Causes and consequences of flow organization during melt transport: The reaction infiltration instability in compactible media

Geochemical and field evidence suggest that melt transport in some regions of the mantle is localized into mesoscale "channels" that have widths of 0. 1-100 m or larger. Nevertheless, the mechanisms for formation of such chann els from a grain-scale distribution of melt are poorly understood. The purp ose of this paper is to investigate one possible mechanism for channel form ation: the reaction infiltration instability (RII). We present numerical so lutions of the full equations for reactive fluid flow in a viscously deform able, permeable medium. We show that dissolution in a compactible solid wit h a vertical solubility gradient can lead to significant flow localization such that > 90% of the melt flux is channelized in < 20% of the available a rea. In particular, the ability of the solid to compact enhances the instab ility by forming impermeable regions between channels. The combination of r eaction, diffusion, and solid compaction leads to strong selection of prefe rred length scales with channel spacing smaller than the compaction length (<delta> similar to 10(2)-10(4) m). We explore the evolution of dissolution channels over parameter space and show that the behavior of the full nonli near solutions is consistent with predictions from linear stability analysi s. We also briefly consider the behavior of the instability in the presence of melting due to adiabatic decompression and demonstrate that significant localization can occur even in the presence of uniform melting and compact ion. Using the linear analysis to extend these results for parameters expec ted in the Earth's mantle suggests that robust channel systems could form t hrough the RII from a homogeneous system in similar to 100,000 years with d ominant channel spacing of 1-200 m.