Multicomponent diffusion in the molten system K2O-Na2O-Al2O3-SiO2-H2O

We have measured multicomponent chemical diffusion coefficients in a melt near to the low pressure water-saturated eutectic granite composition in the system K2O-Na2O-Al2O3-SiO2-H2O at 1.0 GPa and temperatures of 1300 and 1600°C. The measured diffusion profiles can be accounted for within the analytical error by diffusion coefficients, which are not dependent on composition within the range of compositions accessed by our experiments. The multicomponent diffusion coefficient matrix [D] had a highly degenerate set of real, positive eigenvalues that show a regular relation to melt viscosity on an Arhenius diagram. The smallest eigenvalue is that associated predominantly with Si-Al exchange. The larger two eigenvalues are those associated with Si-Na and Si-K exchange and are effectively degenerate, with the result that exchanges of alkalis for silica or for each other can proceed in pseudo-binary fashion without inducing fluxes of other components. The eigenvalue associated with H-Si exchange is smaller than the alkali-silica eigenvalues, but analytical uncertainties make it also effectively degenerate with the alkalis. Uphill diffusion, notably of water and alkalis, was observed in several experiments, and this would lead to transient partitioning of water and alkalis across diffusion interfaces showing large Al2O3 concentration gradients. Such partitioning in natural systems would persist until Al concentration gradients were erased by continued, much slower Al-Si interdiffusion

Numerical modelling of stress generation and microfracturing of vesicle walls in glassy rocks

In the absence of stress-concentrating flaws such as microfractures, vesicular glassy materials can withstand gas pressures within vesicles in excess of 100 MPa; however, vesicles within such materials are known to decrepitate explosively at much lower internal gas pressures, both in natural systems and in the laboratory. Here we present a model that quantitatively predicts the generation of microfractures in vesicle walls during cooling. Cooling of gas-bearing vesicles in glassy rock has little effect on water solubility in the glass, but leads to a rapid decrease in gas pressure in the vesicles. The drop in pressure causes disequilibrium between the water in the glass and in the vesicle. Dehydration of the glass in a diffusive boundary layer around the vesicle leads to elastic shrinkage. The resulting strain generates large tensile tangential stresses which can exceed the strength of the glass, causing microfracturing. Such microfractures present a possible means by which glassy rock surrounding vesicles could be weakened enough to permit explosive decrepitation at low pore vapor pressures. The results have implications wherever hydrous vesicular glasses are formed. For example rocks formed in shallow subvolcanic intrusions or vent plugs may spontaneously disintegrate with explosive emission of vapor; glassy submarine lavas spontaneously decrepitate upon dredging from the ocean floor ("popping rock"); vesicular glasses produced in laboratory experiments investigating vapor-melt phase equilibria have been observed to contain abundant fractures surrounding vesicles and to dehydrate at anomalously high rates