High time resolution by use of the Al-26 chronometer in the multistage formation of a CAI

The Allende calcium-rich inclusion (CAI) 5241 has been found to contain dis tinct initial Al-26/Al-27 in the three consecutive igneous zones that have been identified by extensive petrogenetic studies to have formed in three d istinct crystallization events. The zones in order of sequence of formation from the petrologic observations are: (1) spinel-free islands (SFI) includ ed in (2) a pyroxene-spinel-rich core (SRC) which in turn is included in (3 ) a melilite mantle (MM). The initial (Al-26/Al-27) values of these zones a re respectively (4.6-5.0) x 10(-5), 4.3 x 10(-5), and 3.3 x 10(-5). It is a rgued that these distinct (Al-26/Al-27)(0) values are not the result of met amorphism but reflect the relative times of formation by crystallization fr om melts. Relative to the canonical value of (Al-26/Al-27)(0) = 5 x 10(-5), we find the following chronology: t(SFI) approximate to 01 t(SRC) approxim ate to 10(5) yr and t(MM) approximate to 4 x 10(5) yr. The three-layer CAI is inferred to have sampled a reservoir with an initial uniform Al-26/Al-27 ratio that decreased in value due to radioactive decay. From these observa tions we conclude that we have resolved time differences of a few hundred t housand years at the very early stages of formation of the solar system. Th e SFI, SRC, and MM zones reflect sequential addition of molten CAI material which crystallized rapidly without seriously metamorphosing the previously formed material. These additions took place over a time of about 400 000 y ears. We believe that these sequential events are not compatible with conde nsation in a hot region of the solar nebula. It is proposed that a scenario involving stages of protoplanetary accretion and of melt generation in pro toplanetary sites heated by Al-26 might provide a possible source for CAIs and chondrules. (C) 2000 Elsevier Science B.V. All rights reserved.