THE IMPRINT OF BASALT ON THE GEOCHEMISTRY OF SILICIC MAGMAS

We propose that the fluid mechanics of magma chamber replenishment lea ds to a novel process whereby silicic magmas can acquire an important part of their chemical signatures. When flows of basaltic magma enter silicic magma chambers, they assume a 'fingered' morphology that creat es a large surface area of contact between the two magmas. This large surface area provides an opportunity for significant chemical exchange between the magmas by diffusion that is enhanced by continuous flow o f silicic liquid traversing the basalt through thin veins. A quantitat ive analysis shows that a basaltic magma may thereby impart its trace- element and isotopic characteristics to a silicic magma. Depending on concentration differences and diffusion coefficients for the given com ponents, this new mechanism may be as important as crystal fractionati on and assimilation in producing the compositional diversity of silici c magmas. It may explain concentration gradients in silicic ash-flow t uffs and should be considered when interpreting the isotopic signature s of silicic rocks, even in the overt absence of mixing. For example, we show that, for several well studied, compositionally graded ash-flo w tuffs, the concentrations and isotopic ratios of important geochemic al tracers such as strontium could be largely due to this flow-enhance d diffusion process. (C) 1998 Elsevier Science B.V. All rights reserve d.