MULTIPLE FORMATION OF CHONDRULES IN THE EARLY SOLAR SYSTEM: CHRONOLOGY OF A COMPOUND Al-RICH CHONDRULE

摘要

Chondrules are high-temperature components of meteorites and are formed during flash heating episodes in the early solar system. From the presence of compound chondrules, which consist of an early formed unit enclosed within a later phase, it has been concluded that the chondrule formation event is repeatable. We report on the chronology of one Al-rich compound chondrule from the Allende meteorite, together with its mineralogy, petrography, and oxygen isotope composition. The earlier formed primary chondrule is rich in ~(26)A1 with an initial ~(26)A1/~(27)A1 ratio of (2.7 ± 1.0) × 10~(-5), whereas the later formed secondary chondrule is depleted in ~(26)A1 with an initial ~(26)A1/~(27)A1 ratio of (9.9 + 2.0) × 10~(-6). The difference between the primary and secondary initial ratios corresponds to 1 Myr. We conclude that the primary unit formed during an earlier melting episode and went into a secondary melt that formed 1 Myr later during a later melting episode. The oxygen isotope composition of silicates in the primary and secondary phases shows varying degrees of ~(16)O-depression but is similar to that of single chondrules from Allende meteorite specimens. Therefore, this primary chondrule in Allende stayed in the same dust reservoir for over 1 Myr and experienced multiple heating events, during which secondary and single chondrules were also produced.