The Effect of Volatile Content of Mantle on Planetary Thermal Evolution

The thermal history of terrestrial planets is investigated using a parameterized model that couples thermal convection and volatile cycling. The rate of volatile exchanged between mantle and surface reservoirs is calculated by balancing the amount of volatiles degassed in the atmosphere by volcanic and spreading related processes and the amount of volatiles recycled back in the mantle by the subduction process. We focus on water at this stage. Water recycling into the mantle is assumed to occur principally through subduction of a hydrated serpentine layer whose thickness is self-determined based on serpentine stability. Degassing occurs through magmatism which is parameterized using a volatile dependent solidus and variable content of water in the partial melt zone below the spreading centers. The model stress state is monitored for all parameter sweeps so as to evaluate the potential of changes in the tectonic mode of a planet (plate tectonics or a single plate mode). We present the results that explore model parameter space. We show how negative feedback effect of volatile dependent viscosity produces a self adjustment of the amount of water in the mantle, and we investigate the magnitude of variables that potentially switch the direction of the overall volatile flux of the mantle at some point in time.