We ran several series of two-dimensional numerical mantle convection
simulations representing in idealized form the thermochemical evolution of a
Mars-like planet. In order to study the importance of compositional buoyancy of
melting mantle, the models were set up in pairs of one including all thermal
and compositional contributions to buoyancy and one accounting only for the
thermal contributions. In several of the model pairs, single large impacts were
introduced as causes of additional strong local anomalies, and their evolution
in the framework of the convecting mantle was tracked. The models confirm that
the additional buoyancy provided by the depletion of the mantle by regular
melting can establish a global stable stratification of the convecting mantle
and throttle crust production. Furthermore, the compositional buoyancy is
essential in the stabilization and preservation of local compositional
anomalies directly beneath the lithosphere and offers a possible explanation
for the existence of distinct, long-lived reservoirs in the martian mantle. The
detection of such anomalies by geophysical means is probably difficult,
however; they are expected to be detected by gravimetry rather than by seismic
or heat flow measurements. The results further suggest that the crustal
thickness can be locally overestimated by up to ~20 km if impact-induced
density anomalies in the mantle are neglected.Comment: 29 pages, 10 figure
