Chondrule age distribution and rate of heating events for chondrule formation

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Chondrules are considered to be formed by flash heating events in the protoplanetary disk. In order to evaluate some basic factors in the heating mechanism, we examined the rate of heating events that explains the abundance and the observed age distribution of chondrules in unequilibrated ordinary chondrites. First, we compiled the literature data of ^(26)Al ages of chondrules from the least equilibrated ordinary chondrites (LL3.0-3.1), ranging from 1Myr to 3Myr with a peak at about 1.8Myr relative to the formation time of Ca, Al-rich inclusions (CAIs) in carbonaceous chondrites, the oldest solid materials formed in the solar system. Next, we made a simple phenomenological chondrule formation model assuming that each event heated only a small fraction of existing dust at one time and numerous heating events produced chondrules. Results indicate that (1) an average number of heating events experienced by a dust particle should be 1.2 or higher, (2) more than a half of the present chondrules were reheated, (3) chondrule formation started sometime between 0.4-1.5Myr and ended at 2.2-2.3Myr after the CAI formation, and (4) the rate of heating events has a peak at 0.1-0.8Myr earlier than the peak of the observed chondrule age distribution and should decrease monotonically with time after the peak.

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