Reactive Melt Migration and Channelization in Partially Molten Rocks
Abstract:Several lines of evidence suggest that melt from partially molten regions of the mantle moves rapidly along chemically isolated pathways toward Earth’s surface. In some previous experiments, reactive melt infiltration led to channelization; in others, it did not. To better understand the conditions required for channel formation, we developed an experimental set-up that allows flow of a reactive melt through a rock under a controlled pressure gradient by sandwiching a partially molten rock between a melt reservoir and porous sink.
Hot-pressed 50:50 mixtures of olivine (Ol) and clinopyroxene (Cpx) with either 4 or 20 vol% alkali basalt formed ~4 mm long cylinders of partially molten rock. Source and sink are disks of alkali basalt and porous alumina. A melt reservoir-to-rock volume ratio of 2:1 ensured chemical disequilibrium. We anneal melt:rock:sink triplets for up to 5 h at a confining pressure of Pc=300 MPa with effective pressure Pe=0 to 299.9 MPa (Pe = Pc - Pp) , at T = 1200° & 1250°C.
Two distinct melt migration features form, 1) a planar reaction layer and 2) finger-shaped channels. Both the reaction layer and the fingers contain Ol + melt with little to no Cpx. Planar reaction layers develop at the melt-rock interface in all experiments. Finger-shaped channels form only in experiments in which a fluid pressure gradient exists from source to sink, i.e. Pe>0
The thickness of the planar reaction layer increases with increasing T and with the square root of time, indicating that diffusion controls the growth rate, reaching thicknesses of ~70 µm at 1200°C and ~200 µm at 1250°C in 5 h, though layer thickness is independent of Peand initial melt content. The finger-shaped channels are more closely spaced and thicker in the experiments at higher temperature and higher initial melt content.
To conclude, under our laboratory conditions, channelization of a reactive melt required application of a fluid pressure gradient; scaling conditions for channel formation requires further experiments and connection to theory.