Modelling of volcanic processes is strongly limited by a poor knowledge of the timescales of storage, mixing and final ascent of magmas into the shallowest portions of volcanic 'plumbing' systems immediately prior to eruption. It is impossible to measure these timescales directly; however, micro-analytical techniques provide indirect estimates based on the extent of diffusion of species through melts and crystals. Here, diffusion in olivine phenocrysts from the 1959 Kīlauea Iki eruption is used to constrain the timing of mixing events in the crustal plumbing system on timescales of months to years before eruption. The timescales derived from zonation of Fe-Mg in olivines, combined with contemporaneous geophysical data suggests mixing occurred on 3 timescales: (1) up to 2 years prior to eruption in the deep storage system, (2), in a shallow reservoir, between incoming hot melts and cooler, resident melt for several weeks to months prior to eruption, and (3), in the conduit and summit reservoir, between the resident magma and cooled surface lava, draining back into the vent on timescales of hours to several days during pauses between episodes. Synchronous inflation of the shallow reservoir with deep earthquake swarms and mixing suggests a fitfully open transcrustal magmatic system prior to and during eruption.