High time resolution by use of the ~(26)Al chronometer in the multistage formation of a CAI

摘要

The Allende calcium-rich inclusion (CAI) 5241 has been found to contain distinct initial ~(26)Al/~(27)Al in the three consecutive igneous zones that have been identified by extensive petrogenetic studies to have formed in three distinct crystallization events. The zones in order of sequence of formation from the petrologic observations are: (1) spinel-free islands (SFI) included in (2) a pyroxene-spinel-rich core (SRC) which in turn is included in (3) a melilite mantle (MM). The initial (~(26)Al/~(27)Al) values of these zones are respectively (4.6-5.0) * 10~(-5), 4.3 * 10~(-5), and 3.3 * 10~(-5). It is argued that these distinct (~(26)Al/~(27)Al)_0 values are not the result of metamorphism but reflect the relative times of formation by crystallization from melts. Relative to the canonical value of (~(26)Al/~(27)Al)_0 = 5 * 10~(-5), we find the following chronology: t_(SFI) ≈ 0, t_(SRC) ≈ 10~5 yr and t_(MM) ≈ 4 * 10~5 yr. The three-layer CAI is inferred to have sampled a reservoir with an initial uniform ~(26)Al/~(27)Al ratio that decreased in value due to radioactive decay. From these observation we conclude that we have resolved time differences of a few hundred thousand years at the very early stages of formation of the solar system. The 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 years. We believe that these sequential events are not compatible with condensation in a hot region of the solar nebula. It is proposed that a scenario involving stages of protoplanetary accretion and of melt generation in protoplanetary sites heated by ~(26)Al might provide a possible source for CAIs and chondrules.