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.

How deceptive are microstructures in granitic rocks? Answers from integrated physical theory, phase equilibrium, and direct observations

“…We suggest that the general absence of a significant microstructural modification in granitic rocks results from three factors. The first is the lower temperatures and narrower temperature ranges over which cooling occurs in granitic magmas, in contrast to the situation in higher temperature mafic magmas, many of which do show the evidence of subsolidus grain-shape adjustment (Vernon 2004 , 2010 ; Holness and Vernon 2014 ). The second is that, as a consequence of the chemical compatibility of the granitic rock mineral assemblage, there is no chemical driving force for recrystallisation (Vernon and Paterson 2008 , p. 47).…”

The Granite Aqueduct and Autometamorphism of Plutons

“…Higgins [42] proposed Ostwald ripening, that is, coarsening driven by minimization of surface free energy, but this is unlikely if recrystallization was driven only by surface free energy [45,46]. However, the assertion that crystal coarsening does not happen in coarsely crystalline materials because Ostwald ripening is ineffective (e.g., [47,48]) is clearly contradicted by diverse studies in materials science including petrology (e.g., [49][50][51][52]).…”

The Merensky Cyclic Unit, Bushveld Complex, South Africa: Reality or Myth?

“…It is important to note that within any CSD study, a wide range of kinetic and equilibration processes contribute to the final texture, and the effects of these often competing processes must be considered during the interpretation of CSD patterns (e.g., [58,62]). Detailed discussions on processes that form and modify rock textures can be found in several publications (e.g., [2,5,54,61,[63][64][65] and references therein). Briefly, these have been broadly split into primary, secondary and tertiary processes by Holness, Vukmanovic and Mariani [5].…”

“…However, experimental data suggest that Ostwald ripening can only account for micron scale variations in igneous systems [69,70] and thus would not significantly affect CSD plot shapes. Instead, many other processes can contribute to a downturn at small crystal sizes, including those of nucleation, growth and equilibration [64]. Interpreting equilibration processes in CSD plots is best understood through the application of the Communicating Neighbours (CN) model [71,72], which infers growth rates are not only dependent on crystal size, but are also affected by individual crystal characteristics and the position of the crystal with respect to its neighbouring crystals [58,63].…”

“…The feldspathic orthopyroxenite is characterised by subhedral to euhedral orthopyroxene with many crystals displaying planar growth faces (Figures 6c,d and 9a-c). Some crystals or portions of crystals display curved faces (Figure 6c,d), which are indicative of textural modification during equilibration, as a means of minimising the surface energy of the crystal [54,58,64,65]. All samples, except for EJH/15/069A, have a similar range of grain sizes and Lmax values (Table 1); however, the C i values (Table 1) are variable around the chromitites.…”