Microdistributions of Mg isotopes and REE abundances in a Type A calcium-aluminum-rich inclusion from Efremovka

Abstract

Ion probe measurements were performed in the core and rim of a compact Type A inclusion, E2, from the CV3 chondrite Efremovka. Rare earth element (REE) and selected trace element (Sc, V, Sr, Y, Zr, Nb, Ba, Hf) abundances as well as Mg, Si, and Ti isotopes were measured. The REE are concentrated in perovskite except for Eu which mainly resides in melilite. REE patterns in E2 perovskite are unfractionated and have a Eu depletion. No systematic differences in REE concentrations or patterns between core and rim perovskites were found. In melilite, the concentrations of Ba, La, Ce, and Dy are independent of the distance from the rim. The Mg isotopic composition of melilite and spinel is uniformly heavy (by ~ 10%./ amu) in the interior and smoothly grades to normal in and near the rim. This indicates that the rim must have formed by condensation of minerals onto a solid core at high temperatures. Excesses of 26Mg are present and correlate with the 27 Al 24 Mg ratio, both in the core and rim regions of the inclusion. The slope of the apparent isochron in the rim region is higher than the slope measured in the inner core, providing evidence that 26Al was inhomogeneously distributed in the early solar nebula. In addition, the intercepts differ, pointing to a heterogeneous 26 Mg 24 Mg ratio. No Si isotopic mass fractionation was found. The Ti isotopic composition of E2 perovskite is normal.