Diffusion in magmas at depth in the Earth: The effects of pressure and dissolved H 2O

Abstract

Experimental data on cation diffusion in silicate melts and glasses at atmospheric pressure cannot be quantitatively applied to diffusion in magmas at depth because pressure and dissolved H 2O have significant effects. Diffusivities of Ca and Cs in soda-lime aluminosilicate melt are reduced by roughly an order of magnitude as pressure increases from 1 to 30 kbar at 1100°C (in the case of calcium, the pressure effect is smaller at higher temperatures). The influence of dissolved H 2O is opposite to and (for some ions) far more important than that of pressure: at 700°C, Cs diffusion in granitic obsidian is enhanced by four orders of magnitude by the addition of 6 wt.% H 2O, and Ca diffusivity increases by 2.5 orders of magnitude. Sodium diffusion, on the other hand, is relatively immune to changes in H 2O content. The retarding effect of pressure on diffusion in deep-seated magmas is largely compensated by higher magmatic temperatures, such that overall “depth” effects on diffusion are of secondary importance — diffusion rates in mantle melts are probably well approximated by 1-atm, near-liquidus values. Diffusion rates in crustal magmas, however, are extremely sensitive to H 2O content, so that models of rate-controlled processes in hydrous magmas may be grossly in error if 1-atm diffusivity values are used.