Melt infiltration and advection in microdioritic enclaves

Geochemical investigation of microdioritic enclaves from the centre of the Ross of Mull granite, W Scotland, show that extensive chemical and isotopic exchange can occur between enclave and host magma. A simple physical model is presented for chemical exchange between host magma and enclave where melt infiltration and advection are the dominant transport mechanisms. For this to occur, the enclave itself must be incompletely crystallized and thus open to advective exchange with the host magma. The Ross of Mull microdioritic enclaves are seen petrographically to contain an interconnected three-dimensional network of macroscopic channels filled with acidic melt from the host granite. 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 are taken to suggest that for granitic melt viscosities <<<$I> 10 (super 6) Pa s and channel diameters >><$I> 5 mm, flow (infiltration) velocities are of the order of cm/yr. In these conditions, advection will be more effective than diffusion alone in transporting chemical components between acid magma and enclave.