KINETIC APPROACH TO THE CRYSTALLIZATION OF MAGMATIC MELTS

Abstract

Igneous rocks exposed at the day surface are dominated by two groups: gabbro–basalt and granite–rhyolite. This is typically explained in the following way: the melting of the Earth’s mantle and crustal materials primarily generates the lowest temperature melts with the eutectic-type composition: (i) pyroxene–plagioclase (gabbro–basalt group); (ii) quartz–plagioclase–alkali feldspar (K-Fsp) ± eutectic mafic mineral (granite–rhyolite group). The melt evolves during its ascent from the magma generation area to the crystallization site. Consequently, the melt composition becomes closer to eutectic at the beginning of its crystallization within the Earth’s crust or on its surface. Therefore, the crystallization of such melts is typically analyzed using phase state–temperature equilibrium diagrams, which indicate that cooling of the eutectic melt depending on heat loss should yield either glassy (vitrophyric) or equigranular rock. Noneutectic melts produce a unequigranular or porphyritic texture with (or without) the glass. The diagrams also show that the number of phenocrysts must always be one unit less than the number of components. In these diagrams, the composition of crystallizing minerals is accepted to be constant.