Variation of the chemical diffusivity of oxygen and viscosity of an andesite melt with pressure at constant temperature

Abstract

The chemical diffusivity of oxygen and the viscosity of an andesite melt have been measured at pressures from 3.5 to 20 kbar and 1350°C. Oxygen chemical diffusivity and melt viscosity vary antithetically as a function of pressure. The melt viscosity increases from 1 bar to 3.5–5 kbar, passes through a maximum, and then decreases with further increase in pressure to at least 20 kbar. The chemical diffusivity of oxygen decreases from 1 bar to 5 kbar, passes through a minimum, increases from 5 to 10 kbar, and then remains essentially constant with increasing pressure. In all cases the chemical diffusivity of oxygen is larger than that predicted by the Eyring equation. The results, when combined with data for basalt melts, show that the relationship between the chemical diffusivity of oxygen and melt viscosity is not adequately expressed by the Eyring equation. The data suggest that there are three different relationships between oxygen chemical diffusivity and melt viscosity and that those relationships occur over specific viscosity ranges: (1) at high viscosity the chemical diffusivity of oxygen is related to melt viscosity by the Eyring equation; (2) at intermediate viscosities diffusivity increases more rapidly than the Eyring equation predicts; and (3) at low viscosity the chemical diffusivity of oxygen is constant and independent of melt viscosity. A model is presented that predicts oxygen chemical diffusivities from melt viscosities. The model diffusivities suggest that oxygen chemical diffusion is comparable in magnitude to divalent cation diffusivities in low-viscosity melts, but that oxygen diffuses much more slowly than do cations in high-viscosity melts.