Chemical diffusion of major components in granitic liquids: Implications for the rates of homogenization of crustal melts

Crustal anatexis, coupled with melt extraction and ascent, is the main mechanism by which the continental crust differentiates. During diffusion-controlled melting, and in the absence of melt flow and mechanical mixing, diffusion is the main process by which the melt phase ultimately homogenizes and mineral residuum and melt equilibrate. Knowing the diffusive properties of the elements within a granitoid melts is, therefore, essential for understanding the kinetics and mechanisms of melting, as well as the compositions and nature of chemical heterogeneities in granites. In this contribution we review a series of experimental studies aimed at an improved understanding of the nature and rates of chemical diffusion of major components of H2O-saturated granite melts at anatectic conditions in the continental crust. The hydrous granite system under investigation makes up ≈ 95–99% of natural, restite-free granitic magmas. These experimental results have applications for predicting the composition of granitic liquids during processes such as anatexis, magma mixing, assimilation or crystallization. Regarding the diffusive properties, the major elements of the liquid can be classified into two categories: those whose diffusion is uncoupled with other elements (e.g. Si); and others whose diffusion is highly coupled with the movement and gradients of other ions (e.g. Na and K with H; Na, K, Ca and H with Al). One implication of coupled diffusion is that chemical gradients in fast-diffusing ions, such as Na and K, may persist for long times in the melt due to their chemical coupling with slow-diffusing components, such as Al. The diffusion of components through granitic melts entails two mechanisms. “Local diffusion” corresponds to the classical view of chemical diffusion and refers to the relative and random motion of an ion (e.g. Si, Al) under apparently local gradients in chemical potential. “Field diffusion” corresponds to the long-range and coordinated migration of an ionic species (e.g. Na), driven by a long-range chemical potential gradient created by other melt components with which it associates due to coupled diffusion. This mechanism serves to erase compositional gradients at rates that are orders of magnitude greater than those by local diffusion. This is because the coordinated movement requires individual ions to move only a small fraction of the entire distance over which the chemical potential gradient is established, in order to achieve diffusion over distances that may span the entire liquid system. The significance and implications of major element diffusivities and the occurrence of coupled diffusion and field diffusion in granitic liquids, are highlighted by calculations of the rates of melt homogenization during anatexis of crustal protoliths.