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 models. Nebular formation is indicated by the non-mass-fractionated oxygen isotopic compositions of bulk chondrules, Chondrule formation from a melt is required by the prototypical spheroidal shapes and the presence of euhedral 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 chondrules were hot is constrained by their retention of relict grains and moderately volatile elements. Rapid cooling of chondrules after formation is supported by the presence of zoned phenocrysts, isotopic anomalies and dynamic crystallization experiments. It is clear from the presence of relict grains, enveloping compound chondrules and igneous rims that many chondrules were heated again after cooling. The heating mechanism responsible for chondrule formation seems to have operated at varying intensities over large regions of the inner solar nebula for at least the time it took ambient nebular temperatures to cool from above similar to 900 to below similar to 600 K. The chondrule-formation mechanism provided a repeatable source of energy capable of highly localized melting, characteristic of flash heating. The occurrence of ferroan microchondrules with low melting temperatures within some chondrule rims indicates that chondrule formation did not occur exclusively in high-temperature regions near the Sun as required in bipolar outflow models. Mechanisms for forming chondrules that are consistent with the constraints include various flash-heating models: nebular lightning, magnetic reconnection flares, gas dynamic shock waves and radiative heating. (C) 2000 Elsevier Science B.V. All rights reserved.