Carbon dioxide and argon diffusion in silicate melts: Insights into the CO 2 speciation in magmas

Abstract

To investigate the influence of temperature and composition on the diffusivities of dissolved carbon dioxide and argon in silicate melts, diffusion experiments were performed at magmatic pressure and temperature conditions in (a) albite melts with excess Na 2O (0–8.6 wt%) and a constant Si/Al ratio of 3, and (b) albite 70quartz 30 to jadeite melts with decreasing SiO 2 content and a constant Na/Al ratio of 1. We obtained diffusion coefficients at 500 MPa and 1323–1673 K. In the fully polymerized system Ab 70Qz 30 – Jd, the change in composition only has a weak effect on bulk CO 2 diffusivity, but Ar diffusivity increases clearly with decreasing SiO 2 content. In the system Ab + Na 2O, bulk CO 2 and Ar diffusivity increase significantly with gradual depolymerisation. The relatively small change in composition on molar basis in the depolymerized system leads to a significantly larger change in diffusivities compared to the fully polymerized Ab 70Qz 30–Jd join. Within error, activation energies for bulk CO 2 and Ar diffusion in both systems are identical with decreasing silica content (Ab + Na 2O: 159 ± 25 kJ mol −1 for bulk CO 2 and 130 ± 8 kJ mol −1 for Ar; Ab 70Qz 30–Jd: 163 ± 16 kJ mol −1 for bulk CO 2 and 148 ± 15 kJ mol −1 for Ar) even though this results in depolymerisation in one system and not the other. Although there is a variation in CO 2 speciation with changing composition as observed in quenched glasses, it has previously established that this is not a true representation of the species present in the melt, with the ratio of molecular CO 2 to carbonate decreasing during quenching. Thus, diffusion coefficients for the individual CO 2 species cannot be directly derived by measuring molecular CO 2 and CO 3 2- concentration-distance profiles in the glasses. To obtain diffusivities of individual CO 2 species, we have made two assumptions that (1) inert Ar can be used as a proxy for molecular CO 2 diffusion characteristics as shown by our previous work and (2) the diffusivity of CO 3 2− can be calculated assuming it is identical to network forming components (Si 4+ and Al 3+). This is derived from viscosity data (Eyring eqn.) and suggests that CO 3 2− diffusion would be several orders of magnitude slower than molecular CO 2 diffusion. The systematics of measured bulk CO 2 diffusivity rates and comparison with the Ar proxy all suggest that the faster molecular CO 2 species is much more dominant in melts than measurements on resulting quenched glasses would suggest. This study has confirmed an observation of surprisingly consistent bulk CO 2 diffusivity across a range of natural compositions were Ar diffusivity significantly increases. This is consistent with an actual increase in molecular CO 2 mobility (similar to Ar) that is combined with an increase in the proportion of the slower carbonate in the melt. These results demonstrate that the CO 2 diffusion and speciation model provides an insight into the transport processes in the melt and is promising and an alternative tool to in situ speciation measurements at magmatic conditions, which at the moment are technically extremely difficult. We present the first high pressure high temperature in situ MIR spectra of a CO 2 bearing albitic glass/melt suggesting that molecular CO 2 is a stable species at high temperature, which is qualitatively consistent with the modelled CO 2 speciation data.