MELT INFILTRATION AND ADVECTION IN MICRODIORITIC ENCLAVES

Geochemical investigations of microdioritic enclaves in acid plutons h ave shown that extensive chemical and isotopic exchange can occur betw een enclave and host magma. Although some estimates of the rates of ch emical exchange have been established, most of these models assume tha t diffusion is the mechanism of elemental transport and exchange. We p resent here a simple physical model for chemical exchange between host magma and enclave where melt infiltration and advection are the domin ant 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 microd ioritic enclaves from the Ross of Mull granite, Scotland, contain an i nterconnected three-dimensional network of macroscopic channels filled with acidic melt from the host granite. Channel networks can be chara cterised by their genus, a discrete topological parameter relating por e structure to connectivity. The two most important variables in contr olling the mean flow velocity of an infiltrating granitic melt are the pore diameter and the melt viscosity. Results suggest that for granit ic melt viscosites < 10(6) Pa s and channel diameters > 5 mm, flow (in filtration) velocities are of the order of centimetres per year. Under these conditions, advection will be more effective in transporting ch emical components between acidic magma and enclave than diffusion alon e.