O ISOTOPIC CONSTRAINTS ON THE ORIGIN OF MG- RICH OLIVINES FROM ALLENDE CHONDRULES: PLANETARY OR NOT ?

slopes ( 26 Al/ 27 Al) and the initial (δ 26 Mg0) of 26 Al isochrons in chondrules and CAIs. This can be used to test the homogeneous distribution of 26 Al and Mg isotopes in the accretion disk at the time of formation of CAIs [2] and to try to calculate model 26 Al ages (under the assumption of an homogeneous distribution) for chondrule precursors [2] or for minerals devoid of Al such as Mg-rich olivines in chondrules [3]. This suggests that some of the chondrule precursors and Mg-rich olivines were formed very early, within 1 Myr or less than CAIs [2, 3]. One possibility would be that these Mg-rich olivines are precursors of type I chondrules and could be the fragments of the mantle of preexisting disrupted planetesimals [4]. In the present study we explore what constraints could be derived on the origin of Mg-rich olivines in Allende (either in type I chondrules or isolated in the matrix) from high precision measurements of their O isotopic compositions and from their major and trace elements concentrations. Experimental: The O isotopic compositions were measured with the CRPG-CNRS (Nancy) ims 1270 and ims 1280HR2 multi-collector ion microprobes. In each olivines numerous (from 10 to 25) spots were made in order to reach an analytical precision on the level of ± 0.01-0.02 ‰ (2 sigma) on 17 O excesses relative to mass fractionation (i.e. Δ 17 O).