We study a model for channels of magma flow within mantle undergoing decompression melting. Cylindrical conduits in a viscous, porous, compacting matrix are considered, and it is found that the dynamics of the conduit walls are governed by the competition between melting (caused by decompression) and viscous closure (caused by the reduced pressure in the conduit). There are many similarities with the Röthlisberger channels which transport melt water beneath glaciers. Pressure in these mantle conduits is very nearly magmastatic, and ascent velocities on the order of 100 m a-1 are predicted. Flow from the surrounding porous partially molten matrix into the low-pressure channel is considered and can supply a continual source of melt. The accumulation region is on the order of the compaction length, and the residual matrix is reduced to very low melt fractions, typically <0.5%. Channels form naturally from porous flow in the matrix because of the enhanced melting rate in regions of higher porosity, which have a larger heat flux from below. The vast majority of melt could be expected to flow eventually into one of these channels, which therefore offer a physical grounding for the conceptual near-fractional melting models used to explain field observations. Copyright 2009 by the American Geophysical Union.