The Generation of Barriers to Melt Ascent in the Martian Lithosphere

Planetary mantles can be regarded as an aggregate of two phases: a solid, porous matrix and a liquid melt. Melt travels rapidly upward through the matrix due to its buoyancy. When this melt enters the colder lithosphere, it begins to crystallize. If crystallization happens at a high rate, the newly formed crystals can clog the pore space, reducing its permeability to essentially zero. This zone of zero permeability is the permeability barrier. We use the MELTS family of thermodynamic calculators to determine melt compositions and the crystallization sequence of ascending melt throughout Martian history and simulate the formation of permeability barriers. At lower strain rates (10(-17)-10(-15)s(-1)) permeability barriers form deep in the lithosphere, possibly contributing to the formation of localized volcanic edifices on the Martian surface once fracturing or thermal erosion enables melt to traverse the lithosphere. Higher strain rates (10(-13)s(-1)) yield shallower permeability barriers, perhaps producing extensive lava flows. Permeability barrier formation is investigated using an anhydrous mantle source or mantle sources that include up to 1,000ppm H2O. Introducing even small amounts of water (25ppm H2O) reduces mantle viscosity in a manner similar to increasing the strain rate and results in a shallower barrier than in the anhydrous case. Large amounts of water (1,000ppm H2O) yield very shallow weak barriers or no barriers at all. The depth of the permeability barrier has evolved through time, likely resulting in a progression in the style of surface volcanism from widespread flows to massive, singular volcanoes. Plain Language Summary The surface of Mars has been affected by more than four billion years of volcanic history. It appears that the morphological expression of volcanic activity is not random through time but that there is a progression of styles from numerous and widespread caldera and lava flows and lava plains, to the formation of a few giant shield volcanoes (Hawaii-like volcanoes). We suggest that this evolution of volcanic styles is due to changes in the behavior of magma as the planet ages and cools off. Through computer modeling of the melting and crystallizing of rock ascending from the interior toward the surface, we relate the cooling history of Mars to the depth where magma accumulates on its way to the surface. This provides a possible explanation for the observed changes in volcanic landforms with time.