Modeling of volcanic processes is limited by a lack of knowledge of the time scales 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 time scales directly; however, micro-analytical techniques provide indirect estimates based on the extent of diffusion of species through melts and crystals. We use diffusion in olivine phenocrysts from the A.D. 1959 Kīlauea Iki (Hawai‘i, USA) eruption to constrain the timing of mixing events in the crustal plumbing system on time scales of months to years before eruption. The time scales derived from zonation of Fe-Mg in olivines, combined with contemporaneous geophysical data, suggest that mixing occurred on three time scales: (1) as much as 2 yr prior to eruption in the deep storage system; (2) in a shallow reservoir, between incoming hot melts and 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 time scales of hours to several days during pauses between episodes. Synchronous inflation of the shallow reservoir with deep earthquake swarms and mixing suggests an intermittently open transcrustal magmatic system.