Water, Melting, and Convection in the Martian Mantle

We present models of mantle convection focused on the situation in early Mars in which particular attention is paid to water and its effects on partial melting, melt extraction, and viscosity. We have combined the mantle convection program STAGYY [7] with a parameterized thermodynamic model of martian mantle mineralogy [5] and carried out calculations of convection in a twodimensional compressible model of the planet’s mantle. As the depth to the base of the mantle is known only to within a few hundred kilometers, models with core– mantle boundary depths of 1700 km (LC) and 2000 km (SC), which essentially bracket the range of possible values, are considered. The models are heated from below by a cooling core and from within by the radioactive decay of K, Th, U, and U. At the top, a rigid boundary with a temperature corresponding to the average surface on Mars is imposed as a boundary condition. The model domain is a full spherical annulus [2]. Near-fractional hydrous melting is included in a parameterized form by modifying the dry solidus of martian peridotite [1, 6] by means of a simple method to include the solidus-lowering effect of water in low concentrations [3]; the exhaustion of phases is also taken into account in a simple form. Melt deeper than the density crossover depth is retained. Melting reduces the concentration and changes the distribution of both the water and the heat-producing radionuclides in the solid mantle. Different initial water contents are assumed, and water content is allowed to evolve with time as a consequence of melting and dehydration; the content of the trace components is tracked with tracer particles, and half of the water carried with erupting melts is removed from the model permanently by outgassing. The viscosity of the mantle is dependent on temperature, pressure, water content, the amount of retained melt, and the presence of high-pressure polymorphs of olivine or of basalt/eclogite and therefore also undergoes a secular change as the mantle cools and becomes depleted in some parts and accumulates basaltic crustal material in others. Several aspects of Mars’ internal structure are not well known, so that we have to probe the effects of several model parameters within a certain range; among these, core size, bulk Mg#, initial radionuclide and water content and distribution, and initial potential temperature are particularly important. In this presentation, we show a subset of this survey that centers on two waterfree (D) models and two with an initially homogeneous water content of 200 wppm (W), all with a bulk Mg# of 0.75; radionuclide contents [8] are the same in all four models, and are also initially homogeneous. In either pair, one model has a large core and the other has a small one, so that in the latter case, a dense layer of perovskite (pv)+ferropericlase (fp) exists at the bottom.