On the lower limit of chondrule cooling rates: The significance of iron loss in dynamic crystallization experiments

Abstract

Abstract— Dynamic crystallization experiments performed with different container materials (Fe crucible, pure Pt wire loop, presaturated Pt wire loop) demonstrate the strong influence of Fe loss on texture, mineralogy and chemical zoning in olivine. The use of pure Pt wire loops results in severe Fe loss and prevents the development of strong Fe/Mg zoning in olivine in slower cooled runs (≤ 100 °C/h). Presaturated Pt wire loops reduce Fe loss to some extent but not completely. If severe Fe loss from the melt is avoided by the use of Fe crucibles, then cooling rates between 2000 and 1.2 °C/h yield textures, modal mineral abundances and Fe/Mg zoning in olivine comparable to natural porphyritic olivine chondrules. However, Fe gain from the crucible may possibly enhance Fe/Mg zoning in olivine for cooling rates < 10 °C/h. Therefore, it is concluded that the lower limit of cooling rates of porphyritic olivine chondrules derived from dynamic crystallization experiments is 10 °C/h, perhaps it is even lower, on the order of a few degrees Celsius per hour. This value is not significantly different from estimates for subsolidus temperatures based on the microstructure of chondrule minerals (Weinbruch and Müller, 1995). The lower limit of chondrule cooling rates of 100 °C/h advocated by Hewins (1988) and Radomsky and Hewins (1990) seems to be an artifact of the experimental technique, as their samples were crystallized in pure Pt wire loops.