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.

摘要

为了更好地了解晶体生长过程，对来自西澳大利亚的古生圆形球状类固醇进行了研究。沿生珠与散布在花岗岩宿主岩浆中的tonalitic球状体相对。大多数的小行星至少有两到三个同心带，由带间隙斜长石的细长的和径向取向的角混合组成。每个带由一个富含角闪石的外层和一个富含斜长石的内层组成。双峰带的厚度增加，晶体数密度降低，并且从边缘到核心的晶粒尺寸增加，这表明边缘上的结晶比核心中的结晶更快。尽管存在这些径向差异，但每个谱带的矿物模式和整体组成相似，表明在结晶过程中有限的晶体熔体偏析。这些观察结果使我们认为，小球代表了注入较冷的花岗岩岩浆中的热闪长岩到tonalitic液体的缓慢淬灭斑点。可通过快速的不平衡结晶（过冷）来解释小球中的振荡带。特别地，带宽和到小行星边缘的径向距离之间的线性相关性可以通过传输受限的结晶来解释，其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于生长介质中有效化学扩散率与热扩散率之比的平方根，因此有效化学扩散率为3 可通过快速的不平衡结晶（过冷）来解释小球中的振荡带。特别地，带宽和到小行星边缘的径向距离之间的线性相关性可以通过传输受限的结晶来解释，其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于生长介质中有效化学扩散率与热扩散率之比的平方根，因此有效化学扩散率为3 可通过快速的不平衡结晶（过冷）来解释小球中的振荡带。特别地，带宽和到小行星边缘的径向距离之间的线性相关性可以通过传输受限的结晶来解释，其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于生长介质中有效化学扩散率与热扩散率之比的平方根，因此有效化学扩散率为3 ×  10 ^-8  m ² / s。这些高有效扩散率要求通过流体中的自由挥发相（流体）和/或强对流/对流成分进行静态扩散。无论采用何种机制，这些有效扩散率均可用于估算〜10 ^-6  m / s或0.4 cm / hr的生长速率。我们的结果表明，晶体可以快速生长，可能受流体和动态条件的影响。这些快速的增长速度表明，可以想象在厘米级或更大的晶体中，例如在斑状和岩纹状的系统中，几天之内就可以生长出来。