Transport properties of silicate melts

Abstract

A quantitative description of the transport properties, diffusivity, viscosity, electrical, and thermal conductivity, of silicate melts is essential for understanding melting-related petrologic and geodynamic processes. We here provide a systematic overview on the current knowledge of these properties from experiments and molecular dynamics simulations, their dependence on pressure, temperature, and composition, atomistic processes underlying them, and physical models to describe their variations. We further establish phenomenological and physical links between diffusivity, viscosity, and electrical conductivity that are based on structural rearrangement in the melt. Neutral molecules and network-modifying cations with low electric field strength display intrinsic diffusivity, which is controlled by the intrinsic properties (size and valence) of the species. By contrast, oxygen and network formers with high field strength show extrinsic diffusivity, which is more sensitive to extrinsic parameters including temperature (T), pressure (P), and melt composition (X). Similar T-P-X dependence of diffusivity and electrical conductivity and their quantitative relation reveal the role of intrinsically diffusing species in electrical transport, while viscosity is tied to the extrinsically diffusing species in a similar way. However, the differences in the structural role and mobility of various atomic species diminish with increasing temperature and/or pressure: all transport processes are increasingly coupled, eventually converging to a uniform rate and mechanism. Accurate comprehension of interatomic interactions and melt structure is vital to fully accounting for the compositional dependence of transport properties, and simple polymerization parameters such as nonbridging oxygen per tetrahedrally coordinated cation are inadequate.

Structural changes and anomalous self‐diffusion of oxygen in liquid iron at high pressure

“…Beyond the range of the experiments (P ≳ 25 GPa), however, calculated D O values decrease with pressure. Our findings are similar to those reported for several silicate melt compositions where anomalous maxima in Si and/or O self-diffusivities-or complimentary localized minima in melt viscosity-have been reported to correspond to an increase in Si and/or Al coordination [Reid et al, 2001[Reid et al, , 2003Liebske et al, 2005;Wang et al, 2014;Ni et al, 2015].…”

Melting Inside the Tibetan Crust? Constraint From Electrical Conductivity of Peraluminous Granitic Melt

“…Sodium has been identified to be the predominant charge carrier in granitic melts (Gaillard, ; Ni et al, ). Electrical conductivity of granitic melt is therefore mainly controlled by the concentration and mobility of Na.…”

“…Electrical conductivity of melts is useful for interpreting MT data and constraining the melt fractions in inferred molten zones. Melt conductivity is sensitive to variations in melt composition including H 2 O content (Ni et al, ). Previous estimations of melt fraction were often based on presumed melt conductivity (3–10 S/m) without H 2 O constraint (Bai et al, ; Unsworth et al, ).…”

Atmosphere-Interior Exchange on Hot, Rocky Exoplanets

“…The pool thermal boundary layer grows diffusively for as long as magma takes to move from the substellar point to the pool edge. Therefore δ T ≈ κ T L/Ξv p , where κ T is the diffusivity of heat (which we take to be the molecular thermal diffusivity, 5 × 10 −7 m 2 s −1 ; Ni et al (2015)), and Ξ ("ageostrophic flow fraction") is the scalar product of the magmavelocity unit vector and a unit vector that is directed from the center to edge of the pool. Now we can replace ∇P with g ρ l L −1 κL/Ξv p and solve for thermalflow timescale τ T :…”