Magmatic segregation and volcanic eruptions transport tidal heat from Io’s interior toits surface. Several observed eruptions appear to be extremely high temperature (≥1600 K),suggesting either very high degrees of melting, refractory source regions, or large amountsof viscous heating on ascent. To address this ambiguity, we develop a model that cou-ples crust and mantle dynamics to a simple compositional system. We analyse the modelto investigate chemical structure and evolution. We demonstrate that magmatic segre-gation and volcanic eruptions lead to differentiation of the mantle, the extent of whichdepends on how easily high temperature melts from the more refractory lower mantlecan migrate upwards. We propose that Io’s highest temperature eruptions originate fromthis lower mantle region, and that such eruptions act to limit the degree of compositionaldifferentiation.