Melt infiltration and advection in microdioritic enclaves

Geochemical investigations of microdioritic enclaves in acid plutons have shown that extensive chemical and isotopic exchange can occur between enclave and host magma. Although some estimates of the rates of chemical exchange have been established, most of these models assume that diffusion is the mechanism of elemental transport and exchange. We present here a simple physical model for chemical exchange between host magma and enclave where melt infiltration and advection are the dominant transport mechanisms. For this to happen, the enclave itself must be incompletely crystallised and thus open to advective exchange with the host magma. Using serial sectioning techniques we show that microdioritic enclaves from the Ross of Mull granite, Scotland, contain an interconnected three-dimensional network of macroscopic channels filled with acidic melt from the host granite. Channel networks can be characterised by their genus, a discrete topological parameter relating pore structure to connectivity. The two most important variables in controlling the mean flow velocity of an infiltrating granitic melt are the pore diameter and the melt viscosity. Results suggest that for granitic melt viscosites < 10⁶ Pa s and channel diameters > 5 mm, flow (infiltration) velocities are of the order of centimetres per year. Under these conditions, advection will be more effective in transporting chemical components between acidic magma and enclave than diffusion alone.