  The melt productivity of a differentiated planet's mantle is primarily
controlled by its iron content, which is itself approximated by the planet's
core mass fraction (CMF). Here we show that estimates of an exo-planet's CMF
allows robust predictions of the thickness, composition and mineralogy of the
derivative crust. These predicted crustal compositions allow constraints to be
placed on volatile cycling between surface and the deep planetary interior,
with implications for the evolution of habitable planetary surfaces. Planets
with large, terrestrial-like, CMFs ($\geq$0.32) will exhibit thin crusts that
are inefficient at transporting surface water and other volatiles into the
underlying mantle. By contrast, rocky planets with smaller CMFs ($\leq$0.24)
and higher, Mars-like, mantle iron contents will develop thick crusts capable
of stabilizing hydrous minerals, which can effectively sequester volatiles into
planetary interiors and act to remove surface water over timescales relevant to
evolution. The extent of core formation has profound consequences for the
subsequent planetary surface environment and may provide additional constraints
in the hunt for habitable, Earth-like exo-planets.
