CHONDRULES

Chondrules are ∼1-mm igneous droplets in primitive meteorites, and their abundance suggests widespread melting in the protoplanetary disk. Chondrules with relict unmelted grains or igneous rims record multi111ple melting events. There are two main types of chondrules, type I (FeO-poor and volatile-poor) and type II [FeO-rich and approximately chondritic (solar) in composition]. Type I chondrules in the unmetamorphosed chondrite Semarkona show evidence of evaporative loss with regard to the moderately volatile elements. Loss of S produces much of the FeNi metal in chondrules. Though the finest grained type I and II chondrules in Semarkona are both approximately chondritic in bulk composition, they differ in FeO content of olivine, indicating different precursors. Simulations suggest temperatures of chondrule formation of 1550–1900°C, with short (<1 min) heating times. Short-lived isotopes suggest that chondrules formed relatively late, and nebular shock waves are the current favorite heating mechanism. As chondrules were transported to the midplane more easily than fine dust or fluffy aggregates, they were probably important components in building planets.