Martian crust: a modeling approach

In this study, we use the technique of petrophysical modeling [Sobolev, S.V.. Babeyko, A.Yu., 1994. Modelling of mineralogical composition, density and elastic wave velocities in the anhydrous rocks. Surv. Geophys. 15, 515-544.] to construct mineralogical, density, and velocity models of the deep Martian crust. Calculations are performed in two steps. First, given bulk chemical composition and PT conditions, the equilibrium mineralogical composition is calculated by numerical thermodynamic simulation. Effect of kinetics on mineral transformations was modeled by accepting a ''freezing'' temperature. Second, density and velocities are calculated from the mineralogy. In computations, bulk chemical composition of the crust was accepted to be the average of four basaltic SNC meteorites (Shergotty, Zagami, BETA79001, lithologies A and B). Four various temperature models of the crust were considered: with the temperature gradient of 21, 13, 6, and 2 K/km. The mineral composition of the consolidated crust varies with depth due to a gabbro-eclogite type transition; density and velocities increase correspondingly. The generally accepted data on the structure of the outer layer of the Moon are taken into account in modeling the outer (similar to 11 km), porous layer of the planet. The maximum thickness of the crust was determined from the condition that the density at the crust-mantle boundary equals 3.45-3.5 g/cm(3). The computations showed that, in the case of a cold marsotherm, the crustal density at a depth of 60 km ranges from 3.3 to 3.5 g/cm(3), and at a depth of 120 km, it is similar to 3.5 g/cm(3). In the case of a hot marsotherm, the density 3.45 g/cm(3) is reached at a depth of 150 km; 35 g/cm(3), at a depth of similar to 170 km. The consolidated crust may be divided into several zones according to the distribution of density and its seismic-wave velocities. Regions with a high heat flow have a low velocity (and density) zone in the 10-120-km depth range, underlain by a zone of steep gradients of these parameters. For small temperature gradients, computations yield a qualitatively different picture. In the similar to 10 to (30-65)-km depth range, the density and velocities are approximately constant. From 30-65 to 80-120 km follows a zone of steep gradients. Below velocities and density are nearly constant or slightly decrease down to the crust-mantle boundary. Since density in the lowermost crust exceeds values of similar to 35 g/cm(3), the lower crust is likely to be gravitationally unstable over geologic time intervals. (C) 2000 Elsevier Science B.V. All rights reserved.