PORE SIZE AS A DETERMINANT FOR SOLIDIFICATION MICROSTRUCTURES

The enormous variety of microstructures formed by the solidification of melt-filled pockets in migmatites is mainly controlled by the time-scale of metamorphism. For example, long timescales (e.g. regional vs. contact) permit melt migration and amalgamation into large pockets that crystallise with a coarse grainsize. However, at constant timescales the size of the melt-filled pocket is the controlling variable for microstructure.

The concentration of solute, C, in equilibrium with a spherical crystal of radius r is given by 2γ/r = (RT/V)ln(C/C_o) where γ is the energy of the solid-liquid interface, R the gas constant, T the temperature, V the molar volume of the solid phase, and C_o is the solubility of a liquid in equilibrium with an infinitely large spherical crystal or a planar interface. The thermodynamics of crystallisation in pores is thus different from that in a free fluid: a confined fluid can become more supersaturated before crystallisation begins compared to an unconfined fluid of the same composition. Crystallisation in a liquid-bearing rock with a range of pore sizes may not occur at the point predicted by the phase diagram: nucleation and growth will begin in the largest pores, but will be delayed (or even prevented) in the smallest pores.

Observation of the effect of pore size necessitates fixing the variables time and composition. The quartzo-feldspathic rocks from the gneissic aureole of the Rum Central Complex display a range of microstructures, from igneous-looking textures with euhedral early-formed plagioclase enclosed by interstitial oikocrystic quartz, to highly cuspate, grain boundary interserts with low dihedral angles (melt pseudomorphs). These microstructures occur within 1cm of each other and are directly linked to pore size. The largest pores begin to solidify first and, if sufficiently large to permit unimpeded growth of early-nucleating phases, will result in an igneous texture. The smallest pores retain melt for longer, permitting a greater approach to melt-solid textural equilibrium and a more cuspate shape. Geothermometry using Ti in quartz suggests that the nucleation delay for the smallest pores may be of the order of 100˚C in rocks from the Rum aureole. This may provide an unforeseen opportunity for high mass transport in migmatites.