production of the inclusions by complex sequences of condensation, vaporization and recondensation. A correlation was found between Ca and Mg isotope fractionation effects and inclusion type. A possible correlation between isotope fractionation and rare earth element abundance patterns is discussed.