Thermal and compositional evolution of the martian mantle: Effects of water

We present numerical models of the thermochemical evolution of the mantle of Mars with particular attention paid to the effects of water. With a two-dimensional, anelastic, compressible convection algorithm in combination with a parameterized model of composition and thermoelastic properties, we consider how water in nominally anhydrous minerals may have influenced the melting and rheology of the martian mantle and how it may have been redistributed by melting, melt extraction, and convection over a period of 4 Gy. The strongest effect of water is found to be a marked increase in convective vigor due to the lower viscosity, which leads to faster cooling of the mantle but nonetheless enhances melt production. Modeling results are compared with observations and independent inferences from spacecraft observations and martian meteorite analyses. Some models with radionuclide concentrations from Wänke and Dreibus (1994), but with initial mantle water concentrations between their estimate of 36 ppm by weight and 10 times this value, combined with ∼50% degassing of crust-forming material, partly match observed crustal concentrations of K, Th, and water. In order to account for the observed chemical variation in crustal rocks, however, the formation and survival of ancient mantle heterogeneities appears to have been necessary. Water loss from the mantle since the end of the magma ocean stage has been minor. Elevated water contents notwithstanding, the depth variation of viscosity in the sublithospheric mantle of a given model is likely minor. Depths to the Curie temperature at the end of the Noachian range from ∼40 to 60 km for magnetite and hematite in the models, and present-day total crustal thicknesses lie between 75 and 100 km. The depth to the brittle–ductile transition increases from values around 30 km in the early stages of the models to 170–240 km at present. An estimate of total core entropy renders the existence of a basal perovskite + ferropericlase layer unlikely. The absence of stable superplumes in all of our preferred models suggests that the volcanic provinces of Tharsis and Elysium were formed by a succession of more short-lived events, such as transient plumes, from specific regions at the core–mantle boundary.