Crystal cargoes of oceanic basalts often preserve records of mantle-derived chemical variability that are erased from their carrier melts by magma mixing. However, the impacts of mixing between similarly primitive but chemically variable magmas remain poorly understood despite ubiquitous evidence of chemical variability in primary melt compositions and chemical disequilibrium in erupted crystal cargoes. Here we report observations from magma-magma reaction experiments performed on analogues of primitive Icelandic lavas from chemically and lithologically distinct mantle sources to determine how crystals respond to mixing-induced chemical disequilibrium. Chemical variability in our experimental products is dominantly controlled by major element diffusion in the melt that alters phase equilibria relations and triggers waves of plagioclase resorption in initially plagioclase-saturated magmas. Isothermal mixing of chemically variable oceanic basaltic magmas may thus play a pervasive role in creating and modifying basaltic crystal cargoes by unlocking plagioclase-rich mushes and driving resorption, (re-)crystallisation and solid-state diffusion.