Deciphering magma evolution below ocean basaltic volcanoes is all the more challenging because magma mixing is a common process tending to modify the pristine geochemical diversity during magma ascent. On the western flank of the Piton de la Fournaise volcano, transitional basalts have compositions that testify to origins down to the upper mantle and display a widespread geochemical diversity ranging from a tholeiitic affinity to an alkaline one. There, we show that evolved melt inclusions and matrix glasses (MgO < 9 wt%) record an alkali enrichment coupled with a Ca/Al ratio decrease, which tracks the effect of clinopyroxene crystallization at the depth of the mantle-crust underplating layer. At this depth and shallower, reverse zoning of olivine crystals, clinopyroxene dissolution, and hybrid melt compositions testify to extensive mixing processes leading to a homogenization of the pristine geochemical footprint of melts upon ascent. Enrichment in incompatible trace elements in some evolved melt inclusions suggests that magma ponding at the depth of the mantle-crust underplating layer favours also assimilation of melts originating from low degrees of partial melting of cumulates (wehrlites, dunites). Conversely, the most primitive melt inclusions documented so far at La Réunion Island (MgO up to 11.2 wt%) better preserve a pristine geochemical variability related to partial melting of a slightly heterogeneous mantle source. We suggest that these slightly distinct source components may mirror the compositions of recent melts from the two closely located Piton de la Fournaise and Piton des Neiges volcanoes.