Tracer diffusion in granitic melts: Experimental results for NA, K, RB, CS, CA, SR, BA, CE, EU to 1300°C and a model of calculation

Abstract

Tracer diffusivity of Na, K, Rb, Cs, Ca, Sr, Ba, Eu, and Ce has been measured in natural and dehydrated obsidians in the range 300°–1300°C, 0–4 kbar, using radiotracers and an etching technique. The results complement and extend previous results on similar material. Diffusivity ranging from 10 −16 to 10 −7 cm 2 /s have been observed. An Arrhenius law is observed for all species, with activation energies E ranging from 20 to 120 kcal/mol. A strong correlation of E with ionic radius and squared formal charge (Z 2 ) offers guidelines for model calculation of the activation energy. A mechanistic model of transport through an elastic medium is adapted to the present data. It permits a fair prediction of the activation energy as a function of r and Z according to E = 128(r−1.34) 2 + 33 Z 2 /(r+1.34) + 8 kcal/mol, with a misfit comparable to the experimental uncertainty. The correlation between E and the preexponential factor D o (compensation law) is discussed within the framework of the proposed model. The major conclusion is that local configuration (size, charge, coordination, etc.) is the major control for cationic diffusivity and not the properties of the matrix, such as chemical composition, viscosity, etc.