Petrologic, geochemical and experimental constraints on models of chondrule formation

The petrologic and geochemical properties of chondrules as well as results of experimental studies provide strong constraints on chondrule-formation m odels. Nebular formation is indicated by the non-mass-fractionated oxygen i sotopic compositions of bulk chondrules, Chondrule formation from a melt is required by the prototypical spheroidal shapes and the presence of euhedra l phenocrysts and glassy mesostases. Incomplete melting is indicated by the abundance of porphyritic chondrules (which experiments demonstrate require relict nuclei) and coarse relict grains. The length of rime that chondrule s were hot is constrained by their retention of relict grains and moderatel y volatile elements. Rapid cooling of chondrules after formation is support ed by the presence of zoned phenocrysts, isotopic anomalies and dynamic cry stallization experiments. It is clear from the presence of relict grains, e nveloping compound chondrules and igneous rims that many chondrules were he ated again after cooling. The heating mechanism responsible for chondrule f ormation seems to have operated at varying intensities over large regions o f the inner solar nebula for at least the time it took ambient nebular temp eratures to cool from above similar to 900 to below similar to 600 K. The c hondrule-formation mechanism provided a repeatable source of energy capable of highly localized melting, characteristic of flash heating. The occurren ce of ferroan microchondrules with low melting temperatures within some cho ndrule rims indicates that chondrule formation did not occur exclusively in high-temperature regions near the Sun as required in bipolar outflow model s. Mechanisms for forming chondrules that are consistent with the constrain ts include various flash-heating models: nebular lightning, magnetic reconn ection flares, gas dynamic shock waves and radiative heating. (C) 2000 Else vier Science B.V. All rights reserved.