Transient equilibrium and disequilibrium crystallization drive mafic mineral evolution in alkaline magmas

Our research hypothesis is that oscillations in undercooling and cooling rate are the primary drivers of both morphological transitions (from hourglass skeletal structures to mature dendrites) and compositional evolution (Fo, NiO, CaO, P, etc.) of olivine, clinopyroxene, and Fe Ti oxides in Yangyuan basalts, with equilibrium fractional crystallization (EFC) dominating near the liquidus and disequilibrium processes (e.g., compositional boundary layer effects, dendritic growth) becoming increasingly important during late stage crystallization. Our data show that near liquidus olivines align with EFC trends, while Fo = 79–80 olivines deviate from EFC predictions in NiO depletion and CaO enrichment; phosphorus zoning in olivine and clinopyroxene reveals repeated peaks corresponding to three dendritic growth stages, and integrated cooling rate proxies record a six stage cooling history marked by oscillatory undercooling and cooling rate, linked to punctuated dendritic growth. Notable findings include that EFC is only valid at the initial liquidus stage, disequilibrium processes are pervasive in late-stage crystallization, oscillatory cooling governs both growth morphology and compositional evolution, and stage boundaries in natural crystallization are blurred, with nearly identical compositional features (e.g., P contents) appearing across different stages.