CO ₂ diffusion in dry and hydrous leucititic melt

Using the diffusion couple technique, diffusion of COinline-formula₂ in a leucititic melt from the Colli Albani Volcanic District in Italy was investigated at temperatures between 1200 and 1350 inline-formula^∘C in an internally heated pressure vessel at 300 MPa. To examine the effect of dissolved Hinline-formula₂O in the melt, experiments were performed for a nominally dry melt (0.18 inline-formula± 0.03 wt % Hinline-formula₂O) and for a hydrous melt containing 3.36 inline-formula± 0.28 wt % Hinline-formula₂O. Diffusion experiments were run for 40 to 120 min and terminated by rapid quench. COinline-formula₂ concentration profiles were subsequently measured via attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and fitted with error functions to obtain individual diffusion coefficients.

For the anhydrous and hydrous sample series, seven diffusion coefficients were determined each. Diffusivity was found to increase exponentially with temperature for both melts following an Arrhenius behaviour. The Arrhenius equation for the nominally dry leucititic melt is described by

where inline-formula $M12inlinescrollmathml{D}_{chem{\mathrm{normal\; CO}}_{\mathrm{normal\; 2}}}$ 25pt14ptsvg-formulamathimge119701c34e8c78c02b664699426a72a ejm-35-117-2023-ie00001.svg25pt14ptejm-35-117-2023-ie00001.png is the diffusion coefficient in minline-formula² sinline-formula⁻¹ and inline-formulaT is the temperature in K. In the experimental temperature range, Hinline-formula₂O has an accelerating effect on COinline-formula₂ diffusion. At 1200 inline-formula^∘C, diffusivity increases from 1.94 inline-formula× 10inline-formula⁻¹² minline-formula² sinline-formula⁻¹ in the dry melt to 1.54 inline-formula× 10inline-formula⁻¹¹ minline-formula² sinline-formula⁻¹ in the hydrous melt. The Arrhenius equation for the leucititic melt containing inline-formula3.36±0.28 wt % Hinline-formula₂O is given by

The activation energies for COinline-formula₂ were determined to be 275 inline-formula± 47 kJ molinline-formula⁻¹ for the anhydrous melt and 209 inline-formula± 58 kJ molinline-formula⁻¹ for the hydrous melt.

The high COinline-formula₂ activation energy in the leucititic melt indicates that the diffusion might be partly attributed to the carbonate species. At high magmatic temperatures above 1200 inline-formula^∘C, COinline-formula₂ diffusivity in the leucititic melt is only slightly lower than COinline-formula₂ diffusion in rhyolitic and basaltic melts, suggesting that COinline-formula₂ diffusion in natural melts is relatively independent from the bulk melt composition at such temperatures. COinline-formula₂ diffuses slower than other volatile components such as halogens and Hinline-formula₂O in depolymerized silicate melts. Thus, a fractionation of volatiles can occur during magma ascent and degassing. The experimental data on COinline-formula₂ diffusion can be used for modelling the degassing mechanisms of ultrapotassic mafic melts.