Behavior of Alkalies during Diffusive Interaction of Granitic Xenoliths with Basaltic Magma

Abstract

A series of three experiments was made at ∼125°C and 10 kbar in which a 1-gram reservoir of molten oceanic tholeiite was maintained in contact with one end of a 6.5 mm diameter cylinder of partially-molten, fine-grained granite. Detailed microprobe analysis (450 spots total) of the quenched samples revealed progressive basalt/granite interaction as run duration increased from 0.75 to 8 to 24 hours. Interdiffusion of melt components is limited for structure-controlling species such as and and for divalent cations, which all mix over a zone less than 0.5 mm wide in 24 hours at run conditions. In contrast, K and Na are extremely mobile, resulting in considerable uptake of K by the basalt melt and eventual loss of Na from the basalt to the granite by "uphill" diffusion. The chemical diffusivity of K in the basalt can be accurately estimated at for the specific experimental conditions used. Although this value is somewhat lower than previous estimates K of chemical and tracer diffusivity in molten basalt, it is nevertheless high enough to cause significant K contamination of the basalt well beyond the mixing zone. Sodium mobility is clearly much higher than that of K: in the 24-hour run, the basalt reservoir reached a state of near-uniform depletion in by 30% relative to the initial value of 3.0 wt %. The migration of Na into the already Na-rich granite partial melt indicates that the activity coefficient of this element is lower in silicic melts than in basic ones, and that Na depletion (combined with uptake) may be expected as basaltic magmas ascend through the continental crust.