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.
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We thank Anette von der Handt for priceless help with EMPA measurements and numerous discussions, Brian Bagley for acquiring crystal clear mCT data and help with visualization and analysis, and Chris Frethem for assistance with SEM imaging. Alejandra Quintanilla-Terminel, Amanda Dillman, Marc Hirschmann and members of the Kohlstedt and Hirschman laboratories are gratefully acknowledged for discussions. Yan Liang and Clint Conrad are thanked for providing the alkali basalt used in this study. Insightful reviews by Richard Katz, Yan Liang and an anonymous reviewer have significantly improved this paper and are gratefully acknowledged The Hitachi SU8320 SEM was provided by NSF MRI DMR-1229263. Parts of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the MRSEC program. The funding by the National Science Foundation grant OCE-1459717 is gratefully acknowledged
- Experimental petrology
- Melt migration
- Melt-rock reaction
- Reaction infiltration instability
- Upper mantle