OXYGEN ISOTOPE HETEROGENEITIES IN THE EARLIEST PROTOSOLAR GAS RECORDED IN A METEORITIC CALCIUM-ALUMINUM-RICH INCLUSION

Abstract

Combined petrologic, oxygen and magnesium isotopic and trace element analyses of a compound calcium-aluminum-rich inclusion (CAI) from the Efremovka reduced CV3 carbonaceous chondrite reveal that it consists of a Mg-rich, 16O-rich xenolithic CAI, previously altered in the nebula, that impacted an extensively molten, 16O-depleted, type A host CAI shortly before the end of the host's crystallization. Convoluted regions in the xenolith were probably formed by rapid crystallization of the partial melt produced during impact. Oxygen isotopic ratios in the host CAI are correlated both with melilite chemistry and location in the inclusion. The region immediately inside the Wark-Lovering rim of the CAI consists of 16O-rich gehlenite with Δ17O ranging down to - 20‰ but melilite becomes progressively 16O-poor (Δ17O ~ 0‰) and Mg-rich towards the interior. In the absence of Mg isotopic fractionation, this variation is best attributed to O isotopic exchange between the nebular gas and the partially molten inclusion during its crystallization. This event lasted less than 200 h, which implies that the host CAI was transported between two nebular reservoirs with distinct O isotopic compositions during this time. Examination of possible transport mechanisms suggests that the transport occurred over a distance of less than 1 astronomical unit. The close-to-canonical 26Al/27Al ratio of 4.1 × 10- 5 determined from both inclusions implies that at most 670,000 yr after the birth of the Solar System, the 16O-rich reservoir was spatially limited and an 16O-poor reservoir with typical planetary isotopic composition was available for planet formation. © 2007 Elsevier B.V. All rights reserved.