Textural constraints on the kinetics of crystallization of igneous rocks
Textural constraints on the kinetics of crystallization of igneous rocks
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火成岩结晶动力学的结构约束
DOI：
10.1515/9781501508769-014
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发表时间：
1990
影响因子：
--
通讯作者：
K. Cashman
中科院分区：
文献类型：
--
作者：
K. Cashman
自然系统在实验室无法达到的规模上提供时间控制的实验，其复杂性也是计算机实验无法企及的。由于温度(过冷度)不能随时间直接调整，原位结晶实验提供了其他方法无法获得的信息。简单硅酸盐体系的析晶动力学已被广泛研究用于微晶玻璃体系(例如Uhlmann，1982；James，1982)，但尤其是对成核行为的预测模型仍然不够完善。建立成核模型的部分问题在于难以通过实验分离成核过程；晶体生长测量更容易通过实验获得，因此更容易理解晶体生长(例如Dowty，1980；Kirkpatrick，1981；James，1982；Baronnet，1984)。然而，晶体生长的预测模型(例如Lasaga，1982)需要准确的结晶热力学和动力学驱动力的温度和成分依赖模型，目前的信息有限。Dearnley(1983)提出的将Ymax的位置作为T、TL和T的函数的CSE参数化提供了一个需要进一步检验的潜在有用的模型。同样，将Meyer(1986)的绝热成核模型应用于简单体系，表明Tmax相对于Tl的位置大致为0.55-0.60(除钠长石和石英等低DSF矿物外)。此外，虽然实验结晶速率数据是天然熔体过程和数值模型之间的必要联系，但实验生长速率数据仅限于简单系统和中等大过冷度的条件，而复杂自然系统中的生长速率估计表明，在大多数条件下，过冷度仍然很小。对岩脉的定量结构研究表明，生长速度明显依赖于总结晶时间(例如Ikeda，1977)，而总结晶时间又与有效过冷度有关，而有效过冷度是岩脉总宽度和离岩脉边缘距离的函数。以此类推，如果假设结晶的温度间隔，最小结晶速度可以从程序冷却实验中估计出来。用这种方法测定的生长速度与岩脉研究中看到的生长速度和总结晶时间之间的关系相同，反映了玄武岩体系中斜长石的生长速度和冷却速度(有效过冷度)之间的系统关系。虽然没有其他矿物系统的数据，但没有理由相信其他硅酸盐矿物不会有类似的模式(即Fowler等人1989年估计的树枝状橄榄石生长速度的相似性，使用从程序冷却速度实验估计的快速冷却条件下斜长石的生长速度的分形分析)。对测量到的晶体尺寸频率的分析提供了一种从地质系统中提取有效(大量)动力学信息的手段，这有助于限制岩浆系统物理模型与化学变化速率之间的动态反馈。
Natural systems provide time-controlled experiments on scales unreachable in the laboratory, and with complexities unapproachable by computer experiments. Since temperature (undercooling) cannot be directly scaled with time, in situ crystallization experiments provide information unavailable by other means. The kinetics of crystallization in simple silicate systems have been studied extensively for glass ceramic systems (e.g. Uhlmann, 1982; James, 1982), but predictive models for nucleation behavior, in particular, remain inadequate. Part of the problem in creating models of nucleation lies in the difficulty of experimentally isolating the nucleation process; crystal growth measurements are more accessible experimentally, and for this reason crystal growth is better understood (e.g. Dowty, 1980; Kirkpatrick, 1981; James, 1982; Baronnet, 1984). However, predictive models of crystal growth (e.g. Lasaga, 1982) require accurate models for the temperature and compositional dependence of both the thermodynamic and kinetic driving forces for crystallization, information that is currently limited. The CSE parameterization of the position of Ymax as a function of T, TL and T suggested by Dearnley (1983) provides a potentially useful model that requires further testing. Similarly, application of the adiabatic nucleation model of Meyer (1986) to simple systems suggests that the position of Tmax relative to TL has an approximately constant value of 0.55-0.60 (exceptions are low DSf minerals such as albite and quartz). Additionally, while experimental crystallization rate data is a necessary link between processes in natural melts and numerical models, experimental growth rate data is confined to simple systems and conditions of moderately large undercooling, while growth rate estimates in complex natural systems suggest that under most conditions, undercoolings remain very small. Quantitative textural studies of dikes suggest a pronounced dependence of growth rate on total crystallization time (e.g. Ikeda, 1977), which is in turn related to effective undercooling, a function of both total dike width and distance from the dike margin. By analogy, minimum crystallization rates may be estimated from programmed cooling experiments if a temperature interval of crystallization is assumed. Growth rates determined in this way show the same relationship between growth rate and total crystallization time seen in the dike studies, reflecting a systematic relationship between growth rate and cooling rate (effective undercooling) for plagioclase in basaltic systems. While the data are not available for other mineral systems, there is no reason to believe that similar patterns would not exist for other silicate minerals (i.e., the similarity in dendritic olivine growth rates estimated by Fowler et al.,1989, using fractal analysis to the growth rates of plagioclase under conditions of rapid cooling estimated from programmed cooling rate experiments). Analysis of measured crystal size frequencies provides a means of extracting effective (bulk) kinetic information from geologic systems that can help to constrain the dynamic feedback between physical models of magmatic systems and rates of chemical change.