Diffusive equilibration between hydrous metaluminous-peraluminous haplogranite liquid couples at 200 MPa (H₂O) and alkali transport in granitic liquids

Abstract

This study examines the systematics and rate of alkali transport in haplogranite diffusion couples in which a chemical potential gradient in Al is established between near water-saturated metaluminous and peraluminous liquids that differ only in their initial content of normative corundum. At 800°C, measurable chemical diffusion of alkalis occurs throughout the entire length (∼1 cm) of the diffusion couples in 2–6 h, indicating long range diffusive communication through melt. Alkali transport results in homogenization of initially different Na/Al and ASI [=mol. Al₂O₃/(CaO + Na₂O + K₂O)] throughout the couples within ∼24 h, whereas initially homogenous K* evolves to become uniformly different between metaluminous and peraluminous ends. Calculated effective binary diffusion coefficients for alkalis in experiments that do not significantly violate the requirement of a semi-infinite chemical reservoir (0- to 2-h duration at 800°C) are similar to those observed in previous studies: in the range of (1–8) × 10⁻¹² m²/s. Such a magnitude of diffusivity, however, is inadequate to account for the observed changes of alkali concentrations and molecular ratios throughout the couples in 2- to 6-h experiments. The latter changes are consistent with diffusivities estimated via the x ² = Dt approximation, which yields effective values around 10⁻⁹ m²/s. These observations suggest that Fick’s law alone does not adequately describe the diffusive transport of alkalis in granitic liquids. In addition to simple ionic diffusion associated with local gradients in concentration or chemical potential of the diffusing component described by Fick’s second law (local diffusion), alkali transport through melt involves system-wide diffusion (field diffusion) driven by chemical potential gradients that also include components with which the alkalis couple or complex (e.g., Al). Field diffusion involves the coordinated migration of essentially all alkali cations, resembling a positive ionic current that drives the system to a metastable state having a minimum energy configuration with respect to alkali distribution. The net result is effective transport rates perhaps three orders of magnitude faster than simple local alkali diffusion, and at least seven to eight orders of magnitude faster than the diffusive equilibration of Al and Si.