Diffusion in magmas at depth in the Earth: The effects of pressure and dissolved H2O

Details Diffusion in magmas at depth in the Earth: The effects of pressure and dissolved H2O Diffusion in magmas at depth in the Earth: The effects of pressure and dissolved H2O

Feb 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981e%26psl..52..291w&link_type=abstract

Earth and Planetary Science Letters, Volume 52, Issue 2, p. 291-301.

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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 H2O 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 H2O 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.% H2O, and Ca diffusivity increases by 2.5 orders of magnitude. Sodium diffusion, on the other hand, is relatively immune to changes in H2O 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 H2O content, so that models of rate-controlled processes in hydrous magmas may be grossly in error if 1-atm diffusivity values are used.

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