Basalt contamination by continental crust: Some experiments and models

Abstract

Chemical interaction between molten basalt and felsic minerals of the continental crust (quartz, K-feldspar, and oligoclase) was examined in static and dynamic experiments at 1,200°–1,400° C. Under circumstances of continuous stirring at 1,400°, β-quartz dissolves in tholeiite melt at a rate of 3.3×10⁻⁶ g/s per cm² of contact area; at 1,300°, the solution rate is 1.5×10⁻⁶ g/cm{cm2}s. The feldspars are molten at the experimental conditions, and interact with contacting basalt melt by diffusion in the liquid state. This is a complex process characterized by rapid initial diffusion of alkalies to establish a distribution between felsic melt and basalt similar to that observed in cases of actual two-liquid equilibrium (both alkalies reach concentrations in the felsic melt 1.5–3 times those in the basalt). Alkali diffusion may be “uphill” or “downhill”, depending on which direction of net flux is required to produce a two-liquid type distribution. Once this distribution is attained, subsequent diffusion of all melt species is slow and apparently limited by the diffusivity of SiO₂, which is 10⁻⁹-l0⁻¹⁰ cm²/s at 1,200° C. Interdiffusion experiments involving molten basalt and synthetic granite confirm the behavior illustrated by the feldspar/basalt results, and give similar SiO₂ diffusivities.

The solution rates and interdiffusion data can be used to model basalt contamination processes likely to occur in the continental crust. For the restricted case of solid quartzitic xenoliths, the uptake of SiO₂ in a well-mixed basalt magma is quite fast: appreciable SiO₂ contamination may occur over exposure times of only days to years. If basalt magma induces local melting of crustal rocks, the assimilation process becomes one of liquid-state interdiffusion. In this case, the varying diffusivities of ions and their differing preferences for silicic relative to basaltic melts can produce marked selective contamination effects. Selective contamination of ascending basaltic magmas is particularly likely in the case of K₂O, which may be introduced in substantial amounts even when other elements remain unaffected. The Na₂O content of mantle-derived magmas is buffered against contamination by crustal materials, and K₂O is buffered against further increases once it reaches a level of 1–1.5 wt.%.