MANTLE DIFFERENTIATION AND THE CRUSTAL DICHOTOMY OF MARS

Two thermal evolution models for Mars with crust formation and mantle differentiation are compared. In the first model-termed the homogeneou s differentiation model-we assume that a basaltic crust has grown stea dily in 4.5 Ga as a consequence of pressure-release partial melting of mantle rock. The second model-termed the early differentiation model- incorporates the dichotomy and an early differentiation event. This ev ent is assumed to have resulted in a mantle depleted of radioactive el ements and a primordial enriched southern highland crust. We assume th at the primordial crust acts as an efficient thermal blanket on the so utherly hemisphere mantle. In a second stage of differentiation, a sec ondary basaltic crust in the northerly hemisphere is produced by press ure-release partial mantle melting. Our calculations suggest that the homogeneous differentiation model cannot explain the isotopic characte ristics of the SNC-meteorites, the concentration of Ar-40 in the prese nt Martian atmosphere, the dichotomy, and the long-term stability of t he northerly hemisphere volcanism. The early differentiation model has the required geochemical reservoirs (depleted mantle, enriched crust and, possibly, subcrustal mantle layer) and the calculated volume of t he secondary crust is consistent with the concentration of Ar-40 in th e atmosphere. The thickness of the secondary crust is between 10 and 4 0 km. It depends mainly on the amount of mantle depletion and the crus t production efficiency, but little on the amount of thermal blanketin g of the mantle by the primordial crust. The lithosphere thicknesses i n the two hemispheres, on the contrary, depend to a large extent on th e amount of thermal blanketing and little on the other two of the abov e parameters. The present lithosphere thicknesses are roughly 150-200 km in the northerly hemisphere and 350-500 km in the southerly hemisph ere. The present-day surface heat flow in the southerly hemisphere may be about 15 mW m-2 smaller than in the northerly hemisphere. Mantle t emperature decreases with the amount of depletion of the mantle and in creases with the amount of thermal blanketing, but differs by no more than about 100 K from mantle temperatures calculated in thermal evolut ion models that neglect differentiation. Therefore, earlier core evolu tion and magnetic field generation models [Stevenson et al., Icarus 54 , 466-489 (1983); Schubert and Spohn, J. geophys. Res. 95, 14,095-14,1 04 (1990)] calculated without allowing for mantle differentiation rema in essentially valid.