Disequilibrium crystallization and rapid crystal growth: a case study of orbicular granitoids of magmatic origin

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 <b>× </b>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.

为深入阐明晶体生长机制，我们对西澳大利亚产出的太古代球状花岗岩类（Archaean orbicular granitoids）开展了系统调查。球状结构（orbicules）为闪长质至英云闪长质的球体，分散于花岗质主岩浆之中。多数球状结构至少发育2至3条同心条带，条带由细长且呈放射状排列的角闪石与充填状斜长石构成。每条条带均由富角闪石外层与富斜长石内层组成。从边缘至核部，双层条带厚度逐渐增大，晶体数密度逐步降低，晶粒尺寸持续增大，这表明边缘的结晶速率快于核部。尽管存在上述径向差异，但各条带的矿物模态与整体组成均相似，说明结晶过程中晶体-熔体分离程度有限。基于上述观测结果，我们认为该球状结构代表了注入较冷花岗质岩浆中的高温闪长质-英云闪长质熔体的缓慢淬冷团块。球状结构内的振荡条带可通过快速非平衡结晶（过冷）予以解释。具体而言，条带宽度与距球状结构边缘的径向距离之间的线性相关性，可通过传输限制结晶模型阐释：该模型中，结晶时间尺度短于化学扩散时间尺度。该线性关系的斜率对应生长介质中有效化学扩散率与热扩散率之比的平方根，据此计算得到的有效化学扩散率为3×10⁻⁸ m²/s。如此高的有效扩散率需要通过自由挥发相（流体）实现静态扩散，或流体中存在显著的平流/对流组分。无论具体机制如何，上述有效扩散率可用于估算约10⁻⁶ m/s（即0.4 cm/hr）的晶体生长速率。本研究结果表明，晶体可实现快速生长，这一过程可能受流体与动态环境的促进。如此快速的生长速率意味着，斑岩与伟晶岩体系中的厘米级及更大晶体，理论上可在数天内形成。