The Influence of Interfacial Energies on Igneous Microstructures

Abstract

The thermal history of igneous rocks is encoded in their microstructures via the control by cooling rate on crystal growth. If temperatures remain close to the liquidus, with small undercoolings, growth rates are low and microstructures are strongly affected by constraints imposed by interfacial energies. The minimisation of interfacial energy results in the melt topology becoming a function of dihedral angle and porosity. Further effects include the loss of the smallest crystals (Ostwald ripening), though this process is restricted to very small grainsizes. For large crystals, interfacial energy controls the microstructure by determining the shapes, but not the size or number of crystals. In mafic-ultramafic magmas, minerals with a larger structural anisotropy (e.g., plagioclase) commonly form crystals with low-energy faces, with or without some rounding of corners, whereas minerals with lower structural anisotropy (e.g., olivine) form crystals with rounded to locally planar boundaries. Microstructures in partially solidified material from lava lakes and entrained glassy enclaves suggest that interfacial energies play only a minor role in the supersolidus of rapidly-cooled or coarse-grained rocks. They are most important for slowly-cooled bodies of hot magma crystallizing as crystallographically simple minerals, especially adcumulates in layered complexes. Interfacial energies may also play a role in the sub-solidus, leading to the development of a granular microstructure in some adcumulates. This is, however, only important for very fine-grained rocks such as chill zones, or for monomineralic rocks. Elsewhere there is little or no evidence for significant grain growth in the sub-solidus, demonstrating that the end-stage of microstructural equilibration generally is not reached in crustal layered intrusions.