Oxygen Isotope Anatomy of a Compound Cai- Chondrule Inclusion from Efremovka

Introduction: We recently reported the discovery of E-MNHN-002, a compound Ca-Al-rich Inclusion (CAI)-chondrule object from the reduced CV3 chondrite Efremovka, composed of an amoeboid olivine aggregate (AOA) core surrounded by a type I chondrule mantle [1]. Its petrographic study indicates that a Ca-Al-rich AOA accreted an olivine + metal mantle akin to the precursors of type I chondrules and that this compound precursor underwent partial melting in the conditions of chondrule formation with inward diffusion of SiO 2 condensed from the nebular gas and concomitant outward diffusion of refractory elements. To confirm this interpretation and obtain new insights on CAI and chondrule formation we performed a detailed O isotopic study of E-MNHN-002 and several neighbouring chondrules by multicol-lector SIMS with the IMS 1270 ion microprobe at the CRPG national facility in Nancy, France, using well-established analytical procedures. Typical errors were between 0.3‰ and 1‰ (2σ), allowing a detailed study of small mass fractionation effects. Results and discussion: O isotopes in E-MNHN-002 plot along a slope 1.0 line [2] with the innermost AOA spinels (sp) being the most 16 O-rich minerals (δ 17 O ≈ δ 18 O <-40‰). An O isotope exchange with a 16 O-poor outer reservoir is recorded by 16 O depletions correlated with both mineralogy [sp < olivine < pyroxene (px) < plagioclase (plag)] and position in the inclusion (interior < exterior). Minerals in the chondrule mantle lay at the 16 O-poor end of the trend with low-Ca px being systematically the most 16 O-depleted mineral (apart from mesostasis) and having O isotopic compositions similar to those observed in chon-drules (-13‰ < δ