whose mantle compositions were not located within the MELTS database and were generally those stars with bulk Fe/Si $<$ 0.95 [Bulk Earth Fe/Si = 1; @McD03]. For those stars in the Adibekyan dataset, we find 28% are of compositions likely to produce melt-extracted crust that remains buoyant relative to their mantles ($F_c > 0$), with 69% less chemically buoyant than the Earth (Figure \[fig:histogram\]). The Kepler dataset shows a similar trend, with 22% of stars likely to produce crusts with positive chemical buoyancy and 60% with crustal buoyancy forces greater than the Earth. Each of these systems are therefore *less likely* than the Earth or *completely unlikely* to maintain long-term, steady-state tectonics.

When $\delta_{\rm{Na}}$ is twice $\delta_{\rm{Na-Sun/Earth}}$ ($\delta_{\rm{Na}}$ = 0.52), only 1% of stars within the Adibekyan sample and 4% of the Kepler sample produce any plates with a negative chemical buoyancy force ($F_c < 0$), whereas all but fourteen stellar compositions in our sample of 1186 stars produce negatively buoyant plates when $\delta_{\rm{Na}}$ is half of $\delta_{\rm{Na-Sun/Earth}}$ ($\delta_{\rm{Na}}$ = 0.13; Figure \[fig:Sodium\]). The likelihood of producing negative chemical buoyancy in melt-extracted surface plates is therefore very sensitive to $\delta _{\rm {Na}}$ and weakly dependent upon bulk mantle composition. Stars 