An experimental study of Ostwald ripening of olivine and plagioclase in silicate melts: implications for the growth and size of crystals in magmas

• Published in: Contributions to mineralogy and petrology 2005-08, Vol.150 (1), p.37-53
• Main Authors: Cabane, Hugues, Laporte, Didier, Provost, Ariel
• Format: Article
• Language: English
• Subjects: Basalt; Crystal growth; Crystals; Geological time; Kinetics; Magma; Mineralogy; Monitoring systems; Particle size; Petrology; Silica; Supersaturation
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-005-0002-2

We carried out an experimental study to characterize the kinetics of Ostwald ripening in the forsterite-basalt system and in the plagioclase (An65)-andesite system. Eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution (CSD) with time t; the experiments were performed in a 1-atmosphere quench furnace, at 1,250 degrees C for plagioclase and 1,300 degrees C for olivine. Very contrasted coarsening kinetics were observed in the two series. In the plagioclase series, the mean grain size increased as log(t), from (symbol omitted)3 mum to only 8.7 mum in 336 h. The kinetic law in log(t) means that Ostwald ripening was rate-limited by surface nucleation at plagioclase-liquid interfaces. In the olivine series, the mean grain size increased as t 1/3, from (symbol omitted)mu3 mum to 23.2 mum in 496 h. A kinetic law in t 1/3 is expected when Ostwald ripening is rate-limited either by diffusion in the liquid or by grain growth/dissolution controlled by a screw dislocation mechanism. The shape of olivine CSDs, in particular their positive skewness, indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism, not by diffusion. As the degrees of undercooling deltaT (or supersaturation) involved in Ostwald ripening are essentially