Oxygen isotope heterogeneities in the earliest protosolar gas recorded in a meteoritic calcium-aluminum-rich inclusion

摘要

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, O-16-rich xenolithic CAI, previously altered in the nebula, that impacted an extensively molten, O-16-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 O-16-rich gehlenite with Delta O-17 ranging down to -20 parts per thousand. but melilite becomes progressively O-16-poor (Delta O-17 similar to O parts per thousand) and Mg-rich towards the interior. In the absence of Mg isotopic fractionation, this variation is best attributed to 0 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 0 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 Al-26/Al-27 ratio of 4.1 x 1 0(-5) determined from both inclusions implies that at most 670,000 yr after the birth of the Solar System, the O-16-rich reservoir was spatially limited and an O-16-poor reservoir with typical planetary isotopic composition was available for planet formation. (c) 2007 Elsevier B.V. All rights reserved.