Channels or waves: controls on melt migration through the upper mantle

Understanding how melt is extracted and makes its way toward volcanoes is a fundamental problem in magma dynamics. Geological observations of ophiolites show tabular dunite channels, which are commonly considered to be reactive channels for melt migration. The reaction-infiltration instability has been identified as an important mechanism responsible for the formation of these high-porosity melt channels in the upper mantle. To better understand this mechanism, we have extended previous linear analysis and performed non-linear numerical simulations in a compacting, chemically reactive porous medium.

Strong interactions between compaction and dissolution lead to two interesting unstable features: (1) high-porosity channels and (2) compaction-dissolution waves. The channeling instability that grows monotonically comes from the positive feedback between chemical reaction and melt percolation. The oscillatory compaction-dissolution waves show a checkerboard pattern that migrates upwards in the melting region, driven by the nonlinear feedback between compaction and reaction. These instabilities are controlled by two key dimensionless parameters: the stiffness, which characterizes the system's ability to compact, and the Damköhler number, which describes the relative importance of reaction to advection. The stiffness is strongly affected by the compaction length, which may either follow an inverse power-law dependence on porosity or only a weak dependence on porosity. Here we present a regime diagram with a range of stiffness and Damköhler number values and show that compaction-dissolution waves are favoured in systems with smaller compaction length and lower stiffness relative to high-porosity channels.

The parameter regimes predicted by linear theory align well with the non-linear numerical simulation results. Simulations also show strong interactions between melt channels and oscillatory waves, where the melt channels are focused in the upper domain and porosity waves are in the lower part. The relationships between high-porosity channels and compaction-dissolution waves in this study may shed new light on the geochemical and petrological observations related to magma migration in the mantle.