Martian Crust: What is the Effekt of the Gabbro-Eclogite Transition on the Crustal Gavitational Stability?

Kurzfassung

Getting information about the martian crustal thickness is of primary interest for better understanding Mars dynamics and secular evolution. On a more specific point of view, the crustal thickness is crucial to better constrain how the Mars silicate layer differentiated into crust and mantle. On the one hand, coupled gravity-topography inversion models usually predict a mean crustal thickness of about 50-60 km (Wieczorek and Zuber, JGR 2004) to 100 km (Turcotte et al, JGR 107 2002), local values ranging from basically 0 to 150-200 km. On the other hand, thermal evolution models (Breuer and Spohn JGR 108 2003, Hauck and Phillips, JGR 107 2002) tend to predict a mean crustal thickness close to the highest estimates of the former models, or even significantly larger, depending on the initial temperature, on the viscosity, and on the thermal diffusivity. A very thick crust (from 100 to 250 km) is also inferred from global interior structure models (Sohl and Spohn, JGR 102 1997). From SNC analysis and surface spectroscopy measurements, the martian crust is thought to be mainly basaltic, with some possible andesitic material at the surface of the Northern hemisphere. As a consequence, the gabbro-eclogite type transition is likely to be relevant for the martian crust. Depending on the temperature profile, this transition can be initiated at a depth of about 50 to 100 km (Babeyko and Zharkov, PEPI 117 2000). The density of eclogite is much larger than basalt, and might even be slightly above that of the martian mantle (up to 3.53 kg.m-3 versus 3.4 to 3.55 kg.m-3 for the mantle). In these conditions, the question of a possible recycling of the lower crust back into the mantle is worth investigating: this could possibly lead to important consequences on the global dynamics of the planet, and may limit the crustal thickness. In the framework of 2D thermo-chemical convection models in presence of temperature-dependent viscosity, we will show that key parameters for this problem are: a. the temperature dependence of the viscosity; b. the ratio of crustal to stagnant-lid thickness, Lc / Lst. This ratio is likely to have encountered important variations through time, giving rise to very different situations with regards to the crustal delamination process. We will discuss the consequences of our results for both thermal evolution models on the one hand, and density profile models inferred from gravity inversion on the other hand. Finally, we will also interpret our study in terms of a potential maximal crustal thickness as a function of the global evolution of the planet. This work is funded by the European Community's Human Potential Programme under contract RTN2-2001-00414, MAGE.