Abstract
The Martian surface composition appears mainly mafic but recent
observations have revealed the presence of differentiated rocks, only in
the Highlands. Here, we demonstrate that differentiated melts can form
during the construction of thick crustal regions on Mars by fractional
crystallisation of a mafic protolith, without plate tectonics. On a
stagnant-lid planet, regions of thicker crusts contain more
heat-producing elements and are associated to thinner lithospheres and
to higher mantle melt fractions. This induces larger crustal extraction
rates where the crust is thicker. This positive feedback mechanism is
favoured at large wavelengths and can explain the formation of the
Martian dichotomy. We further develop an asymmetric parameterised
thermal evolution model accounting for crustal extraction, where the
well-mixed convective mantle is topped by two lithospheres (North/South)
characterised by specific thermal and crustal structures. We use this
model in a Bayesian inversion to investigate the conditions that allow
crustal temperatures to be maintained above the basalt solidus during
crustal growth, resulting in the formation of evolved melts.
Among the thermal evolution models matching constraints on the structure
of the Martian crust and mantle provided by the InSight NASA mission, a
non-negligible fraction allows partial melting and differentiation of
the crust in the south, which can occur very early (<100 Myr)
as well as during the Hesperian ; partial melting in the north appears
unlikely. Although crustal differentiation may occur on a hemispheric
scale on Mars, its vertical extent is limited to less than a third of
the crustal thickness.