DYNAMIC CRYSTALLIZATION OF CHONDRULE MELTS OF PORPHYRITIC OLIVINE COMPOSITION - TEXTURES EXPERIMENTAL AND NATURAL
DYNAMIC CRYSTALLIZATION OF CHONDRULE MELTS OF PORPHYRITIC OLIVINE COMPOSITION - TEXTURES EXPERIMENTAL AND NATURAL
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DOI：
10.1016/0016-7037(89)90397-9
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发表时间：
1989-02-01
影响因子：
5
通讯作者：
LOFGREN, G
中科院分区：
文献类型：
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作者：
LOFGREN, G
与斑状橄榄石球粒定义的成分相同的熔体在受控冷却条件下结晶，动态结晶。在这种陨石球粒中观察到的全部纹理都在实验产生的类似物中得到了复制。在生产这一范围的纹理的主要控制因素是异质成核行为。一般来说，如果冷却开始时有原子核存在，生长会在低过冷度下立即开始，并产生斑状结构。的phenocyrs的形状取决于生长速度，这反过来又取决于冷却速率。具有高的成核密度和适度到低的冷却速率，纹理是微斑状的，通常具有等长的自形斑晶。随着成核密度的降低，斑晶变得更大，随着冷却速率的增加，它们变得越来越骨架化。如果冷却开始时核不存在，则生长被延迟，直到亚临界尺寸的胚胎逐渐变成超临界核。生长通常在一定程度的过冷时开始，这取决于冷却速度和胚胎变成核所需的时间。晶体的形状取决于生长开始时的过冷程度和随后的冷却速率。随着过冷度和冷却速率的降低，所产生的纹理从板状枝晶（禁止橄榄石球粒）到随机取向的细长骨架晶体。矩阵的实验结果设置限制球粒形成材料的最高熔化温度和随后的冷却速率。主要基于晶体形状，对于这种组合物，在熔化温度低于1590°C和冷却速率接近100°C/hr并且肯定低于1000°C/hr时，斑状结构发展最好。在冷却速率显著低于100°C/hr时，斑晶通常比在球粒中观察到的大，并且当冷却速率接近1000°C/hr时，它们变得更骨架化。棒状橄榄石球粒结构最好由在较高温度和1000至3000°C/hr范围内的冷却速率下熔化的材料形成。
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.