Dynamic cyrstallization of chondrule melts of porphyritic olivine composition: Textures experimental and natural

Abstract

Melts of the same composition as has been defined for porphyritic olivine chondrules have been crystallized under controlled cooling conditions, dynamic crystallization. The full range of textures observed in such chondrules has been duplicated in the experimentally produced analogs. The primary controlling factor in producing this range of textuures is the heterogeneous nucleation behavior. In general, if nuclei are present when cooling begins, growth starts immediately at low degrees of supercooling and porphyritic textures result. The shapes of the phenocyrsts depend on the growth rate, which in turn depends on the cooling rate. With high nucleation densities and modest to low cooling rates, the textures are microporphyritic with generally equant, euhedral phenocrysts. As the nucleatio density decreases, the phenocrysts become larger, and as the cooling rate increases, they become increasingly skeletal. If nuclei are not present when cooling begins, growth is delayed until subcritical-sized embryos graduate to supercritical nuclei. Growth usually begins at some significant degree of supercooling depending on the cooling rate and how long it takes the embryos to become nuclei. The shapes of the crystals depend on the degree of supercooling when growth is initiated and on the subsequent cooling rate. The resulting textures vary from plate dendrites (barred olivine chondrules) to randomly oriented, elongated, skeletal crystals as the degree of supercooling and cooling rates decreases. The results of a matrix of experiments set limits on the maximum melting temperatures of chondrule forming materials and the subsequent cooling rates. Based primarily on crystal shape, the porphyritic textures develop best for this composition at melting temperatures less than 1590°C and cooling rates near 100°C/hr and definitely less than 1000°C/hr. At cooling rates significantly less than 100°C/hr, the phenocrysts are generally larger than observed in chondrules, and as cooling rates approach 1000°C/hr, they become more skeletal. Barred olivine chondrule textures form best from materials melted at higher temperatures and cooling rates in the range 1000 to 3000°C/hr.