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

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.

KEYWORDS:

• Orbicular granite
• crystal growth
• kinetics
• diffusion
• pegmatite

Acknowledgments

We thank Ming Tang, Xu Chu, Chenguang Sun, Hehe Jiang, Patrick Phelps and Eytan Sharton-Bierig for useful discussions. Ben Clausen, Chris Ballhaus, David London, and Calvin Barnes are thanked for reviews or editorial comments although this does not necessarily mean that they all agree with the interpretations.

Disclosure Statement

No potential conflict of interest was reported by the authors.

Supplementary material

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Additional information

Funding

This work was supported by the National Science Foundation [EAR 1753599].