The crystal-scale characteristics of crystal mushes: crystal shape as a record of dynamic magmatic processes

All magmas transition through a crystal mush stage during solidification, and long-lived crystal mushes are thought to be common in crustal magmatic systems. The efficacy of many crystal mush processes depends primarily on the permeability and porosity of the mush, and thus on the shape and size of crystals in the mush framework. Plagioclase is one of the most common minerals to form in igneous rocks and is commonly a framework-forming phase.

 

Crystal shape is determined by the relative growth rates on different crystal faces and can be readily measured in thin section. Holness (2014) showed a relationship between plagioclase crystal shape and cooling conditions, but without a mechanistic underpinning. Here, we show experimentally that plagioclase 3D crystal shape evolves from prismatic to tabular (platy) with the thermodynamic driving force for crystallisation (undercooling) that is experienced by the crystal. This occurs because crystallisation on the long and intermediate axes transitions to faster, higher-energy growth mechanisms with increasing undercooling, relative to the short axis. This is a general crystallographic processes and we anticipate that similar relationships can be found for other anisotropic minerals.

 

We combine our experiments with numerical forward modelling to produce a quantitative measure of the evolving undercooling at any instant that is experienced by the crystal. This instantaneous undercooling varies over the course of a single experiment or stage of crystallisation. Its maximum value controls the majority of growth and correlates with 3D crystal shape.

 

An evolution from more tabular to more prismatic crystal shape occurs with decreasing undercooling (or cooling rate). This framework can also be extended to decompressing systems. As the thermal state (maturity) of the crust is a critical control on the local cooling rate, magma intruded into cooler crust and in smaller batches will have more tabular crystals than large batches of magma intruded into warmer crust. Repeated episodes of intrusion, resorption and remobilisation will also affect crystal shape by decreasing crystal aspect ratio and increasing the crystallinity at which the mush becomes immobile.

 

Overall, crystal shape is a strong control on igneous rock texture, and quantitative investigation of crystal shape has great potential to uncover the details of magmatic plumbing and volcanic processes.