Why are Martian volcanoes so lonely?

First view of the Tharsis Montes from Mariner 9. The poor image quality is due to a global dust storm that was occurring at the probe's arrival.

Martian volcanoes were confirmed to exist in the early 1970s, from pictures taken by the Mariner 9 orbiter. These first images revealed massive shield volcanoes, like Hawaii on Earth but taller than Mount Everest and wider than Ohio.  One of the first observations was that three of these larger volcanoes, the Tharsis Montes, exist in a straight line, each separated by about 750 km. Rather than a volcanic chain like Hawaii, the line of the Tharsis Montes appears to be related to a rifting feature, but why are they spaced with a certain distance?

Topographic map of the Tharsis province.

My research focuses on the processes occurring beneath the surface of Mars that result in the spacing of volcanoes. Specifically I am looking at the role of permeability barriers and decompaction channels in the Martian lithosphere. As hot mantle melt rises up into the Martian lithosphere it cools and crystallizes out in the surrounding rock matrix. At a certain horizon, the crystals will clog the matrix preventing further upwards movement. This horizon is the permeability barrier. Melt from below continues to rise up until it encounters the barrier. Pressure builds as the melt accumulates, so a horizon of high porosity is forced open below the barrier, like air forcing a balloon to expand. This is the decompaction channel.

I use the alphaMELTS computer program to simulate this process. Starting with an estimate of mantle composition, the virtual material is brought up from great depth, where it melts from decompression. When the material reaches the bottom of the lithosphere the melt continues to rise and the crystallization is simulated. The minimum depth of the permeability barrier is assumed to be where the crystallization rate is a maximum.

Results of one dimensional crystallization simulation. The bottom of the graph represents the bottom of an imagined 50 km lithosphere. The red line is the crystallization rate of melt as it ascends. The permeability barrier in this simulation would likely be at about 38 km depth.

Permeability barriers and decompaction channels have been studied in terms of mid ocean ridges on Earth. The slope of the mid ocean ridge combined with the barrier and channel focus a large region of melt to the ridge axis. On Mars a similar focusing could be occurring which would focus melt below surface volcanoes. The act of crystallization actually releases heat, which would allow melt to rise above the normal permeability barrier layer, like thermal erosion. Anywhere in the layer that is a little thermally eroded would focus melt to that point, increasing the erosion rate there. The overall instability of the layer would have a preferred wavelength due to the balance of barrier rise and melt supply rate. This instability is to be modeled in two and three dimensions and compared to the actual distribution of volcanoes on the surface of Mars.

Diagram of permeability barrier instability. Melt (red arrows) releases heat (green arrows) and is focused by the slope of the unstable permeability barrier (thin black line). The initial permeability horizon is represented by dashed black line.