Reaction infiltration instabilities in mantle rocks: An experimental investigation

Melt extraction from partially molten regions of the mantle occurs along high-permeability pathways. Melt-rock reactions can lead to the formation of high-permeability channels due to a positive feedback between melt flow and reaction. To study this process, we performed a series of Darcytype experiments in which a cylinder of partially molten rock sandwiched between a melt source and a porous sink was annealed at high pressures (P=300 MPa) and high temperatures (T=1200° or 1250°C) under a controlled pressure gradient (∂P/∂x=0-100MPa mm^-1) for up to 5 h. The partially molten rock was formed from 50:50 mixtures of olivine (Ol) and clinopyroxene (Cpx) plus 4, 10 or 20 vol. % of alkali basalt. The melt source was a disk of alkali basalt undersaturated in silica with respect to the partially molten rock, and the sink was a disk of porous alumina. During an experiment, melt from the source dissolved Cpx in the partially molten rock and precipitated Ol, thereby forming a Cpx-free reaction layer at the interface between the melt source and the partially molten rock. The melt fraction as well as the grain size in the reaction layer increased significantly compared with that present in the starting material, confirming that the reaction increased the local permeability of the partially molten rock, one of the prerequisites for the reaction infiltration instability process to operate. In experiments carried out under a small pressure gradient (and hence slow melt flow velocity), the reaction layer remained roughly planar and no channels developed. However, if the melt flow velocity by porous flow exceeded ~0·1 mms^-1, the reaction layer locally protruded into the partially molten rock forming finger-like, melt-rich channels. The morphology and spacing of the channels depended on the initial melt fraction. In a partially molten rock with 20 vol. % melt, multiple, voluminous channels with an elliptical core of pure melt developed. At lower melt contents, fewer and thinner channels formed. Our experiments demonstrate that melt-rock reactions can lead to melt channelization in mantle lithologies, consistent with general predictions of the reaction infiltration instability theory.