Decompaction channel formation and elevation in planetary lithospheres

As melt travels upwards from the convecting mantle and enters the colder lithosphere of planetary bodies, it likely becomes trapped below an impermeable horizon, or permeability barrier. Melt accumulates beneath this barrier in a high porosity channel known as the decompaction channel, too deep to be readily accessed by dikes or fractures. Yet, melt must be able to reach the surface to create the varied volcanoes and lava plains we observe throughout the solar system. Using the finite element code ASPECT, we model the formation and evolution of permeability barriers and decompaction channels in two dimensions. The absence of slope on the layer represents the base of the lithosphere in a one-plate planet. Designed to represent Mars in the early Hesperian, the model features a 150 km thick lithosphere above a 50 km thick segment of upper mantle. The temperature in the lithospheric domain follows a curved geotherm, while the mantle domain has an adiabatic profile of a convecting mantle. The adiabat of the mantle crosses the liquidus to allow for melt generation. Melt is generated in the first time step of the model, creating no more than 15% melt in any cell in the mantle. Over the first 1 million years of the model run, melt rises and freezes at a depth of 110 km. Melt then accumulates beneath the barrier in a 20 km thick layer where porosity can be as high as 35%. As melt collects and crystallizes near the top of the decompaction channel, downwellings of cold, melt-poor mantle descend from the top, subsequently heating up and becoming more melt rich before ascending. The cycle of melt-poor downwellings and melt-rich upwellings forms a convective system. This system is not thermally driven, as the convection occurs even if the thermal Rayleigh number is zero. Instead the convection is primarily driven by the effect of melt content on the density of the solid-melt mixture. As the convection progresses, melt-rich regions appear at the top of the channel, allowing melt to rise into the lithosphere are regularly-spaced intervals. This spacing approximately matches the nearest neighbor spacing of volcanoes on Syria Planum Mars, whose conditions during volcano formation match this model.