Formation conditions of plagioclase-bearing type I chondrules in CO chondrites: A study of natural samples and experimental analogs

About ten percent of type I, FeO-poor chondrules in unequilibrated CO chondrites contain plagioclase that appears to be igneous in origin, crystallizing at a late stage of solidification of host chondrule melts. We have studied plagioclase-bearing chondrules in detail, and compared them with plagioclase-free chondrules, in order to determine the formation conditions of plagioclase and the constraints that the presence of plagioclase places on the conditions of chondrule formation. Plagioclase-bearing chondrules have similar textures, mineral compositions and bulk compositions to plagioclase-free chondrules. The only possible chemical difference that might control the presence or absence of plagioclase in a given chondrule is that most plagioclase-bearing chondrules have a slightly higher bulk Al/Ca ratio than plagioclase-free chondrules. We carried out dynamic cooling experiments on a type IAB chondrule analog in order to investigate chondrule formation conditions. Our experiments at slow cooling rates, <25 °C/hr, reproduce natural type I chondrule textures as well as mineral and glass compositions very closely. We attempted to facilitate nucleation and growth of plagioclase by optimizing several parameters, including using a bulk composition for our experiments comparable to natural plagioclase-bearing chondrules, using slow cooling rates, quenching from low temperatures, seeding the experiments with anorthite crystals, and maintaining a Na-rich atmosphere around the experimental charges. Of all these parameters, we only succeeded in growing plagioclase in the slowest cooled experiment, which included multiple linear cooling steps with a final cooling stage of 1 °C/hr between 1000 and 800 °C. Our experiments indicate that type I chondrules can plausibly be formed at slow cooling rates, and slow cooling rates may actually be a requirement for production of plagioclase. The cooling histories we examined are very similar to those predicted in recent shock wave models for chondrule formation.