26Al records in chondrules from unequilibrated ordinary chondrites: II. Duration of chondrule formation and parent body thermal metamorphism

Fossil records of the now-extinct short-lived nuclide 26Al were analyzed in twelve chondrules from four unequilibrated ordinary chondrites (UOCs), Semarkona (3.0), Bishunpur and Y-791324 (both 3.1) and ALHA76004 (3.3), belonging to the LL group to infer their time of formation. The present data along with those reported by us previously firmly establish a short duration of less than a million years for the major episode of UOC chondrule formation. Nebular processes responsible for chondrule formation started ~ 1 Ma after formation of CAIs and effectively stopped within another one and a half million years in the inner region of the asteroidal belt. Modeling of thermal evolution of chondritic parent bodies suggest that the longer duration of UOC chondrule formation inferred in some of the earlier studies could be attributed to thermal metamorphism experienced by some chondrules in UOC parent bodies. The duration of formation of UOC chondrules is significantly shorter than that for carbonaceous chondrite chondrules.