Chemical Diffusion in Magmas: An Overview of Experimental Results and Geochemical Applications

Abstract

The subject of chemical diffusion in magmas has attracted the interest of penologists and geochemists seeking to place time constraints on phenomena ranging from magma mixing to crystal growth. Experiments have been devised to examine chemical diffusion effects during such processes as interdiffusion of two liquids, growth and dissolution of crystals, exchange of halogens with oxygen in the air, reduction or oxidation of dissolved iron oxide, and introduction of dissolved volatiles. A few experiments have even been done using a temperature gradient to induce thermal migration.

Many of the studies carried out to date have incorporated variations in temperature, pressure, and dissolved H₂O content, so the collective results allow diffusivities in magmas to be estimated quite well for most geologically- realizable conditions. In general, the following major characteristics appear to hold:

• (1) Network-forming species, most notably SiO₂, are the slowest-moving magmatic components, although network-modifiers that form stable complexes in the melt may be equally sluggish;

• (2) alkalies, divalent cations, oxygen, and fluorine are the most mobile magmatic components when their transport is not rate-limited by counterdiffusion of slower species; and

• (3) the effect of H₂O content on chemical diffusion of most components (includ- ing H₂O itself) is extremely large, sometimes amounting to several orders of magnitude at crustal melting conditions.