Disequilibrium crystallization and rapid crystal growth: a case study of orbicular granitoids of magmatic origin
Disequilibrium crystallization and rapid crystal growth: a case study of orbicular granitoids of magmatic origin
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DOI：
10.1080/00206814.2020.1734975
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发表时间：
2020-03
影响因子：
2.6
通讯作者：
Julin Zhang;Cin-Ty A. Lee
中科院分区：
文献类型：
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作者：
Julin Zhang;Cin-Ty A. Lee
摘要为了更好地了解晶体生长过程，对澳大利亚西部的类花岗岩进行了研究。球状体是闪长质到英云闪长质的球状体，分散在花岗质寄主岩浆中。大多数圆形体至少有两到三个同心带，由细长的、放射状的角闪石和填隙的斜长石组成。每一条带由富含角闪石的外层和富含斜长石的内层组成。双峰带厚度增加，晶体数密度降低，晶粒尺寸增加，从边缘到核心，这表明结晶是更迅速的边缘比在核心。尽管这些径向差异，矿物模式和散装组成的每个带是相似的，表明有限的结晶过程中的结晶-熔体偏析。这些观察结果使我们认为，这些球状体代表了缓慢淬火的热闪长质到英云闪长质液体注入到较冷的花岗岩岩浆中的斑点。球状体中的振荡带可以用快速不平衡结晶（过冷）来解释。特别是，带宽和径向距离之间的线性相关性，可以解释为传输限制结晶，其中结晶时间尺度比化学扩散时间尺度短。该线性关系的斜率对应于生长介质中的有效化学扩散率与热扩散率之间的比率的平方根，导致有效化学扩散率为3 × 10−8 m2/s。这些高效扩散率需要通过自由挥发相（流体）和/或流体中的强平流/对流组分的静态扩散。无论机制如何，这些有效扩散系数都可以用来估计约10−6 m/s或0.4 cm/hr的生长速率。我们的研究结果表明，晶体可以快速生长，这可能是由流体和动力学条件促进的。这些快速的生长速度表明，厘米或更大的晶体，如在斑状和伟晶岩系统中，可以想象在几天内生长。
ABSTRACT Archaean orbicular granitoids from western Australia were investigated to better understand crystal growth processes. The orbicules are dioritic to tonalitic spheroids dispersed in a granitic host magma. Most orbicules have at least two to three concentric bands composed of elongate and radially oriented hornblendes with interstitial plagioclase. Each band consists of a hornblende-rich outer layer and a plagioclase-rich inner layer. Doublet band thicknesses increase, crystal number density decreases, and grain size increases from rim to core, suggesting crystallization was more rapid on the rims than in the core. Despite these radial differences, mineral mode and bulk composition of each band are similar, indicating limited crystal-melt segregation during crystallization. These observations lead us to suggest that the orbicules represent slowly quenched blobs of hot dioritic to tonalitic liquids injected into a cooler granitic magma. The oscillatory bands in the orbicules can be explained by rapid, disequilibrium crystallization (supercooling). In particular, a linear correlation between bandwidth and radial distance from orbicule rim can be explained by transport-limited crystallization, wherein crystallization timescales are shorter than chemical diffusion timescales. The slope of this linear relationship corresponds to the square root of the ratio between effective chemical diffusivity in the growth medium and thermal diffusivity, resulting in effective chemical diffusivities of 3 × 10−8 m2/s. These high effective diffusivities require static diffusion through a free volatile phase (fluid) and/or a strong advective/convective component in the fluid. Regardless of the mechanisms, these effective diffusivities can be used to estimate growth rates of ~10−6 m/s or 0.4 cm/hr. Our results indicate that crystals can grow rapidly, possibly facilitated by fluids and dynamic conditions. These rapid growth rates suggest that centimetre or larger crystals, such as in porphyritic and pegmatitic systems, can conceivably grow within days.