Constraining noble gas abundance and isotopic diffusion in silicate liquids
Pete Burnard and Julien Amalberti
Volatile components play a fundamental role in the ascent of magmas to the Earth’s surface and in the generation of explosive eruptions. The solubility of a volatile component into a silicate melt — that is, the maximum amount that can be dissolved into the melt phase — increases with increasing pressure. Thus at the high pressures prevailing in magma sources or deep magma chambers beneath volcanoes, silicate melts contain large amounts of volatiles. Pressure decrease during magma ascent to the Earth’s surface causes volatile exsolution and the formation of gas bubbles: volcanic eruptions are powered by the tremendous expansion that results from volatile exsolution, and the eruptive style is controlled by the kinetics of magma vesiculation (bubble nucleation and growth). Because they are inert and have systematically varying physical properties (solubilities, diffusivities), the noble gases are excellent tracers of the processes of volatile exsolution and migration during magmatic processes. However, in order to be able to apply this tool to natural systems, we first need to constrain their basic physical behaviour in well constrained laboratory experiments.
We are currently undertaking a series of experiments to constrain diffusivities of the noble gases in silicate glasses and liquids, in collaboration with the Experimental Petrology lab at CRPG and with the Laboratoire Magmas et Volcans, Clermont-Ferrand. Samples are synthesised and doped with noble gases either in controlled atmosphere (1 bar) vertical furnaces or in piston cylinder presses. Analyses are performed by stepped heating, total fusion (infra red laser) or laser ablation (UV laser).
This project is financed by the “DEGAZMAG” project, Agence National de la Recherche.
Example of a diffusion measurement in a silicate liquid. A bead of silicate liquid approximately 2mm in diameter is suspended in a He-enriched atmosphere at high temperature for a carefully timed period (here, 1350 °C, 15 minutes); this induces a diffusion profile in the bead (with a concentration at the rim equal to that of the He solubility and falling to zero at the center of the bead). After quenching, the bead is sectioned and analysed by laser abalation in the noble gas lab (photo on the right). The He diffusivity under these conditions (1 bar, 1350 °C) can be estimated from the error function of the diffusion profile (graph, left).