Noble Gas Diffusion in Silicate Glasses and Melts

Abstract

Studies of noble gas diffusion and solubility provide information on the internal structure of silicate glasses, i.e., the size distribution and interconnection of cavities within the framework (Doremus 1994). Additionally, such diffusion data are of interest for understanding degassing kinetics of glasses and melts. Water and carbon dioxide, the most important volatiles in magmas, can be present in silicate melts both as unreacted (molecular) species and as dissociated species (hydroxyl groups or carbonate groups, respectively) (e.g., Holloway and Blank 1994; Kohn 2000). The contributions of the different species to the overall transport of the volatiles are difficult to separate. In the case of rhyolitic melts it was found that the diffusivity of Ar is similar to that of molecular CO2 (Watson 1994; Behrens and Zhang 2001). Assuming that this similarity is a general property for silicate melts, Nowak et al. (2004) analyzed the diffusion behavior in synthetic rhyolitic to basaltic melts and estimated the relative abundance of carbonate and molecular CO2 in the melt. Knowledge of noble gas diffusivity is also of importance for interpretation of geochemical findings both on short and on large scales. For instance, the isotopic compositions of noble gases have been extensively used to identify long-lived heterogeneities within the Earth’s mantle (Allegre et al. 1986; Graham 2002). The noble gases He, Ne and Ar are highly incompatible elements, which will be preferentially incorporated into a melt during melting of the mantle, so at partial melt fractions relevant to mid-ocean ridges a primary melt will inherit the relative noble gas abundance pattern of the mantle source (Burnard et al. 2004). Helium, on the other hand, is a geochemical tracer for mantle signatures in rocks. Measurements of both helium abundance and the 3He/4He ratios in volcanic rocks, gases and waters …