An analytical, mathematical model is discussed for the transport of trace element and isotopic ratios during magma migration, when no major dissolution or precipitation of minerals occurs, but diffusive exchange of atoms between magma and solid is important. This model is then used to elucidate the effect of channel spacing on the transport of trace element and isotopic ratios during the migration of basaltic magma. It is found that, when total reequilibration occurs, as it will in migration through closely spaced channels on grain boundaries, isotopic ratios are transmitted to the surface unchanged but trace element ratios may vary wildly. When channels are far enough apart so that only partial reequilibration occurs, variations in both isotopic and trace element ratios are reduced or eliminated during magma migration. Finally, when channels are sufficiently distant so that negligible reequilibration occurs, both trace element and isotopic ratios are transmitted to the surface unchanged. ¿ 1998 American Geophysical Union