<p>Basaltic melts may variably disaggregate macrocrysts (large crystals) from crystal mushes during the assembly of magma bodies beneath ocean-island volcanoes. The entrained macrocrysts modulate the crystallinity, density, and rheology of magmas, parameters that control magma system architecture and eruptive dynamics. However, the timescales, drivers, and consequences of inconstant crystal mush incorporation into carrier melts require quantification. Here, we use a suite of plagioclase-rich basalts to show that the entrainment efficiency (the ability for melts to disaggregate and entrain macrocrysts from crystal mushes) is temporally variable on inter-eruption timescales at ocean-island volcanoes. Macrocryst cargoes are predominantly out of equilibrium with their carrier melts in both chemistries and mass proportions (ratios of different macrocryst phases). Geochemical and petrological evidence reveals that macrocryst mass proportions are established in a density-stratified melt-rich reservoir shortly before eruption, whereas the absolute crystallinity is a function of crustal physics, likely driven by fluctuations of annual-to-millennial melt supply. Variations in entrainment efficiency explain several universal, but enigmatic, features in oceanic volcanic systems, such as decoupled crystallinity–temperature–time relationships and the dearth of plagioclase-rich basalts at fast-spreading mid-ocean ridges. Systematically studying temporally constrained eruptive products offers a unique window into the evolution of crystal-rich magma storage zones.</p>