Short time interval for condensation of high-temperature silicates in the solar accretion disk

摘要

Chondritic meteorites are made of primitive components that record the first steps of formation of solids in our Solar System. Chondrules are the major component of chondrites, yet little is known about their formation mechanisms and history within the solar protoplanetary disk (SPD). We use the reconstructed concentrations of short-lived Al-26 in chondrules to constrain the timing of formation of their precursors in the SPD. High-precision bulk magnesium isotopic measurements of 14 chondrules from the Allende chondrite define a Al-26 isochron with Al-26/Al-27 = 1.2(+/- 0.2) x 10(-5) for this subset of Allende chondrules. This can be considered to be the minimum bulk chondrule Al-26 isochron because all chondrules analyzed so far with high precision (similar to 50 chondrules from CV and ordinary chondrites) have an inferred minimum bulk initial (Al-26/Al-27) >= 1.2 x 10(-5). In addition, mineral Al-26 isochrons determined on the same chondrules show that their formation (i.e., fusion of their precursors by energetic events) took place from 0 Myr to similar to 2 Myr after the formation of their precursors, thus showing in some cases a clear decoupling in time between the two events. The finding of a minimum bulk chondrule Al-26 isochron is used to constrain the astrophysical settings for chondrule formation. Either the temperature of the condensation zone dropped below the condensation temperature of chondrule precursors at similar to 1.5 My after the start of the Solar System or the transport of precursors from the condensation zone to potential storage sites stopped after 1.5 My, possibly due to a drop in the disk accretion rate.