Separation of a liquid from the partially melted solid residue is a form of diversification because it involves partitioning and separation of chemical constituents, and it can produce a variety of melt compositions from a single source.
When a rock begins to melt, a tiny fraction of initial melt forms discrete liquid drops at the junctions of mineral grains, usually at the points where three or four grains meet. only when a critical quantity of melt is produced will there be a sufficient liquid volume that –
- The liquid forms an interconnected network.
- The interior body of the liquid can be free from the restraining effects of crystal surface adsorption.
Only when these two conditions meet, some of the melt can be separable from the solid.
The critical melt fraction required to from an interconnected network depends upon the dihedral angle (θ), formed between two solid grains and the melt.
When the interfacial energy (surface tension) of the melt is similar to that of the minerals, the dihedral angle is low, and the melt forms an interconnected network at a low melt fraction. If θ < 600 the melt may form a network with as little as ~1% melt. As the dihedral angle increases above 600, the amount of melt required to establish connectivity also increases. In mafic systems, θ has been found to be less than 500, allowing very small melt fractions to be extractable. Some experimental evidences suggests that rhyolitic melts have higher angles but still in the range of 50-600.
Separation of melt :
Separation of a melt may require higher melt than indicated by dihedral angle and permeability threshold, because of different viscosities of different melts. The critical melt fraction or rheological critical melt percentage (RCMP), is the percentage of melt at which a crystal dominated, more rigid granular framework gives rise to a melt dominated, fluid suspension, commonly called a crustal mush. (Theoretical system of spheres, the RCMP is 26%, but for more irregular shapes and sizes, this amount may vary between 30-50% for static situations involving viscous granitic composition).
Liquid separation is commonly started by gravitational effect as the buoyant liquid seeks to rise. As the melt sources are generally deeper and under pressure, the separation may be aided by filter pressing or compaction, in which the crystal-liquid system is squeezed like a sponge. For systems being deformed or compacted the RCMP drops from the theoretical value of 26%.
- Classic models based on dihedral angles are of limited use because of other factors like temperature, pressure, viscosity, composition, volatile content, and interfacial energy is also anisotropic. RCMP ranges from 1-7% for basalt/ peridotite and may be considerably higher 15-30% for more viscous silicic magmas.
References :
- Lecture Notes in Earth System Sciences, Metasomatism and the Chemical Transformation of Rock (pp.535-598).
- JD, winter, Principles of igneous and metamorphic petrology
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