Influence of water on granite generation: Modeling and perspective

Abstract

Water influences the physics and chemistry of magmatic differentiation because it reduces the melting point, decreases melt viscosity, modifies phase equilibria, and controls how latent heat is released. Here, we explore how compositional trends of fractionating magmas can be used to evaluate the amount of water in the crystallizing system. Of interest is the role of water in generating silicic magmas such as granites and rhyolites. Water expands the crystallization window over which silicic melts are stable, such that granites are limited to small residual melt fractions (F < 0.2) under dry conditions but stable over a wider range of residual melt fractions (F < 0.6) at high water contents. Variations of SiO2 versus F are relatively insensitive to oxygen fugacity and can be used as a hygrometer for granitoids, where F is estimated through relative enrichments in incompatible elements, such as Th and K. We also show that generation of granitoids in arc environments follow distinctly different Mg#-SiO2 trends compared with anorogenic or intra-plate granites. We confirm that arc magmas differentiate at higher water contents and possibly higher oxygen fugacities than anorogenic/intra-plate magmas. Finally, Archean TTGs (tonalite, trondhjemite, and granodiorite) show similar Mg#-SiO2 systematics as Phanerozoic arc-related granitoids, suggesting similar petrogenetic physical and chemical conditions.